UNLOCKING THE POTENTIAL OF CIRCADIAN BIOLOGY FOR CARDIOVASCULAR HEALTH

Abstract

Circadian rhythms, governed by the body’s endogenous clock mechanism, regulate daily fluctuations in cardiovascular function, optimizing physiological processes like blood pressure regulation, cardiac metabolism, and myocardial repair. Rhythms also align cardiovascular reactivity with predictable environmental and behavioral cycles, enabling normal function and affecting disease susceptibility. Major adverse cardiovascular events, including myocardial infarction, ventricular arrhythmias, and stroke, exhibit a distinct morning peak, highlighting circadian regulation in cardiovascular health. Controlled human laboratory studies demonstrate that beyond the influences of sleep and other behaviors, endogenous circadian rhythms independently regulate blood pressure, autonomic nervous system activity, blood clotting, vascular tone, and metabolic function. Additionally, the kidney plays a critical role in circadian sodium handling, fluid balance, and blood pressure control, with disruptions in renal circadian rhythms contributing to hypertension and progression to heart failure. Chronic circadian misalignment resulting from shift work, irregular sleep-wake cycles, or misaligned lifestyle habits is strongly associated with increased cardiovascular risk and disease progression. The emerging field of Circadian Medicine applies circadian principles to clinical care, leveraging interventions such as optimizing light exposure, meal timing, and physical activity to restore biological alignment. Chronotherapy, the strategic timing of medications or procedures to align with a patient’s diurnal or circadian rhythms, offers further potential for enhancing treatments and reducing adverse effects. By integrating circadian biology into cardiovascular medicine, novel strategies are emerging to help prevent disease, improve patient outcomes, and enhance therapeutic precision. Understanding the interplay between circadian regulation and cardiovascular physiology provides a foundation for advancing cardiovascular prevention and treatment strategies.

1. INTRODUCTION

“Our body is like a clock; if one wheel be amiss, all the rest are disordered; the whole fabric suffers; with such admirable art and harmony is a man composed”

(Robert Burton in The Anatomy of Melancholy, 1621).

1.1. The endogenous circadian system

Biological systems are not static; they are dynamically regulated to anticipate environmental and behavioral changes. While early physiologists such as Claude Bernard (1813–1878) and Walter Cannon (1871–1945) emphasized homeostasis as a key principle, modern research demonstrates that biological functions also incorporate endogenous variability, for instance, ~24 hour rhythmicity as governed by the endogenous circadian system (1). Rather than simply maintaining a fixed internal state, cells and tissues rely on this circadian system to coordinate metabolic pathways, enzymatic reactions, and physiological responses, ensuring that they occur not only in the right location but also at the right time and to the right extent. Harmony among molecular rhythms within the cell, in tune with the external environment as well as ongoing and anticipated behaviors, is essential for healthy organ biology (2). Thus, cell physiology is inherently 4-dimensional, organized across space and time (3, 4) (FIGURE 1).

Figure 1. Cell Physiology is 4D: Organized in Space and Time.

This figure depicts that all cellular processes, including chromatin loops, messenger ribonucleic acid (mRNA) activity, gene transcription, protein expression and metabolism have a rest/activity cycle across the 24-hour period. While these cellular processes may respond to changes in behaviors and the environmental, these processes are also endogenously regulated across the 24-hour cycle by the circadian system, as can be demonstrated under constant behavioral and environmental cycles. The circadian system therefore facilitates the expected physiological changes based on the daily changes in the external environment as well as anticipated regular daily behavioral cycles, such as wake/sleep, fasting/feeding and activity/rest. Thus, physiological function (including metabolism, neural activity, and endocrine secretions, etc.) is inherently four-dimensional, organized across both space and time (1, 91, 228), and at the core of this daily variation is the endogenous circadian timing system.

The fundamental discovery of the molecular circadian clock was recognized with the 2017 Nobel Prize in Physiology or Medicine, awarded to Jeffrey C. Hall, Michael Rosbash, and Michael W. Young for identifying the core circadian genes period, timeless, and doubletime in Drosophila, and for elucidating the transcriptional-translational feedback loops that drive circadian rhythms at the cellular level. Similar components of this molecular clock system have been conserved across 600 million years of evolutionary opportunity from flies to mammals (5, 6), and these clock mechanisms are present in virtually all tissues and cells. The circadian system can be thought of as a hierarchical network of synchronized clocks that optimize physiological functions across the day and night based on anticipated changes in the environment, and the daily regular rest/activity, fasting/feeding and sleep/wake cycles (7, 8). In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus functions as the central circadian pacemaker, synchronizing peripheral clocks in cells, tissues and organs through a combination of neural pathways and circulating hormones, such as glucocorticoids and melatonin (9, 10). In turn, these peripheral clocks regulate local oscillations in gene and protein expression, metabolism, and tissue homeostasis (11). The SCN itself synchronizes with the outside world mostly by daily light cues (12). This coordination ensures that local clocks in peripheral tissues, such as the heart and vasculature, remain synchronized with one another and with the external environment. Of note, when this harmony breaks down, as can occur with night shift work, irregular feeding schedules or artificial light at night, peripheral clocks may drift out of phase with the SCN or with each other (13). In the cardiovascular (CV) system, such misalignment dampens or disrupts the normal circadian oscillations of clock gene expression. The result is a blunting or loss of daily rhythms in heart rate, myocardial contractility, substrate utilization, and the response to stress (14, 15), as will be discussed below..

1.2. Interaction between reflex cardiovascular control and circadian cardiovascular control

Classical physiological studies have shown how the CV system reflexively responds, via neurohumoral pathways, to almost every psychological, behavioral and environmental change in highly characteristic ways (16). However, the hierarchical circadian system also affects CV physiology via similar neurohumoral connections from the SCN to the entire CV system including the peripheral clocks in CV tissues (17, 18). Indeed, over the last ~15 years there has been growing evidence that these control systems interact such that the magnitude of responses to a standardized psychological, behavioral and environmental may be different at different times and day and night, i.e., at different phases of the circadian cycle (14, 19–22). This demonstration of interactions between the circadian influences and the psychological, behavioral, and environmental influences on the CV system has led to the concept that these circadian influences optimize physiological function and reactions in an anticipatory way (19). For instance a circadian system driven increase in circulating cortisol towards the end of the night before waking helps prepare an individual for appropriately elevated CV responses to the activities expected upon awakening (23). This is a particularly salient because it takes many minutes to increase circulating cortisol after an acute behavior change such as waking up, changing posture, exercising or eating, hence increasing cortisol well before expected awakening ‘primes the system’ (23). Further evidence that the circadian system promotes overall CV health in normal situations is provided by the adverse effects that occur when this optimal alignment is perturbed when behaviors occur at unusual circadian phases (i.e., circadian misalignment). Numerous epidemiological studies have shown that chronic circadian misalignment between behaviors and the circadian system, caused by night shift work, irregular sleep-wake cycles, or nighttime light exposure, is strongly associated with increased risk of cardiovascular diseases (CVD), including hypertension, atherosclerosis, myocardial infarction, and heart failure (24, 25). Human laboratory studies in healthy individuals are consistent with the epidemiology studies and demonstrate that circadian misalignment alters CV function (26). Further, numerous animal models provide mechanistic insights into how even brief circadian disruption can have lasting effects on organ function and long-term CV risk (27, 28). Thus, further below we discuss how the circadian system aids optimal CV function, how CV health is adversely affected by chronic circadian misalignment, and how various approaches aimed at enhancing alignment of behaviors to underlying circadian rhythmicity (i.e., ‘Circadian Medicine’) can reduce CVD risk and improve CV health.

1.3. Circadian system as a cardiovascular risk factor

CVD is the leading cause of death worldwide (29, 30), and while the circadian system may help prepare for optimal CV responses in healthy people, these same circadian CV changes may themselves contribute to the robust day/night pattern of major adverse CV events (MACE), including myocardial infarction, sudden cardiac death, ventricular arrhythmias, and stroke in vulnerable individuals with underlying CV pathology (19). These events occur most frequently between 6 AM and noon (31, 32) (FIGURE 2). For instance, circadian-regulated morning peaks in coagulation factors and vasoconstrictive hormones, while perhaps evolutionarily adaptive, may be counterproductive in individuals with underlying atherosclerosis and could contribute to increased CV risk (19, 33–35). In contrast, these same behaviors at other circadian phases do not put the vulnerable individual at risk. Further below, we describe a theoretical model whereby increased CV risk is most likely to occur based on the physiological responses to an ongoing stressful activity (e.g., exercise) occurring at a specific circadian phase in people with underlying CV vulnerabilities (19, 23).

Figure 2. Diurnal variation in the risk for major adverse cardiovascular events.

There is a marked diurnal variation in the occurrence of major adverse cardiovascular event. For example, ischemic strokes, myocardial infarction, ventricular tachycardias, and sudden cardiac death all tend to cluster in the morning between 6 AM and noon (indicated by the red bracket on the x-axis). The gray rectangles indicate assumed approximate nocturnal sleep periods in these populations (i.e., 23:00–07:00). Note, these epidemiological studies reported adverse events relative to clock times, and the corresponding internal circadian phases are unknown. Redrawn from (136, 512–514).

1.4. Scope of this review

In this review we synthesize findings from epidemiological studies, controlled human laboratory experiments and preclinical animal studies to provide a comprehensive understanding of: (1) the mechanisms by which molecular clocks across the CV system help regulate CV physiology (in combination with CV responses to behaviors, stress and the environment); (2) how misalignment between daily behaviors and the underlying circadian system affects CV health; (3) how the circadian system may be involved in the morning increase in adverse CV events in vulnerable people; (4) the role of renal circadian rhythms in BP regulation, sodium handling, and fluid balance, and how disruptions in renal circadian rhythms contribute to hypertension, renal dysfunction, and long-term CV pathology; (5) the emerging field of Circadian Medicine, which applies principles of circadian biology to CV care, including strategies for optimizing circadian alignment through lifestyle interventions, including light exposure, meal timing, and physical activity, including a special focus on the elderly; (6) opportunities for chronotherapy to enhance drug efficacy or reduce side-effects by timing medications according to circadian timing; and (7) gaps in the field and future research directions that could facilitate the clinical translation of circadian science into CV medicine. By integrating fundamental circadian biology with human physiology, preclinical models, and clinical applications, this review aims to advance the implementation of circadian principles in CVD prevention, diagnosis, and treatment.

2. MOLECULAR, NEURAL, AND ENDOCRINE CONTROL OF CIRCADIAN RHYTHMS, AND CARDIOVASCULAR REGULATION

2.1. Molecular mechanisms of the circadian clock

Molecular circadian clocks, with a period of ~24 hours, are evolutionarily conserved across diverse taxa, including insects, fish, birds, and mammals, with species-specific adaptations. In Drosophila melanogaster, core clock genes such as period and timeless regulate circadian cycles through autoregulatory feedback loops, coordinating behavioral patterns like foraging and mating (36, 37). Insects and aquatic species exhibit light-sensitive cells capable of generating rhythmic behaviors independently of a central pacemaker (38–41). As noted above, in humans and other mammals, the primary circadian pacemaker, or the central clock, is located in the SCN of the hypothalamus, a network of heterogeneous neurons situated above the optic chiasm. The SCN synchronizes circadian rhythms across the body by integrating external light cues via the retino-hypothalamic tract and coordinating organ function through autonomic nervous system projections plus hormonal control. In addition to this direct neuroendocrine control, the SCN also synchronizes downstream peripheral clocks, and thereby 24-hour rhythms in physiological function, through similar neuroendocrine time cues to many tissues such as the heart, vasculature, and kidneys (42–44). Although peripheral clocks in these tissues can oscillate autonomously, SCN-driven synchronization is essential for maintaining coordinated organ function across the body, including CV function.

At the cellular level, the circadian system operates through a transcription-translation feedback loop (TTFL) (5). In mammals this core mechanism involves the heterodimerization of circadian locomotor output cycles kaput (CLOCK) and brain and muscle ARNT-like 1 (BMAL1), which bind to E-box response elements to activate transcription of key circadian genes, including the genes that encode the PERIOD (PER1, PER2, and PER3) and CRYPTOCHROME (CRY1 and CRY2) proteins. PER and CRY proteins accumulate in the cytoplasm, then translocate to the nucleus where they inhibit BMAL1-CLOCK activity, closing the negative feedback loop that sustains circadian oscillations with ~24-h rhythmicity (45, 46). The complex molecular machinery of the mammalian circadian clock is now well established (5, 47, 48). This ~24-hour rhythm of transcriptional activities drives oscillations in clock-controlled genes (CCGs), influencing nearly all physiological functions (FIGURE 3). While peripheral clocks in CV and renal tissues can cycle autonomously (49, 50), they require synchronization from the SCN via neural, endocrine, and physiological pathways for normal 24-hour cycling of integrated physiological function (47, 51, 52). Disruptions to the core clock machinery are associated with systemic dysfunction, CVD, and other pathologies, as discussed further below.

Figure 3. The core circadian proteins in the transcription translation feedback loop.

Circadian rhythms are driven by a highly conserved molecular mechanism involving core clock genes that regulate rhythms of their own transcription/expression via negative/positive feedback loops. In mammals, the central loop consists of CLOCK, BMAL1, PERs, CRYs genes. The rhythmic expression of these core clock genes leads to the downstream regulation of clock-controlled genes, which govern a wide range of cellular processes. Created in BioRender. Gumz, M. (2025) https://BioRender.com/zblplqs

2.2. Control of cardiovascular rhythms by the circadian system

The SCN exerts direct neural control over CV function via autonomic pathways that regulate sympathetic and parasympathetic tone in mammals (FIGURE 4). Research in rats shows that the SCN projects to the paraventricular nucleus (PVN) of the hypothalamus, where it modulates autonomic output through excitatory (glutamate) and inhibitory (gamma-aminobutyric acid, GABA) neurotransmission (53–56). Neural outputs of the SCN are important for coordinating CV rhythms. For instance, parabiosis experiments in SCN-lesioned and intact mice revealed that while behavioral and bloodborne cues alone may be sufficient to sustain some diurnal rhythmicity in the liver and kidney, direct SCN signaling is necessary to maintain rhythmicity in core clock gene rhythms in the heart and spleen (57). Earlier SCN transplantation and lesion studies in rats and hamsters confirmed that while transplantation restores ~24-hour locomotor rhythms, endocrine or temperature rhythms are not recovered, emphasizing the importance of SCN projections (58–60). SCN cells can be subdivided based on their principal neuropeptide expression, such as those that release vasoactive intestinal polypeptide (i.e., VIP cells), or those principally expressing arginine vasopressin, and these have differential effects on downstream CV targets. For instance, VIP cells in the SCN are either directly or indirectly essential for circadian locomotor rhythms, environmental light resetting, and day-night rhythms in heart rate (HR) and corticosterone levels as discovered using both rat and mouse models (61–66). Moreover, the SCN has a multi-synaptic projection to the heart (67), plus projections to pre-autonomic neurons in the PVN, which regulate adrenal and hepatic sympathetic activity, increasing sympathetic tone before the active phase in rats (analogous to daytime in humans) (68, 69). The SCN also projects to the medial hypothalamus and medial forebrain bundle in rats, integrating circadian control across multiple physiological systems (70).

Figure 4. Circadian Organization and Cardiovascular Physiology.

The molecular clocks as illustrated in Figure 3 are present not only in the suprachiasmatic nucleus (SCN)—the central pacemaker—but also in virtually all peripheral organs and tissues, including cardiac/vascular tissues, adrenal glands, kidneys, and the immune system (middle column). Rhythms in these central and peripheral oscillators are represented by the circled cosine waves. Timing signals, also called Zeitgebers, from the environment (e.g., light, as indicated by the image of the sun) and from behaviors (e.g., food intake, as indicated by an apple, and physical activity/rest, as indicated by the stationary bike/bed) affect the rhythms of the central clock (i.e., the SCN [red cosine]) and/or peripheral clocks (shown as black cosine curves for heart, lung, blood cells, adrenal gland, kidneys, liver, muscle, gastrointestinal track). The timing of the Zeitgebers determines the direction and magnitude of phase shifts (top left inset, light phase response curve as an example, adapted from (141)). The temporal signals can be transmitted between the SCN and the peripheral clocks via the autonomic nervous system, hormonal signals, and body temperature (shown by the white double arrow). Through this relay of timing information, the body’s rhythms can shift in response to environmental and behavioral cycles, and modify CV functions/physiology accordingly (right column).

Numerous hormones, such as epinephrine, cortisol and melatonin have endogenous circadian rhythms and have broad effects on the CV system with these effects changing with circadian phases (19). For example, circulating melatonin is a robust circadian hormone used by many to define the phase of the central circadian pacemaker (71) and is regulated by the SCN via a multi-synaptic pathway including the PVN, the intermediolateral nucleus (IML) and the superior cervical ganglion (SCG) and the pineal gland where melatonin is produced and released in circulation. The SCN modulates the SCG, suppressing melatonin production during the daytime and promoting its release at night (72, 73). Light exposure at night also acutely suppresses melatonin and can phase shift the SCN rhythms. Melatonin, in turn, regulates CV physiology by reducing sympathetic vasomotor tone through enhanced GABAergic activity in the PVN (74) and directly influences myocardial clock gene expression (75, 76) in rats and mice. In humans, melatonin may also influence CV function through feedback inhibition of multi-unit activity in the SCN itself, thereby changing the SCN output to the autonomic nervous system (71, 77, 78). These neural circuits position the SCN as the central regulator of CV function, integrating autonomic, hormonal, and genetic pathways in response to environmental light cues (FIGURE 4).

Beyond neuro-humoral pathways from the central circadian pacemaker, circadian clocks are also present in nearly all organs of the CV system, including the heart, vasculature, and kidneys (79). They have been identified in both non-human (rodent) and human hearts (80–82), where they govern numerous key processes such as transcription, translation, nutrient storage, metabolism, growth and repair, ventricular repolarization, contractile function, and tolerance to ischemia-reperfusion injury, to name a few (83–87).

2.2.1. Circadian clocks in the heart

There are daily rhythms in cardiomyocyte function which involve intrinsic regulation mediated by a cell-autonomous circadian clock, which operates through transcription/translation feedback loops (81). In vivo, these cardiomyocyte circadian clocks govern the timing of many essential cardiac muscle cell functions including preferred metabolic pathways, and changes in contractility, growth, repair and responsivity to external influences (88). One of the earliest demonstrations of the molecular circadian mechanism in the heart came from studies of fatty acid metabolism. Circulating fatty acid levels vary over 24-hour day, with generally higher levels during sleep. Young and colleagues first showed that cardiac responsiveness to fatty acids varies with time of day, enabling metabolic adaptation (89). This work led to the discovery of oscillating circadian genes in adult rat cardiomyocytes in vitro. These oscillations were associated with rhythmic cardiac metabolism (81, 85) establishing a new mechanism for myocardial metabolic control. Microarray studies of murine hearts across diurnal (27, 80) or circadian (90) cycles further revealed that ~10% of the cardiac genome undergoes daily rhythmic regulation, not only in metabolism but also in cardiac growth, renewal, transcription, translation, and repair pathways. Human cardiomyocytes in culture were later confirmed to possess 24-hour oscillations of core circadian mechanism genes (82). These circadian changes in the heart are presumably evolutionarily adaptive. For instance, it has been postulated that since the heart has a high workload during the waking part of the day, cardiac protein synthesis is minimized then to avoid competition for energy substrates, whereas protein translation rises across the sleep period - when contracting workload is lower - to enable appropriate cardiac growth and remodeling (91). The fact that the circadian clock has broad influence on myocardial gene expression, cardiac contractile function, and metabolism was further evidenced by studying cardiomyocyte-specific circadian clock mutant mice (CCM) (92, 93). In terms of circadian effects on the reactivity to stresses, CCM mice exhibit a striking time-of-day dependence in ischemia-reperfusion tolerance, mediated by the cardiomyocyte clock (94). In an analogous finding in humans, circadian variation in infarct size was reported in a retrospective study of ST-elevation myocardial infarction (STEMI) (95). A BMAL1 cardiomyocyte knockout mouse (CBK) (96), studied alone or in combination with other models, further demonstrated circadian control over responsiveness of the heart to hypertrophic stimuli, cardiac metabolism, and contractile function (83, 97–104). These studies also implicate BMAL1 loss in the pathophysiology of CVD (87, 105–112).

In addition, the potential role of circadian changes on cardiac function on the timing of sudden cardiac death is thought to arise from the interaction between behavioral triggers and associated increased sympathetic nervous system signaling on top of an endogenous variation in the susceptibility of the heart to arrhythmias (113–115). Indeed, susceptibility to arrhythmias can be linked to electrophysiological properties and circadian control in the heart, which extends to ion-channel expression and electrophysiological properties. As one example, the transcription factor Krüppel-like factor 15 (KLF15) regulates rhythmic expression of Kv channel-interacting protein 2 (KChIP2), a key modulator of transient outward potassium current, contributing to diurnal variation in the electrocardiogram’s QT-interval duration (116). However, overall, the precise mechanisms and dynamics of these interactions between central and peripheral circadian control of the CV system are essentially unknown at the molecular level.

2.2.2. Circadian clocks in the vasculature

Peripheral circadian clocks have also been found in all layers of the vasculature, including the coronary arteries, the aorta, endothelial cells, vascular smooth muscle cells, and fibroblasts, where they regulate vascular resistance, signaling, and homeostasis (27, 111, 117–122). For example, circadian transcription factors regulate endothelial nitric oxide synthase (eNOS) expression, modulating vascular tone and endothelial function in mice (123). Disruption of BMAL1 (e.g., global Bmal1-KO) impairs eNOS signaling, contributing to endothelial dysfunction and increased CV risk. The BMAL1-CLOCK complex is also responsive to mechanical stress, modulating smooth muscle cell growth and extracellular matrix remodeling as seen in mice, key processes in CV health and disease (123–125). Tissue Inhibitors of Matrix Metalloproteins display profound diurnal rhythms under normal 24 hour light and dark cycles (80), and Matrix Metalloproteinases (MMPs) including MMP-2 and MMP-9, display circadian oscillations. MMP dysregulation due to loss of functional CLOCK or BMAL1 has been shown to contribute to vascular stiffness and pathologic remodeling in mice (126).

2.2.3. Other peripheral circadian cardiovascular clocks

As with other organs, approximately 20% of genes in the kidney are rhythmically expressed under circadian control, influencing sodium reabsorption, urine volume and concentration, and BP regulation (127, 128). The renal molecular clock plays a central role in daily BP rhythms, with various disruptions shown to lead to impaired fluid balance (129–131), abnormal cardiac metabolism, signaling, and contractile function (87, 93), worsened carotid atherosclerosis (132), and exacerbated renal injury and hypertension in salt-sensitive mice (133), even while BP rhythms are maintained.

Circadian control extends to coagulation pathways, with BMAL1 directly regulating prothrombotic factors and thrombomodulin, an anticoagulant protein. Loss of BMAL1 in mice leads to a prothrombotic phenotype, increasing susceptibility to thromboembolic events (117, 134). These findings highlight that circadian disruption, whether through genetic mutations or external misalignment, can profoundly impact vascular integrity, coagulation homeostasis, and overall CV health.

2.2.4. Integration of central and peripheral circadian control of the cardiovascular system, and interaction with behavioral and environmental stressors

The fact that circadian rhythmicity can be demonstrated in many isolated CV tissues begs the question of how these cells interact with the central circadian pacemaker in vivo, and how they interact with other inputs related to behavioral and environmental stressors. It is certainly possible that direct or indirect circadian neuroendocrine signals from the central circadian pacemaker, such as circadian variations in norepinephrine, may synchronize these peripheral circadian rhythms in some circumstances (81). It also seems likely that the varying states of the peripheral clocks across the day and night will result in different CV responses to the same inputs (e.g., behavioral, environmental or psychological stresses, or even direct or indirect neuroendocrine signals from the SCN). This may underlie much of the variability in responsiveness of the CV system seen at the physiological level, as described in the next section. What is currently known about the optimization of circadian cardiac function, how external versus cellular-specific circadian regulation shapes CV physiology, and how their interaction or misalignment affects CV function and pathological consequences has been recently reviewed (88). At present, the precise molecular mechanisms by which endogenous clocks in the heart and vasculature interact with the central circadian pacemaker in the SCN or other extrinsic factors such as behavioral or environmental factors, is mostly unknown, and this presents an important opportunity for the field to address.

3. IMPORTANCE OF CIRCADIAN CARDIOVASCULAR RHYTHMS TO HUMAN HEALTH AND IMPLICATIONS FOR DISEASE

In this section, we discuss: (1) how adverse CV events occur more frequently at specific times of day; (2) approaches to studying CV circadian rhythms in humans with highly-controlled laboratory studies; (3) independent effects of the endogenous circadian system on CV function at rest; (4) endogenous circadian modulation of CV reactivity in response to behavioral/environmental factors; (5) how these circadian changes in CV reactivity could contribute to the morning increase in MACE; and (6) adverse effects of circadian misalignment (mistiming of behavioral/environmental cycles relative to the central circadian clock) on CV function and CV risk markers.

3.1. Major adverse cardiovascular events at vulnerable times in vulnerable people

As noted above, MACE do not occur randomly across the day (135) but are clustered around a morning vulnerable period (31, 32, 136) (FIGURE 2). In people who have an underlying predisposition to MACE (e.g., due to CVD), this daily pattern of MACE likely emerges from three sources. First, CV responses to rapid changes in behavior during the morning—including waking from sleep, posture changes, and sudden physical activity—can precipitate adverse events (137). Second, the time of day itself represents a time of vulnerability to CV events due to the phase of the circadian rhythms in physiology and behavior (138). Third, the likeliest scenario, is a mixture of these in which a behavior like exercise is harmless at most times of day, but may trigger an adverse event in this clock-defined window in those with underlying vulnerability. This theoretical model of the role of the circadian system in increased morning risk for MACE is shown in FIGURE 5 which depicts a continuous interaction between environmental factors (e.g., temperature), behavioral factors (e.g., exercise), stressors (e.g., psychological stress) and the underlying circadian rhythm in a CV risk marker (e.g., epinephrine). In healthy people, the circadian system primes the CV system for optimal functioning with increased reactivity in the morning which matches the anticipated morning behaviors, and without CV risk. In people with existing risk factors, the resting CV risk markers may be elevated, and the reactivity in the morning hours can cross a theoretical risk threshold for triggering MACE. Thus, the circadian system may be potentially implicated in the morning increase in adverse CV events.

Figure 5. Conceptual diagram for the role of the circadian system in increased morning risk for adverse CV event.

There is a continuous interaction between environmental factors (e.g., temperature), behavioral factors (e.g., exercise), stressors (e.g., psychological stress), and the underlying circadian rhythm in a CV risk marker (e.g., epinephrine or blood pressure). A sample of the resting basal rhythms (blue lines) and responsivity rhythms (yellow lines) is shown for each potential risk profile. In healthy people (a), the circadian system primes the CV system for optimal functioning with normally increased reactivity in the morning (orange shaded area). But there is no increased CV risk (i.e., the rhythms do not cross the red CV risk zone). We also present three conceptual models to help explain the increased risk in the morning of an adverse CV event in “unhealthy” people with existing risk factors: (b) a normal basal rhythm but increased responsivity in the morning hours; (c) an elevated basal rhythm with normal responsivity; and (d) an elevated basal rhythm plus increased responsivity in the morning hours. In these last 3 cases, the individuals cross into the risk zone for adverse CV events in the vulnerable morning hours. Adapted from figure 1,(19).

3.2. Approaches to studying circadian rhythms in humans

Virtually every part of the CV system responds to almost every psychological, behavioral, and environmental change as instigated by autonomic and endocrine responses. Since many behaviors have a general day/night pattern of occurrence, such as the daily sleep/wake cycle and the fasting/feeding cycle, then CV variables could display day/night variations based solely on the 24-hour patterns of these behaviors. But, as noted above, the SCN connects with all parts of the CV system also causing ~24 changes in all CV variables, even under constant behavioral and environmental conditions. These separate behavioral/environment vs. circadian factors can be revealed experimentally by controlling the environment and scheduling all behaviors, such as sleep opportunities, meals and exercise (20, 139). Specifically, to assess the contribution of the endogenous circadian system to CV physiology, dissociation of circadian effects from those caused by behaviors and environmental factors can be achieved through two established experimental protocols in human research (FIGURE 6). The first method, known as the constant routine (CR), requires participants to remain in a controlled environment with consistent, spaced-out behaviors for at least 24 hours (140). In this protocol, participants maintain wakefulness while lying in a semi-recumbent position and consume identical iso-caloric snacks at regular intervals (e.g., every 2 hours). The environment is kept under dim lighting (ideally <10 lux to minimize any effect of light on the circadian pacemaker (141, 142), constant temperature, and without external time cues. While circadian rhythms can be observed in various physiological variables under these conditions, the potential confounding factor of sleep loss can also influence the results (143, 144). To overcome this limitation, these two effects—circadian rhythms and continuous wakefulness (i.e., sleep loss)—can be mathematically separated by assuming a 24-hour sinusoidal circadian rhythm superimposed on the monotonically increasing effects of sleep loss (145), however the dynamics and magnitude of the accruing effect of continuous wakefulness on a particular variable is rarely known. The second, more rigorous protocol is termed forced desynchrony (FD), which extends over several days and avoids the issue of accumulating sleep loss (146). Participants follow a schedule that deviates from the natural 24-hour day, living on recurring sleep-wake cycles of identical duration that are not aligned with the body's internal circadian clock. In this approach, sleep/wake cycles and other behaviors (e.g., fasting/feeding, activity/inactivity) are intentionally shifted and desynchronized from their natural timing. The idea is to ensure that by the end of these multiple sleep/wake cycles, all behaviors will have been scheduled to occur evenly across the full circadian cycle, and still allowing sufficient time for sleep (often preserving the ratio of wakefulness being twice the duration of the sleep opportunity – akin to a normal 16-hour wake period and 8-hour sleep period per 24 hours). Many different cycle lengths (non-24-hour “days”) have been tried e.g., 28 hours (allowing 9 hours 20 mins sleep opportunity every 28 hours). This forced desynchrony offers a more robust experimental design than the constant routine, allowing for a comprehensive analysis of circadian influences on CV function. Controlling for all behavioral variables across the circadian cycle enables statistical separation and quantification of two key effects: (1) the average circadian rhythm (averaged across all behaviors that are evenly distributed across the circadian cycle), and (2) the average behavioral effect (estimated across all phases of the circadian cycle). In addition, it is possible to examine the type of interaction between circadian effects and behavioral effects. Specifically, if responses to behaviors are the same at all points across the circadian cycle then we would infer a simple mathematical summation (i.e., linear interaction), whereas if responses differ at various points in the circadian cycle this would infer a more complex nonlinear interaction (e.g., caused by a physiological ceiling effect, such as a maximum heart rate).

Figure 6. Comparison of diurnal rhythms and circadian rhythms.

Diurnal rhythms are daily patterns that occur across the day and night and are an end-result of additive and interacting influences by the endogenous circadian system, behavioral rhythms, and/or environmental changes. Circadian rhythms are internal, biological processes that follow an approximately 24-hour cycle, driven by the endogenous circadian system that persist even in the absence of rhythms in behavioral or external cues like sleep, nutrient intake, physical activity, and light. To determine if a diurnal pattern (left) is—at least partially—caused by an endogenous circadian rhythm (right), it is necessary to measure the outcome of interest under constant environmental and behavioral conditions (i.e., Constant Routine protocol) or under conditions in which masking factors are uniformly distributed across the circadian cycle (i.e., Forced Desynchrony protocol). In this figure, the diurnal rhythm (grey line) in systolic BP (top left) and heart rate (bottom left) (515) is caused by an underlying circadian rhythm (right, orange line) (20) in concert with changes caused by activity, eating, and light (during the daytime), and sleeping, fasting, and darkness (during the nighttime). The middle panel shows diurnal rhythms with constant rest in supine posture, and with sleep in darkness at night, and wakefulness and eating during the day (blue line). The circadian rhythms were assessed in forced desynchrony protocol (20). The vertical line in the top right corner indicates 10% (brown) change from the mesor. Substantially redrawn from figure 2, (515) and figure S2, (20)

Using such laboratory experiments in humans it has been shown that the CV system exhibits strong circadian oscillations at multiple levels, such as governing BP regulation, myocardial metabolism, renal function, cardiac cycle regulation, vascular resistance, vascular reactivity and platelet aggregation across the 24-hour daily cycle (19, 34). Studies using constant routine protocols, forced desynchrony paradigms, and time-isolated environments confirm that circadian-driven increases in BP, and immune factors, occur independently of ongoing behaviors (20, 22, 147–150). Of note, these CR and FD studies that reveal endogenous circadian rhythms in physiology have almost exclusively been performed in healthy humans, with very few validated circadian studies in more vulnerable populations, with a few notable exceptions in people with asthma (151), epilepsy (152), obstructive sleep apnea (OSA) (153, 154), and in midlife (148) and older adults (155). Thus, future studies will be required to investigate the influence of population characteristics on circadian control of CVD, including interactions with other disorders, medication usage, age, sex, and ethnicity and race (156).

3.3. Circadian rhythms in cardiovascular physiology at rest

3.3.1. Blood pressure, heart rate and the autonomic nervous system

In humans, blood pressure (BP) follows a well-defined day/night pattern, typically dipping at night and rising in the morning (25). The relatively decreased BP during the sleeping hours is related to the influence of changes in posture, sleep state, and temperature upon BP, summated with an underlying endogenous circadian rhythm of BP that also declines across the night (and is actually lowest in the biological morning) (20, 139). These summated day/night patterns presumably provide for optimal physiological function, including increased reactivity when awake and decreased reactivity during the rest and recovery phase which is aligned with sleep (34).

In the first CR studies to test the effect of the endogenous circadian system on BP, no evidence for an endogenous circadian rhythm was found (157). These studies were conducted when there was less widespread recognition of the high sensitivity of the human circadian system to light. These early CR protocols allowed light levels up to 100 lux, an illuminance that can have a substantial effect on the circadian phase and on physiology (158), and allowed participants to leave the bed every 3 hours to visit the restroom. Both of these study limitations may have influenced the findings. In the first FD study to investigate the influence of the circadian system on BP regulation, a significant rhythm was detected using an FD protocol with twelve 20-h ‘days’ in dim light conditions (20). This study, together with two other independent circadian protocols, including an FD protocol with 20-h cycles and a CR protocol, showed that BP has an endogenous circadian rhythm with a consistent peak in the circadian evening at about 8 PM, a trough around 8 AM, and a peak-to-trough difference of 3 to 6 mm Hg for systolic BP and 2 to 3 mm Hg for diastolic BP (20, 139). Using a short-cycle FD protocol (recurring 5 hours 20-minute cycles) in middle-aged healthy people, an endogenous circadian rhythm in BP was also observed, but with a ~3 hour earlier circadian peak (and a lack of a detectable circadian rhythm in HR [see further below] (148). Given that lower BP is generally protective against MACE, these circadian studies indicate that the endogenous circadian modulation of BP, with a trough in the circadian morning, is unlikely to contribute to an increased risk for MACE in the morning, that is, unless the timing of circadian BP regulation is dramatically different in certain vulnerable populations. Indeed, as proof of concept of altered circadian CV rhythmicity in a patient group, it was recently found in a small study that people with OSA have a substantially altered circadian rhythm in BP compared to a control group without OSA (153).

The existence of a circadian rhythm in HR had been demonstrated by CR protocols already in the 1990’s, showing a peak-to-trough amplitude consistently of about 6 bpm and a peak during the middle of the circadian day (159, 160). These rhythms were also confirmed by more recent FD protocols (20) (139), showing a trough during the middle of the circadian night and a peak during the middle of the circadian day. Of note, in midlife adults, this rhythm was not observed (148). While the circadian rhythm in HR shows the steepest rise during the circadian morning, the peak occurs at a circadian phase equivalent to between noon and 6 pm, such that any contribution to the increased morning risk is less clear. HR is influenced by both the sympathetic and parasympathetic branches of the autonomic nervous system (ANS), raising interest into the role of each in circadian control of HR and CV function.

These circadian effects on BP and HR are at least partly mirror the underlying strong circadian effects on the autonomic nervous system (ANS). As noted above, there is evidence for a multi-synaptic pathway from the SCN to the heart involving the sympathetic nervous system that indicates a neural substrate for one of the ways by which the SCN may regulate the heart (67). Moreover, while cardiomyocyte-specific clock gene mutation in rodents causes bradycardia relative to wild-type mice, they still have a similar amplitude of the diurnal rhythm in heart rate (93), suggesting that this rhythm is not solely dependent on the cell-autonomous clock in the cardiomyocytes, but is driven by neural and/or endocrine factors presumably emanating from the molecular clock in the SCN.

Parasympathetic nervous system control of the heart (i.e., cardiac vagal modulation, mostly caused by respiratory sinus arrhythmia) is typically estimated from HR variability (HRV) measures of high-frequency (HF) power in the spectral analysis of inter-beat-interval. These measures include HF power (HF), root mean square of successive differences (RMSSD), and percentage of sequential inter-beat intervals varying by more than 50 msec (pNN50). The reason why HF power is specific for parasympathetic nervous system modulation is that those high frequencies (~0.15–0.40 Hz) are too fast for the slower sympathetic effects to take effect. For sympathetic nervous system activity, circulating epinephrine and norepinephrine concentrations have been used, and the ratio of low-frequency to high-frequency power (LF/HF ratio) from HRV analysis has been used as a marker of sympatho-vagal balance. Using an FD protocol, Hilton and colleagues, revealed an endogenous circadian rhythm in pNN50, a measure of cardiac vagal modulation, with a peak during the middle of the circadian night (149). Also using an FD protocol, it was shown that the sympathetic nervous system has a robust endogenous circadian rhythm, with plasma epinephrine and norepinephrine peaking during the middle of the circadian day, reaching a trough during the circadian night, and having large circadian peak-to-trough amplitudes of ~70% and ~35%, of the 24-h average, respectively (20). This same study further confirmed the endogenous circadian rhythm in cardiac vagal modulation markers (using both pNN50 and the HF power), with the circadian peak during the biological night, and with large 20–35% peak-to-trough amplitudes. The rhythm in circulating epinephrine mirrors the circadian rhythm in HR, with similarly timed peaks and troughs (lowest concentrations in the first half of the circadian night, increasing concentrations in the circadian morning, and a peak in the middle of the circadian afternoon), but interestingly, the circadian BP rhythm is shifted later by a few hours, presumably due to other BP regulatory factors beyond immediate ANS effects. The circadian rhythm in cardiac vagal modulation is inverted and slightly delayed compared to the sympathetic markers, with the timing of the circadian peak occurring during the habitual waketime and the trough in the evening, which matches the circadian BP rhythm (i.e., BP high when cardiac vagal modulation is low, and vice versa).

Just like for HR, the circadian peak in sympathetic nervous system markers and the trough in parasympathetic markers do not align precisely with the circadian morning (i.e., the time of highest risk for MACE). However, the rate of change in these markers could be important. Notably, the steepest rise in sympathetic activity and the sharpest decline in parasympathetic tone occur during this vulnerable morning period. Whether this rapid shift, potentially involving adrenergic or muscarinic receptor sensitization or desensitization, contribute to the morning surge in MACE remains an important unanswered question.

3.3.2. Vascular function and blood flow

Vascular endothelial function plays an important role in BP and blood flow regulation: higher endothelial function allows for higher blood flow reactivity and is protective. Vascular endothelial function, measured by brachial flow-mediated dilatation (FMD), is a robust marker of CV health (161). With regards to the increase in CV risk in the morning, diurnal studies have shown that FMD is impaired in the biological morning (162, 163). However, contrary to expectations, one well-controlled laboratory study showed that FMD actually increases from evening to morning independent of whether sleep or wakefulness occurred during the night between both measurements (164). In a separate study using a short-cycle FD protocol (recurring 5 hours 20 minutes), it was subsequently shown that vascular endothelial function has an endogenous circadian variation with the lowest FMD in the circadian evening, an increase across the biological night, a morning decline and the highest FMD in the circadian late afternoon (148). This effect occurred whether or not the results were adjusted for shear rate, which is a surrogate measure of the downstream resistance vessel function and can affect the upstream dilation. These results suggest that while the circadian system influences FMD, it may not play a critical role in the increased risk for MACE in the morning compared to the evening. In a similar way to these peripheral vascular results, in a circadian FD study of coronary microvascular function in healthy people, assessed with cardiac ultrasound during drug induced vasodilatory challenges (intravenous injection of Regadenoson, which produces coronary vasodilation and hyperemia by adenosine A2a receptor–mediated vasodilation), it was found that coronary microvascular function is also higher in the circadian morning in healthy people, which would likely be protective against MACE in this healthy group (165). A recent study showed that in people with OSA, the endogenous circadian system impairs FMD in the biological night in people with OSA (154), in whom MACE is also more likely to occur across the night as opposed to in the morning as in the general population (166, 167). This finding could implicate the circadian system in the increased risk for MACE. At this point, studies of the circadian influences on vascular function in people with underlying CVD are lacking.

Apart from those circadian effects on peripheral vasculature and the heart, brain blood flow regulation is critical to maintaining optimal brain oxygen supply and brain BP. In a modified CR (i.e., admission on the morning of the CR, <25 lux light exposure, and permission to leave bed for bowel movements) by repeatedly measuring middle cerebral artery blood flow by transcranial Doppler, it was found that there exists a circadian trough in cerebral blood flow velocity in the biological morning i.e., ~6 hours after the core body temperature minimum, which coincides with the period of increased risk of stroke in the morning (168). Thus, the impaired CBF regulation in the circadian phase corresponding to the late morning, may relate to increased risk for MACE at that time.

3.3.3. Platelet activation, platelet aggregability and fibrinolysis

In addition to the importance of cardiac and vascular control, another key factor that can modulate the risk for MACE relates to blood clotting and—the opposing force—the breakdown of blood clots. Platelet activation and aggregation play a key role in blood clotting while fibrinolysis regulates the breakdown of blood clots. Both these processes have recently been shown to be under endogenous circadian control.

Using an FD protocol with twelve 20-hour behavioral cycles, it was shown that platelet activation (as measured using flow cytometry of repetitive plasma samples) is under circadian control with 6–17% peak-trough amplitude, and peaks in platelet surface activated glycoprotein (GP) IIb-IIIa, GPIb, and P-selectin at a circadian time consistent with the increased morning risk for MACE (150). In the same study, circadian rhythms in platelet ATP release and platelet aggregometry were also observed, but with a later circadian phase equivalent to the late afternoon or early evening. These data indicate that the circadian system influences several aspects of platelet function and aggregation and may play a role in increasing CV risk in the morning hours.

On the other hand, fibrinolysis (the breakdown of blood clots) is important in the recovery process after blood clotting and important to keep the blood clotting process in check. It had been observed that the concentration of plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue plasminogen activator (tPA), is higher in the morning than in the evening in healthy individuals but also in patients with coronary artery disease; and it has been speculated to play a role in the increased risk in the morning for thrombotic events and MACE (169). It was subsequently shown, using an FD protocol in healthy humans, that PAI-1 is under robust control by the circadian system while controlling for any additional effects of sleep, physical activity, eating, and light upon PAI-1 (147). These data show that the relative amplitude and the timing of the primary peak were remarkably similar between the endogenous circadian rhythm and the rhythm observed on the baseline day when the same individuals were awake and eating during the day and sleeping, fasting, supine, and resting during the night. The data showed a robust peak in the circadian morning. Thus, the circadian rhythm in PAI-1 may increase CV vulnerability in circadian morning and play a contributing role in the increased risk at that time for MACE. Together, these data indicate that both platelet function and fibrinolysis are under robust circadian control and that they may combine to increase vulnerability for MACE in the morning in vulnerable patient populations.

3.3.4. Other endocrine effects

Numerous hormones that have CV effects have been shown to be under robust endogenous control including melatonin, cortisol, and epinephrine. Melatonin is often described as the hormone of darkness as it contributes to signaling the circadian night to the rest of the body (and the duration of elevated melatonin is relevant for coding seasons in many species). Thus, melatonin is one of the mechanisms linking light with CV function. Circulating concentrations of melatonin are synthesized and released from the pinealocytes of the pineal gland which is under SCN control via a multi-synaptic pathway – as described above (10, 170). Melatonin has one of the largest amplitude circadian rhythms with multiple functions, including across the CV system, such as promoting vasodilation, reducing oxidative stress, and supporting endothelial function (170). Moreover, given its antioxidant effects, the administration of exogenous melatonin may reduce hypertension, and attenuate myocardial damages following cardiac ischemia/reperfusion (171). With relevance to BP regulation, repeated but not acute evening melatonin administration one hour prior to bedtime resulted in a reduction in BP at night and an increase in the 24-h BP rhythm amplitude in patients with untreated hypertension (78, 172, 173). Given the hypotensive effects of melatonin, although evidence is mostly derived from using melatonin at pharmacological concentrations, these studies suggest that elevated melatonin concentrations at night may contribute to the decrease in BP across the circadian night. Despite these widespread CV effects, further research is still needed in larger groups to investigate the effect of melatonin on daytime and nighttime BP, interactions with demographics, sustained vs. immediate release formulations of melatonin, dose-dependency, interactions with antihypertensive medications, and whether CV effects are mediated through modulation of circadian function, sleep, or antioxidant mechanisms (174–176).

Cortisol, released by the adrenal cortex, is frequently referred to as the ‘stress hormone’. However, the magnitude of circadian control of cortisol is typically much stronger as compared to the effects of the sleep/wake, fasting/eating, rest/activity cycles, and even very stressful events such as explanation of open-heart surgery plus chest shaving in anticipation of open-heart surgery (26, 177, 178). This large-amplitude circadian rhythm in cortisol, with a rapid rise starting about four hours prior to the habitual wake time, a circadian peak at the habitual wake time, and a gradual decline across the habitual 16-hour wake episode, may potentiate the effects of catecholamines on CV function and contribute to changes in CV functions, such as the increases in HR and BP during the circadian morning.

Epinephrine, also called adrenaline, released by the adrenal medulla, has been shown to be under robust endogenous circadian control, with a rapid rise in the circadian morning, a peak during the early circadian afternoon, and a trough during the circadian night (20, 139). The circadian rhythm in epinephrine, together with the circadian rhythm in norepinephrine and cardiac vagal modulation described above, likely drives the circadian rhythm in HR and may contribute to the circadian rhythm in platelet function (150) and PAI-1 (147, 179).

3.3.5. Other markers

There are many other CV-related function and/or risk factors that have been shown to be under circadian control in human studies, including kidney function (e.g., urine secretion (147, 180); QT interval and PR interval on electrocardiogram, the latter of which reflects atrioventricular conduction time, both of which follow the same relative timing as the R-R interval (the time between successive heartbeats (181); cholesterol concentrations (182); microbiota composition (183); and immune function (184). Other variables related to CV function and risk requiring further study include endogenous circadian control of human baroreceptor function, estimated glomerular filtration rate, vascular function related to smooth muscle sensitivity, nitric oxide production, cardiac/vascular remodeling, atherosclerotic plaque build-up and vulnerability, and ion channel function, etc..

3.4. Circadian rhythms in cardiovascular reactivity to behavioral and environmental stimuli/stressors

In humans a typical day starts with waking up in the morning, ocular exposure to light, rising up from bed, engaging in physical activity, eating, being exposed to the psychological and physical stressors throughout the day, periods of waking restfulness, and many other factors, and then sleep at night. Of course, the exact sequences can differ and many of these transitions are gradual and/or variable. Below, we summarize the effect of the main behavioral and environmental factors on CV markers and provide details, where data are available, of changes in the reactivity to these factors across the endogenous circadian cycle. In many instances we address whether the specific physiological circadian changes would appear evolutionarily adaptive in healthy people based on the anticipated 24-hour environmental and behavioral cycles, and whether such changes could also play a potential role in triggering MACE at specific times in people with existing CV vulnerabilities, due to underlying CVD. This model of the circadian system and its interaction with disease is discussed further below.

3.4.1. Awakening

The cortisol awakening response is thought to optimize physiological responses, including CV responses, to the anticipated stressors related to awakening. Bowles et al examined the cortisol awakening response across the circadian cycle in healthy adults using two complementary FD protocols (10 recurring 5-hour 20-minute sleep/wake cycles, and 5 recurring 18-hour sleep/wake cycles) (23). In both protocols there were significant differences in magnitude of the cortisol awakening responses, with peaks occurring at circadian phases corresponding to 3:40–3:45 AM. Surprisingly, there was no detectable cortisol awakening response during the circadian afternoon. This circadian effect could have implications for shift workers who wake up at unusual circadian phases and thereby would likely have diminished responses to stressors after awakening. Similarly, using an ultra-short FD (recurring cycles of 1 hour wakefulness and 1 hour scheduled sleep), Boudreau and colleagues studied how circadian phase influences the CV response to scheduled awakening in healthy participants. They found the largest increase in HR and in LF/HF ratio (a marker of sympatho-vagal balance) in the circadian morning (185). These data suggest that the circadian system may increase physiological preparedness following scheduled morning awakenings compared to other times of awakening. However, as acknowledged by the authors, actual awakening from sleep was not assessed in this study, thus follow-up studies of CV reactivity to awakenings while using simultaneous polysomnography for assessment of sleep are recommended.

3.4.2. Posture

Changing from lying to standing is a daily occurrence for most people, for instance, soon after awakening. Except in particular patient populations, e.g., those with orthostatic, neuromuscular, or vestibular disorders, people do not usually give this transition much thought even though it is accompanied by dramatic shifts in body fluids requiring counterregulatory hemodynamic changes to ensure blood flow to the brain. A passive head-up tilt table test (HUTT) can test for orthostatic hypotension and other conditions related to syncope (fainting). It turns out that the time of day of testing is an important factor to consider. Hu and colleagues, using a two-week FD protocol enabling twelve HUTT performed in each participant every 20 hours around the endogenous circadian cycle, discovered that the same HUTT resulted in a dramatically increased risk for presyncope if performed during the circadian night compared to the circadian day. In fact, while none of the healthy participants had a history of syncope, half of them experienced at least one presyncope event during the FD, and these were invariably across the circadian night. In those participants, the group-average probability of presyncope was 1–3% during the circadian day and 9–30% during the circadian night, thus a ten-fold increased vulnerability (21). This is consistent with increased susceptibility to syncope in healthy humans at 6. AM vs. 4 PM under normal diurnal conditions of a sleep/wake and fasting/eating cycle (186). Thosar & Shea discovered that compared to midlife adults, in older adults, the BP reactivity to active standing is greatly impaired during the biological night (155). These studies suggest that the circadian system plays a major role in daily rhythms in orthostatic vulnerability and is perturbed in aging. From an evolutionary point of view, the stronger resilience of an individual to an orthostatic challenge during their active phase, when they are more likely to be upright vs. supine, would be adaptive. However, these observations may also have clinical relevance, especially to individuals with nocturia, sleep disorders, and older age, as well as those engaged in shift work, irregular sleep/wake cycles, on-call emergency personnel, and nursing mothers in whom there is an increased likelihood of being awake and assuming an upright body posture during the circadian night.

3.4.3. Exercise

It is well established that regular exercise has beneficial effects on CV fitness and health. On the other hand, acute physical exertion can trigger adverse CV events in individuals susceptible to CV risk (19). To optimize the long-term beneficial effects of physical activity while minimizing negative acute effects, there is a broad interest in the best time of the day to engage in exercise for optimal performance, optimal training effects, and minimal risk (187, 188). To determine whether the endogenous circadian system influences the reactivity of CV risk markers, a two-week FD protocol with twelve 20-hour ‘days’ in dim light was performed in healthy adults. Participants performed the same 15-minute exercise bout on a stationary bike (calibrated to ~60% of maximal HR) every 20 hours throughout the FD protocol. This study revealed that the magnitude of response of certain CV risk markers depended on the endogenous circadian phase. Measures of cardiac vagal modulation showed the largest decrease with exercise in the circadian morning, concurrent with the largest increase in plasma epinephrine and norepinephrine. This suggests that the increased risk for adverse CV events in the morning may, in part, be due to the circadian system causing a larger fall in parasympathetic nervous system activity and a larger increase in sympathetic nervous system activity in response to physical actions. This is reminiscent of pressing the gas pedal and releasing the break at the same time in a car. In this way, this shift to sympathetic dominance during the circadian morning may increase CV preparedness for the morning activities, in a similar manner to the cortisol awakening response. However, this same increase in CV reactivity in the morning may be related to increased CV risk in people with underlying CVD. Of note, in contrast to expectations, the largest increase in BP in response to exercise occurred in the circadian evening and the smallest increase in the circadian morning, although the slowest recovery after exercise was observed in the circadian morning (20, 189). Other markers, including HR, platelet aggregometry, and platelet count, did not show circadian phase-dependent changes in response to exercise (20, 189). Recently, Brito and colleagues (190) studied the circadian CV mechanisms underlying post-exercise BP responses using a short 30-hour FD with ‘6- hour’ days (4 hours of wakefulness and 2 hours of sleep). During each wake period, participants performed 30 minutes of moderate-intensity aerobic exercise at 40% heart-rate-reserve. These authors discovered that systolic BP reduction post-exercise had significant circadian rhythms, with the greatest decreases at the circadian phase corresponding to ~1 PM (-10 mmHg). Systemic vascular resistance and leg blood flow also had significant circadian rhythms, with the lowest resistance and greatest blood flow at ~11 AM suggesting that post-exercise BP reductions were likely due to skeletal muscle vasodilatory mechanisms (190). FIGURE 7 shows the influence of the circadian system on cardiovascular risk factors during rest and in response to exercise.

Figure 7. Influence of circadian system on cardiovascular risk factors during rest (left) and in response to exercise (right).

Displayed here are CV risk factors that exhibit endogenous circadian rhythms, as identified through circadian unmasking protocols (i.e., forced desynchrony) in humans. These factors are categorized based on their temporal alignment with the biological morning—a time window associated with increased vulnerability to adverse CV events in epidemiologic studies (red bracket). Risk factors (black lines) are those whose elevation may be associated with increased acute risk of adverse CV events. These are grouped into three categories: “Peak”: Factors that reach their endogenous circadian peak during the vulnerable window; “Rising”: Factors that are actively increasing during this time; and “Trough”: Factors that are at their lowest levels during this window. Protective factors (blue lines), by contrast, are those whose elevation is associated with reduced acute CV risk. These are grouped as: “Rising”: When they are decreasing during the vulnerable window, potentially indicating a rise in risk; and “Trough”: When their peak occurs during the vulnerable window, suggesting a trough in protection. Circadian rhythms were also identified in the reactivity (i.e., magnitude of change) in response to standardized exercise of certain risk factors to exercise (right). The vertical line in the top right corner indicates 10% (brown) change from the mesor (except for coronary vascular function, for which the mesor is a negative value). This figure provides a qualitative synthesis of quantitative findings from controlled laboratory studies, offering insight into the circadian timing of CV risk profiles. Substantially redrawn from figure S2, S3, (20), figure 1, (516), figure 2, (150), figure 1, (147), figure 2, (148), figure 3, (165).

3.4.4. Psychological stress

Psychological stressors can act as triggers for adverse CV events (191). There is very little data on the interaction of psychological stressors and the endogenous circadian system on CV effects. The data from one study, using a standardized mild mental stressor of a 10-minute computerized addition test during an FD protocol in healthy men and women showed that the effect of the mild stressor and the circadian system were mostly additive for cardiac vagal modulation, plasma epinephrine, BP, and HR, although there was a suggestion for a larger effect on epinephrine during the circadian day than the night (22). Clearly, more work is needed in this area.

3.4.5. Meals

There have been many studies that have investigated the association between and the effect of meal timing on CV and metabolic function and risk factors (14, 192–195). For example, in a seminal proof-of-concept randomized control trial, 5-weeks of early time-restricted eating was found to lower morning levels of systolic and diastolic BP by 11 mmHg and 10 mmHg in men with prediabetes (196). Similarly, restricting feeding time to the active phase in mice mitigated the adverse effects of high fat diet on BP dipping, aortic stiffness, and renal medullary fibrosis (197). Further details are discussed in Section 5.2. As far as we are aware, there have been no studies testing if the acute effect of food intake on CV risk markers depends on the endogenous circadian rhythm of food intake.

3.4.6. Environmental factors (e.g., light, temperature, oxygen)

Light is the most important Zeitgeber, or time cue, that synchronizes the circadian system to the solar day via its circadian phase-shifting effects on the SCN. However, light also has acute effects on physiology, and the magnitude of this effect may depend on the circadian phase of exposure to light. The most well-known example is the acute suppression of production and circulating concentrations of melatonin by light exposure. Light also has acute and time-of-day-dependent effects on CV function. In 1999, Scheer and colleagues, using repeated assessments at rest, in a supine posture, and following exposure to darkness across the day and night showed that light exposure caused a time-of-day dependent increase in resting HR with a stronger effect in the middle of the night and morning, and no effect during the middle of the day (198). These effects were accompanied by acute and time-of-day dependent effects of light on autonomic nervous system markers, specifically cardiac sympathetic modulation as measured by the pre-ejection period (199). Work by Cajochen and colleagues (200) showed that the acute effects of light on HR were dependent on the spectral composition of light with a stronger effect of a blue vs. green light at equal photon density, consistent with an important role of melanopsin in a photopigment in intrinsically photosensitive retinal ganglion cells [ipRGCs] (201). The effects of light on CV function have been comprehensively reviewed (202).

There are other environmental factors that vary in a somewhat predictable fashion, besides light, such as daily variations in environmental temperature (even though dampened in our ‘built environment’). These environmental temperature fluctuations influence physiology in part due to changes in body temperature. Daily variations in body temperature are a combined consequence of environmental changes, behaviors (e.g., sleep/wake, rest/activity, fasting/eating, postural, and psychological changes), and endogenous circadian control. These factors influence body temperature differentially also depend on location within the body dependent on the source of heat production and heat dissipation e.g., core vs. shell and organ-specific. While circadian rhythms in CV function typically vary in parallel with the endogenous circadian control of core body temperature (20) and even though biochemical processes generally speed up at higher temperatures, to what degree the circadian control of body temperature plays a causal role in the circadian rhythm in various aspects of CV function is not well established, and in most cases is likely secondary to direct neural and humoral control. For other outcomes, the direct effect of body temperature has been carefully studied, e.g., for sleep propensity at night, by thermal manipulation of core temperature, proximal skin temperature, and distal skin temperature (203). Such careful temperature manipulations to study the circadian phase-dependent effects on CV function have not been performed. Furthermore, body temperature variations within the physiological range in mammalian models have been shown to be able to phase shift peripheral oscillators (204). Environmental temperature effects also include other, more extreme aspects, including heat and cold stress, that should be considered also in the context of climate change. Heat stress is accompanied by distal and proximal skin vasodilatation and perspiration to enhance heat dissipation, while cold stress is accompanied with skin vasoconstriction to protect the core body temperature and thereby brain and vital organs. Such changes in vascular tone can lead to decreases and increases in BP, respectively, that drives baroreflex-mediated counterregulatory changes in HR, stroke volume, and kidney function and can contribute to increased CV risk (205, 206). Whether and how such diverse thermal effects on CV markers depend on the endogenous circadian phase requires more research.

Another environmental factor of (internal) milieu that can influence CV function and shift the timing of peripheral clocks is oxygen concentration (15, 207, 208). Future studies are needed to determine whether in humans' oxygen concentration variations, including hypoxemia, affect CV function and peripheral clock function differently depending on the endogenous circadian phase of exposure, and how they may be relevant in conditions such as OSA, characterized by recurrent nocturnal hypoxia. For instance, it is noteworthy that although the general population has a characteristic morning peak in MACE, MACE is more likely to occur across the night in OSA (167) (166).

3.5. Circadian Misalignment

The circadian effects on CV physiology are coordinated with the expected day/night pattern of behaviors – but this optimal alignment is perturbed when behaviors occur at unusual circadian phases. Numerous epidemiological studies have shown that chronic circadian misalignment between behaviors and the circadian system, caused by shift work, irregular sleep-wake cycles, or nighttime light exposure, is strongly associated with increased risk of CVD, including hypertension, atherosclerosis, myocardial infarction, and heart failure (24, 25). The longer an individual experiences circadian misalignment, the greater the CVD risk (209–213). Moreover, shift workers and individuals with misaligned sleep-wake patterns often exhibit disrupted natriuresis and nocturnal hypertension, leading to loss of the dipping BP at night. The normal BP drop across the night of >10% (usually during sleep) is considered a healthy pattern known as nocturnal dipping BP (214). In non-dipping hypertension, this nighttime drop is less or non-existent and this “non-dipping hypertension” is associated with an increased risk of CVD (214). Shift work studies show that misaligned sleep-wake cycles result in elevated 24-hour BP, increased sympathetic activity, and impaired glucose metabolism, all of which contribute to CVD risk (19, 209, 212, 215–217). Disruptions in renal circadian rhythms in humans are implicated in BP dysregulation and increased CVD risk (14, 19, 34, 91, 113, 156, 218–232). Human laboratory studies in healthy individuals are consistent with the epidemiology studies and demonstrate that circadian misalignment alters BP rhythms and impairs endothelial function, both critical determinants of CV health (26). Animal models provide mechanistic insights into how even brief circadian disruption can have lasting effects on organ function and long-term CV risk (27, 28). Genetic studies in rodents demonstrate that deletion of specific circadian clock genes in the kidney disrupts sodium handling and promotes hypertension (229). Furthermore, in mice, knockout of BMAL1, a core circadian clock protein, leads to endothelial dysfunction via impaired nitric oxide signaling (233). Bmal1 deletion in vascular smooth muscle cells disrupts BP rhythms (234), while its deletion in cardiomyocytes alters myocardial metabolism and accelerates age-related cardiomyopathy and heart failure (235). Additionally, genetic mutation of the cardiac Clock gene in cardiomyocyte-specific circadian clock mutant mice abolishes the protective time-of-day variation in ischemia-reperfusion, highlighting the circadian system’s role in cardiac repair and resilience (94). At the cellular level, misalignment can cause reduced amplitude of core clock and downstream gene rhythms, loss of time-of-day variation in calcium handling and contractile reserve, and impaired oscillations in metabolic processes such as glucose and fatty acid selection, which may disrupt homeostasis, increases CV risk, and impair cardiac repair (91). These alterations increase pro-inflammatory signals and impair cardiac performance. Over time, circadian desynchrony may promote the development of MACE and heart failure (236).

These human and animal models are described in more detail further below.

3.5.1. Extent of circadian misalignment in society

Circadian misalignment occurs when the light-dark cycle (environmental), sleep-wake, rest-activity and fasting-eating cycles (behavioral), and/or peripheral oscillators are out of synchrony with the central circadian clock (FIGURE 8). In modern industrialized society, circadian misalignment has become highly prevalent due to societal obligations, occupational demands and lifestyle changes. Many essential services and industries, such as the healthcare, public safety, emergency service, transportation, and energy sectors, rely on shift work to ensure continuous operation beyond traditional daytime hours. Besides, the globalized economy also requires shift work to maintain services across different time zones. According to the US Bureau of Labor Statistics, approximately 15% of the US workforce is engaged in shift work (evening/night shift, rotating shift, early morning shifts, or other work schedules that differ from the traditional 9 am-5 pm day) (237). This forces millions of workers to stay awake at night and sleep during the day, thus chronically exposing themselves to circadian misalignment. Additionally, social jet lag, the difference in sleep timing between workdays and free days, has become common as people maintain different sleep schedules on workdays and free days, largely due to rigid work or school schedules combined with late-night social or digital entertainment. It is estimated that up to 70% of individuals suffer at least 1-hour of social jetlag (238–241) and nearly one-third of the general adult population experiences ≥2-hour of social jetlag (238, 240). Time zone-related jet lag contributes to acute circadian misalignment. This may affect about 80 million of international travelers annually in the US alone (242), particularly frequent flyers, international business travelers, and flight crews, as many of their flights likely cross time zones. Globally, the number of affected individuals is likely to be significantly higher. With technological advancements and urbanization, up to 83% of the population on earth and >99% of the people in the USA and EU may experience artificial light at night due to outdoor light pollution as measured by satellite data (243, 244). Regarding personal indoor lighting environments, data remain relatively limited. In the Sister study, which includes a cohort of approximately 48,000 women, around 82% reported some form of indoor night light exposure. Specifically, about 30% indicated the light source originated from outside the room, 2.5% from within the room, 9.4% from a TV (245). Such night light exposure suppresses melatonin release, delays circadian phase, and prolongs sleep onset latency, contributing to circadian misalignment and sleep disruption. In addition to light at night, other factors, such as lack of sunlight exposure caused by indoor-heavy lifestyle and social events that encourage nightlife, can give rise to nocturnal lifestyles, another form of widespread circadian misalignment.

Figure 8. Circadian misalignment has significant implications for adverse cardiovascular outcomes.

(Top) Examples of circadian misalignment in the real world. (Middle) Different types of circadian misalignment. Circadian misalignment often manifests when environmental cycles (brown square wave, e.g., light/dark cycle; which we refer to as ‘environmental misalignment’) and/or behavioral cycles (blue square wave, e.g., sleep/ wake, fasting/eating, rest/activity cycle; ‘behavioral misalignment’) are misaligned relative to the central clock in the SCN (red cosine) or to the peripheral oscillators in other tissues (black cosine). At an organismal level, circadian misalignment can materialize as internal misalignment between the central clock and peripheral clocks, which can be induced by, for example, misaligned eating. It also refers to misalignment among peripheral clocks in different organs, where peripheral clocks are in abnormal phase relationships with each other, substantially redrawn from (517). (Bottom) Circadian misalignment can increase the risks of CVD and disorders. Although not directly indicated in the figure, poor CV health may also result in circadian misalignment.

Other than these above external factors, internal biological factors can also contribute to circadian misalignment such as circadian rhythm sleep disorders and genetic predispositions. Individuals with a late chronotype are particularly susceptible to circadian misalignment, as society predominantly follows a morning-oriented schedule. In more extreme cases, Delayed Sleep Phase Disorder (DSPD) is typically characterized by a 2─6 h delay in the major sleep episode relative to the desired or required sleep-wake cycle. The overall prevalence of DSPD in the general population likely range between 0.2–10% (246), but is much higher in adolescents with estimates of approximately 7─16% (247). Polymorphisms in clock genes such as CLOCK, CRY1, PER2 and PER3 have been linked with DSPD (248). It has been reported that about half of the DSPD patients experience circadian misalignment, as their circadian phase is delayed relative to desired bedtime, while the other half shows a delay in the sleep/wake cycle without a delay in the circadian phase (249). On the other hand, Advanced Sleep Phase Syndrome (ASPS) is characterized by a ≥2h advancement of the major sleep episode relative to the desired regular sleep time (250). ASPS starting at a young age is often familial and linked to autosomal dominant mutations in circadian clock genes like PER2, Casein Kinase 1 (CK1), PER3, CRY2, TIMELESS, and Basic Helix-Loop-Helix Family Member E41 (DEC2 or BHLHE41). It is tempting to speculate that this is a more manageable condition as compared to DSPD, because many patients can easily adapt to an early schedule, although many will also seek medical help. Notably, it still can cause circadian misalignment, especially when individuals attempt to join evening activities.

It is worth pointing out that circadian misalignment is a significant issue in healthcare settings, particularly in intensive care units (ICUs) and other 24/7 medical environments. ICU patients are constantly exposed to bright artificial lighting and frequent disturbances—including during the night—and irregular care schedules, all of which disrupt their circadian rhythms. Even in general inpatient care, around-the-clock procedures and monitoring such as vital sign checks, medication administration, imaging studies, can fragment sleep and contribute to circadian misalignment. Indeed, a multi-center randomized controlled trial found that premature infants exposed to a light/dark cycle had greater weight gain and a shorter hospital stay compared to those kept under constant light condition (251). Furthermore, critically ill patients often receive enteral or parenteral nutrition continuously. Given that food intake is a strong Zeitgeber for many peripheral oscillators, the delivery of 24-h nutrition support is expected to exacerbate circadian disruption. The historical practice of the 24/7 medical care is to ensure continuous patient care, emergency response, and life-saving interventions. The growing recognition of circadian health have sparked discussion on the impacts of circadian misalignment in hospitalized patients and the development of strategies to optimize circadian function and promote patient recovery.

3.5.2. Epidemiological evidence linking circadian misalignment with increased incidence of cardiovascular events and mortality

Epidemiological evidence strongly links circadian misalignment to higher incidence of CV events and increased mortality risk. For example, numerous systematic reviews and meta-analyses have consistently shown that individuals engaged in night and/or rotating shift work face a significantly higher risk of CVD and events compared to non-shift workers. Specifically, shift work is associated with a ~17% increased risk of any CVD events and a ~20% higher risk of CVD/coronary heart disease (CHD) mortality (209, 252, 253). This elevated risk includes a 10–31% higher odds ratio of hypertension [(215, 254, 255), as well as increased risks of ischemic stroke (4–5%) (252, 256, 257), ischemic heart disease (13–26%) [(252, 258)], and myocardial infarction (23–27%) (252, 256). Night shift work also associates with impaired kidney function as measured by eGFR (259). Importantly, these associations exhibit a dose-dependent relationship, meaning that the longer an individual engages in shift work, the greater their risks for CVD events (253, 258, 260). It is noteworthy that the above associations likely underestimate the risk of shift work, as they are based on cumulative lifetime shift work history. This approach may introduce a selection bias known as the “healthy worker survivor effect”, where individuals who continue working night shifts tend to be generally more resilient and healthier than those that are no longer engaged in shift work. This is particularly relevant since quitting shift work has been shown to reduce these risks (212).

Even milder forms of circadian misalignment, such as due to having a late chronotype, social jet lag, or nighttime light exposure, are associated with significant CV consequences, despite their lesser severity. Cross-sectional and prospective studies found that individuals with late chronotype as compared to those with early chronotype have elevated risk for hypertension, CVD, CHD, and stroke (261, 262) (263), although not all did (264). One study also reported a slight increase in CVD mortality with greater “eveningness” (265). Larger social jet lag tends to associate with worse CV risk markers, including higher adverse cholesterol/triglycerides suggestive of worsening plaque buildup in the arteries (266–268). A systematic review examining the relationship between social jet lag and BMI reported inconsistent findings across studies, probably due to differences in the study populations and methodological variability. A more recent meta-analysis looking at a larger set of variables related to obesity and adiposity reported a positive relationship (269). Nevertheless, overall, there is a scarcity of studies investigating the relationship between chronotype, social jet lag and CV outcomes. Studies deriving outdoor night light exposure from satellite data reported inconsistent associations between night light exposure and risk of CVD (270–272). This is probably because such population-level outdoor night light data has limited spatial resolution and does not account for indoor light data, making it an unreliable measure of individual light exposure. Studies using portable/wearable light meters found night light exposure is associated with higher BP in older adults (273, 274). Notably, even subtle misalignment caused by relative position in a time zone may have significant effects on health. Life expectancy decreases and risk of CVD increases from the eastern to the western border of time zones, as sunset occurs at a later clock time the further west one moves (275, 276). Furthermore, transient forms of circadian misalignment can have deleterious consequences. For example, people are more likely to experience sudden cardiac death on Monday morning, typically associated with an earlier waketime vs. the prior weekend days, than any other days of the week (137, 277). Incidences of myocardial infarctions (278) and ischemic stroke (279) also increase on the first days after the DST change in spring (1-hour advance in external clock time). Although the above cases only show associations and do not assess endogenous circadian phase, they provide strong epidemiological evidence for a link between circadian misalignment and worse CV health. Moreover, circadian misalignment in people who already have CVD is very likely to exacerbate clinical outcomes, and this concept of different outcomes of circadian-behavioral desynchrony in people with and without underlying CVD is shown in FIGURE 9.

Figure 9. Role of circadian-behavioral synchrony in health and disease.

Normal alignment between the circadian system and behaviors (e.g., sleeping at night) leads to robust circadian rhythms and healthy outcomes, including optimal sleep, physical and mental performance, growth, recovery and repair (green text indicates healthy timing and healthy outcomes). On the other hand, abnormal alignment between the circadian system and behaviors (e.g., sleeping during the day, because of night-shift work) leads to desynchronized, blunted circadian rhythms, and unhealthy outcomes (red text indicates unhealthy timing and unhealthy outcomes). On the right-hand side, circadian misalignment in people who already have CVD is very likely to exacerbate clinical outcomes.

3.5.3. Components of circadian misalignment

The above real-life scenarios of circadian misalignment often involve the misalignment of myriad rhythms, including the central clock, peripheral oscillators, different behavioral cycles, and environmental cues. In many cases, the degree of mismatch among these components can vary considerably across different scenarios. For example, night shift workers typically have misaligned sleep-wake and fasting-eating cycles relative to their central clock, yet most of them still experience a natural light-dark cycle that remains aligned with their central clock (280, 281). In contrast, individuals exposed to artificial light at night often have a delayed central clock and later sleep onset, but their “9-to-5” work and social schedule may keep their wake time and fasting-eating cycle largely aligned with the solar day (282). Similarly, those who frequently consume late evening meals may maintain a sleep-wake cycle aligned with the light-dark cycle, while experiencing misalignment in their metabolic rhythms (283). Recognizing these distinct patterns of circadian misalignment is crucial for understanding their potential health implications and developing targeted interventions to mitigate their effects.

3.5.4. Potential mechanisms underlying adverse effects of circadian misalignment

Internal circadian misalignment, causing a mismatch in timing of biological processes between SCN, organs, tissues, cells

One possible mechanism through which circadian misalignment impairs health is internal desynchrony. This is in part attributed to the Zeitgeber properties of the environmental/behavioral factors. As mentioned in the Introduction, light serves as the strongest entrainment signal for the central clock (SCN), while eating and exercise can reset the circadian clocks more effectively in peripheral oscillators compared to the central clock, such as those in the liver and skeletal muscle (284, 285). Consequently, when the environmental and behavioral cycles are misaligned, it can create a mismatch between the central clock and different peripheral oscillators, disrupting the synchronization of biological rhythms among multiple organ systems and leading to circadian misalignment across various physiological processes. Such phenomenon was well-established in animal models (286–288). Whole body clock mutant mice (ClockΔ19/Δ19) model systemic circadian desynchrony, which is analogous to chronic shift work in humans. This model demonstrates the necessity of an intact clock for maintaining cardiac health, enabling efficient repair, and protecting against ischemia-reperfusion injury (28, 121). These mice have also been used to study circadian contributions to cardiac aging (e.g., (289, 290)).Studies in mice have shown that modulating circadian timing through adenosine A2b receptor signaling or light-induced stabilization of PER2 protein enhances cardiac glycolytic metabolism and confers protection against ischemia-reperfusion injury, whereas Per2−/− mice lose this cardioprotective effect (291) (. Perhaps most strikingly, the tau/+ hamster model provides compelling evidence for the importance of alignment between the external environment and the internal circadian clock (292). Although circadian phase of CV rhythms could not be measured in these arrhythmic animals, heart-weight to body-weight ratios were significantly higher in tau/+ than wild-type (WT) animals under 14-hour:10-hour L:D conditions, as expected. Strikingly, SCN-lesioned tau/+ animals had significantly lower heart-weight to body-weight ratios than intact tau/+ animals and were not different from WT. Thus, such tau/+ hamster experiments illustrate that circadian organization is important for heart health, and that misalignment itself can be an etiologic cause of disease. Moreover, with the SCN lesion findings, these studies also suggest that targeted manipulation of circadian alignment can, in some contexts, mitigate disease phenotypes and support the concept that purposeful circadian misalignment may have therapeutic relevance.

These preclinical animal studies have recently gained proof-of-concept evidence in humans. For example, Skene and colleagues demonstrated that after a simulated night shift schedule, many 24-h rhythms of circulating metabolites are disassociated from the SCN rhythms (293). Additionally, two separate studies manipulating meal timing found that the phase alignment between circulating glucose/insulin rhythms and the central clock was disrupted (283, 294), and in one of these studies, the oscillation of some clock genes in adipose tissues was slightly delayed (283). Although internal desynchrony was widely hypothesized to be a key driver of the adverse health consequences of circadian misalignment, direct experimental evidence remains limited. In a genetically modified mouse model, van der Vinne and colleagues were able to lengthen the SCN circadian period without disrupting the ones in peripheral oscillators, thus creating internal desynchrony in the absence of environmental timing cues. Surprisingly, the internal desynchrony failed to induce metabolic dysfunction (295). However, the study highlights that it remains possible for different types of internal desynchrony to induce adverse health outcomes. The potential causal role of internal circadian misalignment in causing negative health consequences requires further study.

Behavioral circadian misalignment: sleep, physical activity, eating vs. central and/or peripheral clocks

Another potential mechanism underlying the adverse effects of circadian misalignment is due to the circadian variation in the physiological responses to the behavior factors. When misaligned, certain behavior factors can occur at the endogenous circadian phase that are not optimized for its responses. For example, rapid eye movement (REM) sleep propensity and total sleep duration depend on the circadian phase (296), with individual experiencing poorer sleep quality and reduced sleep efficiency when trying to sleep during their biological daytime. This sleep loss can accumulate over time, contributing to increased CV risk. Similarly, glucose tolerance declines from morning to evening or night in healthy individuals, increasing the likelihood of postprandial hyperglycemia when consuming meals in the biological late evening (178, 297). Over time, this can contribute to the development of type 2 diabetes which is a major risk factor for CVD (298, 299).

3.5.5. How to study circadian misalignment in the laboratory, the findings, and the benefits of isolating the different components in the development of countermeasures

As previously mentioned, the epidemiological studies only show associations. To determine a causal relationship between circadian misalignment and impaired CV functions, highly controlled in-laboratory protocols have been used to experimentally test the effect of circadian misalignment on CV outcomes. The FD protocol described further above is one method to induce circadian misalignment. In this approach, the participants live in a dim light, time-free environment while following a recurrent sleep-wake schedule that is much longer or shorter than 24 hours. Since their internal circadian system cannot entrain to the imposed schedule, it begins to free-run, causing the behavioral cycle to gradually drift out of phase with the circadian system. This controlled misalignment allows the isolation and examination of the effects of circadian misalignment. Indeed, the first FD study to investigate the consequence of circadian misalignment on BP found that acute exposure to circadian misalignment led to a 3 mmHg increase in mean arterial pressure in healthy adults while awake (26). Notably, this change is clinically significant, as its magnitude is comparable to the BP reduction achieved in the 3-week Dietary Approaches to Stop Hypertension (DASH) study (300). The observed increase in BP did not seem to be explained by waketime epinephrine (decreased), norepinephrine (unchanged), waketime HR (unchanged), or cardiac vagal tone (unchanged). While FD protocols provide proof-of-concept evidence for the adverse effects of circadian misalignment on CV functions, their translational value is limited due to the highly artificial experimental conditions. These include the use of sleep-wake schedule that are significantly longer or shorter than 24 hours and the removal of light cues, which deviate from real-world shift work schedules. Thus, carefully controlled simulated night shift work protocols provide a more realistic approximation to the circadian disruption experienced by shift workers, while still maintaining control over behavioral, environmental, and interindividual factors to effectively isolate the effect of circadian misalignment (e.g. see FIGURE 10). In a randomized, cross-over study design with two highly controlled 8-day in-laboratory protocols, circadian misalignment was achieved by a rapid 12-h shift of the behavioral and environmental cycles – relevant for real-life night shift workers. Consistent with the FD results, acute circadian misalignment increased waketime BP, as well as 24-h systolic and diastolic BP (301). The latter effect was primarily driven by elevations in sleeping BP. Importantly, sleep-opportunity associated dipping in systolic BP was significantly reduced. A blunted dipping in BP during sleep is an independent predictor of adverse CV events and all-cause mortality (214, 302). Cardiac vagal (parasympathetic) activity is generally considered to be cardioprotective (303). In the same protocol, wake cardiac vagal modulation, as assessed by HRV analysis, was diminished under circadian misalignment, which may help explain the increased wake-period BP. On the other hand, circadian misalignment increased urinary epinephrine excretion rate, as marker of sympathetic activity, across the sleep episodes, which may partially explain the elevated sleeping BP. Furthermore, acute circadian misalignment also increased blood biomarkers of inflammatory stress, including interleukin-6 (IL-6), high-sensitivity C-reactive protein (hsCRP), resistin, and tumor necrosis factor alpha (TNF-α). In a 6-day simulated shift-work protocol designed to investigate how circadian misalignment influences circulating proteins linked to CV physiology, the results indicated that circadian misalignment may contribute to a hypercoagulable state by increasing morning abundance of coagulation factor VII combined with decreased tissue factor pathway inhibitor and increased tissue factor (304). As above studies were all conducted in non-shift workers, this raised the question if these effects would also be observed in chronic shift workers, or if this population would be protected against those adverse effects, e.g., due to the healthy worker effect (i.e., selection bias to become or stay employed in a shift work setting) or due to some type of adaptation to recurrent circadian misalignment. Therefore, chronic shift workers were studied in another randomized-crossover study with simulated night work vs. day work protocols. This study showed, similarly, increases in BP and C-reactive protein under circadian misalignment conditions. This finding supports the notion that shift workers are not necessarily resilient against the adverse effects of circadian misalignment on CV system.

Figure 10. Effects of acute circadian misalignment on circadian markers and cardiovascular risk factors.

This figure presents a qualitative summary of quantitative data collected under experimentally induced circadian misalignment (simulated night shift conditions, including ~90 lux during scheduled wakefulness) in health human participants. On the left two panels, the effect of acute circadian misalignment on two classic circadian phase markers—melatonin and cortisol—are illustrated. Misalignment leads to a notable dampening of their circadian amplitude and a ~12-hour offset relative to the sleep/wake cycle (178). This offset reflects the inertia of the circadian system, which does not immediately adapt to the abrupt change in behavioral schedule. The blunting of melatonin reflects in part an acute suppression by light, while the blunting of melatonin and cortisol in part may also reflect a blunting of the output of the SCN. On the right two panels, the figure shows the effects of acute misalignment on two CV risk factors: high-sensitivity C-reactive protein (hsCRP) and systolic blood pressure (BP) (301)]. Unlike melatonin and cortisol, the daily rhythms of these CV risk markers are more strongly influenced by behavior rather than by the central circadian clock. Consequently, during circadian misalignment, these markers are not offset by 12 hours but instead show overall elevations. These observations may help explain the underlying mechanisms by which shift work increases CV risks. The vertical line in the top right corner indicates 10% (brown) change from the mesor. Substantially redrawn from figure S1, (178), figure 2, (301), and figure 2, (518).

One potential confounding factor of the aforementioned simulated night shift protocols is sleep disruption. This is because sleeping during the biological day is at odds with the endogenous circadian clock and leads to reduced sleep duration (296). Poor sleep is a well-established risk factor for CVD. Sleep restriction has been shown to result in elevated BP, reduced nocturnal BP dipping, increased sympathetic markers, and elevated proinflammatory markers (305). While the above studies statistically account for the potential influences from sleep disruption, a parallel group design with two 11-day interventions combining circadian misalignment with sleep restriction was the first to induce circadian misalignment while experimentally controlling for the amount of sleep restriction. It was found that sleep restriction with circadian misalignment, but not circadian alignment, elicit increased 24-hour urinary norepinephrine, greater reduction in vagal indices of HRV during sleep, and increased levels of inflammatory marker hsCRP, although no changes were observed for BP (306, 307).

Given that circadian misalignment, particularly in the form of shift work, is unlikely to disappear, there is growing interest in identifying effective countermeasures to mitigate its adverse CV effects. One promising strategy is to decouple meal timing from other misaligned behaviors (e.g., sleep, activity) to maintain an eating window that remains aligned with the central clock. Preclinical works have demonstrated that aligning feeding times with the active phase protects against cardiac tissue aging in flies (308) and leads to the greatest lifespan extension in calorie-restricted mice (309). However, experimental evidence in humans is just emerging. A recent randomized parallel-arm trial compared CV risk markers between two CR protocols, one before and one after circadian misalignment imposed by four 28-h FD ‘days’ and tested if restricting meals to the circadian day during the circadian misalignment can mitigate adverse effects. This study found that restricted food intake to the circadian day alleviated adverse changes in CV risk factors, including preventing decrease in cardiac vagal modulation, preventing increase in LF/HF as sympatho-vagal marker, preventing increase in PAI-1 and lowering BP (310). This approach also prevents internal circadian misalignment and adverse changes in glucose regulation which may have CV benefits (294). This study provides proof-of-principle evidence for optimizing circadian meal timing as a promising countermeasure against the adverse effects of circadian misalignment on CV risk factors.

Other potential strategies to mitigate the adverse effects of circadian misalignment include adjusting timing of exercise, light exposure, and naps. However, research in these areas remains very limited. Although not directly testing exercise timing, one study found that exercise before a shift can lower BP in healthy individuals throughout the subsequent 8-h simulated night shift (311), suggesting that the hypotensive effect of exercise can be used to lower BP in night shift workers. Timed bright light exposure is a widely accepted method to manage circadian rhythm disorders, including shift work disorder. So far, most studies utilizing light therapy in shift worker populations focus on its effect on enhancing alertness during work and improving sleep after shifts (312, 313). Its potential to alleviate the adverse CV consequences of circadian misalignment remains untested. While light exposure is typically associated with wakefulness, evidence suggests that flashes of light during sleep can entrain the human central clock, thereby expanding the potential window for light therapy application (314). Yet, this approach has also not been explored for its potential CV benefits. Several studies have examined the relationship between on-shift naps, nap duration, and CV outcomes in shift workers. For example, cross-sectional studies indicated that shift workers who do not take on-shift naps as compared to those who do, exhibit lower 24-h parasympathetic activities as measured by HRV indices (315), are more susceptible to increased BP when working more than four night shifts per two weeks (316), and have a higher odds ratio for hypertension (317). Additionally, findings from a randomized crossover trial with simulated night work conditions suggest that taking an on-shift nap can restore nighttime BP dipping, which is typically blunted in non-nappers (318, 319). However, few studies have directly investigated whether nap timing modifies these beneficial effects in shift workers ─ an emerging frontier worth exploring.

4. INSIGHTS INTO CIRCADIAN MECHANISMS OF BLOOD PRESSURE REGULATION AND CARDIOVASCULAR DISEASE FROM PRECLINICAL MODELS

Both pre-clinical and clinical research indicate that circadian misalignment drives risk for CVD and mortality. Regarding studies in humans, a number of candidate gene studies have found associations between single nucleotide polymorphisms in core clock genes potentially linked to hypertension, coronary artery disease and susceptibility to myocardial infarction ((320–325). However, we note that such candidate gene approaches are not statistically rigorous (risk of false positives due multiple testing) and await confirmation via more robust genome-wide association studies. At present, deeper insights have been provided with pre-clinical models. For example, ClockΔ19/Δ19 mice, which lack a functional CLOCK protein due to genetic deletion of exon 19, have dampened diurnal feeding rhythms and tend to overeat, developing obesity and metabolic syndrome (326). Though interestingly, these underlying metabolic conditions did not result in heart disease in the ClockΔ19/Δ19 mice (109) suggesting that additional factors are needed to trigger CVD. Other investigations using ClockΔ19/Δ19 mice as well as mice with global BMAL1 knockout demonstrated diminished glucose tolerance, low levels of insulin, and maladaptive morphological changes in pancreatic islet cells, which become worse as the animal’s age (327). These studies investigating links between glucose homeostasis, and pancreatic clocks, are important although their direct connection to the pathogenesis and pathophysiology of diabetes or CVD remain unclear.

Tamoxifen-inducible Bmal1 knockout mice, which express the gene only during embryogenesis but not after birth, provide additional insights. In these mice, oscillatory rhythms in activity, physiological parameters, and molecular outputs were abolished, yet age-dependent abnormalities commonly seen in constitutive Bmal1 KO mice did not develop (291). Moreover, when subjected to a jet-lag circadian disruption protocol, these inducible knockouts adapted their behavior more effectively than controls and were protected from circadian disruption-induced insulin resistance compared to their conventional littermate controls (290). This model highlights that while circadian disruption is generally pathological, under certain conditions it may paradoxically reveal adaptive responses; controlled blunting or misalignment of circadian rhythms could hold therapeutic value. Importantly, they also highlight that the phenotypes observed in circadian disruption are highly dependent on the type of experimental model being studied.

Significantly more work has investigated the role of hypertension is a major risk factor for serious and at times fatal CVD. Preclinical research clearly links circadian biology to CV health, namely BP regulation, allowing for the investigation of mechanisms behind this connection. This has been revealed through the characterization of rodent knockout (KO) models of clock genes. In fact, every clock gene KO exhibits a BP phenotype, highlighting the significance of an intact clock mechanism in CV function and health (FIGURE 11). The overwhelming evidence demonstrating the importance of clock genes in CV physiology is outlined in the following sections.

Figure 11. Evidence from preclinical studies outlining the role of the molecular clock in blood pressure regulation.

The graphs show representative blood pressure measurements over a 24-hour period in knockout models with controls in black and knockouts in red. The gray shaded component represents the dark cycle, which is the rodent active phase. Clock genes are outlined by a gray box, while clock-controlled genes are outlined by a magenta box. The left panel shows global knockouts in mice and rats. Mice are indicated by the small brown mouse icon, while rats are white and larger. Presence of a saltshaker indicates a model for salt-sensitive hypertension. Each graph has the rodent model above, and the name of the clock gene knocked out. BMAL1 knockout (C) is a constitutive knockout model while BMAL1 knockout (I) is representative of inducible knockout studies. The middle panel shows evidence from tissue-specific knockout studies, all done in mice. These include the kidney, adrenal, liver, adipose tissue, and smooth muscle cells. The right panel indicates the implication of evidence collected from preclinical studies. There is a close relationship between circadian biology (clock genes and clock-controlled genes) and blood pressure regulation. Dysregulation of circadian biology can disrupt blood pressure regulation, increasing risk for and exacerbating cardiovascular disease. This relationship is bi-directional, with blood pressure regulation as one of the major mechanisms through which this vicious cycle continues. Created in BioRender. Gumz, M. (2025) https://BioRender.com/9uflsd6

4.1.1. Pressure Regulation in Global Knock Outs in Rodents

The outcomes of studies using rodents for investigation of the role of the circadian mechanism on CV health and disease in vivo have shown direct involvement in BP regulation. These pre-clinical studies provide valuable insights into potential mechanisms regulated by these clock genes, which likely contribute to the regulation of BP particularly in the context of adapting to changes in the environment. For example, in 2007, Curtis et al. showed that male mice with global constitutive BMAL1 KO have lower BP and a non-riser BP phenotype, meaning their surge in BP leading up to the active phase was blunted (328). This study found that this phenotype may be due, in part, to a lack of appropriate coordination within the adrenal gland, which integrates sympathetic nervous system activity and glucocorticoid signaling to align with daily changes in the external environment. Changes in the external environment can include light cycle modifications as well as other stressors. The role of BMAL1 in blood pressure regulation is also evident in inducible knockout models, in which tamoxifen is administered to knock out BMAL1 in adult mice rather than throughout development (329–331). Consistent with the constitutive knockout results, induced knockout of BMAL1 in adult mice lowers BP and disrupts rhythmicity of BP. Hou et al found that this impact of loss of BMAL1 on BP could be partially resolved by a combination of a regular 12/12 LD cycle and time restricted feeding, but not by either of these circadian manipulations independently (329). These data highlight the role of clock genes in coordinating multiple systems and endocrine factors to anticipate changes in the external environment, particularly in CV function. Furthermore, this demonstrates the role of BMAL1 in regulating BP circadian rhythms.

This role of clock genes in endocrine coordination for BP regulation at the level of the adrenal is further supported by studies investigating other clock genes. In 2008, Sei et al. found that loss of CLOCK protein function in mice led to dampened diurnal variations and shifts in the timing of arterial pressure and HR (332). This was associated with low plasma aldosterone, a key adrenal hormone responsible for maintaining sodium and fluid balance, and is, therefore, critical for BP regulation. In contrast, Doi et al. found that mice with double KO of CRY1 and CRY2 exhibited high levels of plasma aldosterone and salt-sensitive hypertension that was ameliorated with spironolactone treatment to inhibit the aldosterone receptor, the mineralocorticoid receptor (333). This elevation in aldosterone was likely due to increased expression of hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 6 (Hsd3b6), which is highly expressed in the zona glomerulosa of the adrenal gland where aldosterone is produced. The difference in the direction of changes in aldosterone depending on the clock gene is in line with our understanding of the clock mechanism, as CLOCK drives the positive regulation of the clock while CRY1 and CRY2 function in the negative regulation. The main point taken from these studies is that both the positive and negative arms of the clock are crucial for regulating endocrine factors to properly time BP changes with the demands of the external environment.

Clock genes regulate BP via tissue-specific mechanisms. This includes the heart and the kidney. Global KO of CLOCK protected against the impacts of a chronic kidney disease model on cardiac inflammation and remodeling, likely through modulation of clock machinery in monocytes (334). Loss of PER2 function lowered HR and diastolic BP under normal light:dark conditions (335). When light cues were removed by placing the animals in total darkness, the PER2 mutant mice exhibited impaired circadian rhythms in BP, with a decrease in the night/day difference, or dip, in BP. Gumz and colleagues demonstrated a role for PER1 in regulating the BP response to a high salt diet plus mineralocorticoid treatment (336). Specifically, male mice with global PER1 KO exhibited non-dipping hypertension compared to wild-type controls, which maintained a normal BP rhythm. Subsequent studies by Douma et al. later linked this phenotype to a defect in renal sodium handling (337). These studies implicate CLOCK and PER proteins in BP regulation and cardiac as well as kidney function.

Mouse studies have also shown that the importance of the molecular clock in BP regulation and CV health extends beyond the core clock genes. Gachon and colleagues found that KO of direct circadian mechanism-controlled proteins - D-box binding protein (DBP), hepatic leukemia factor (HLF), or thyrotrophic embryonic factor (TEF) - collectively known as PAR-domain basic leucine zipper (PAR bZip) transcription factors, shortened lifespan, a phenotype involving cardiac dysfunction, low BP, and a maladaptive decrease in aldosterone (338). These results indicate that all components of the clock mechanism are critical to cardiac and endocrine function, ultimately impacting BP regulation and CV health. This highlights the importance of clock-controlled pathways in maintaining CV health through the coordination of regulatory systems.

In addition to mice, rat models for clock gene knockouts provide crucial information regarding the role of the molecular clock in CV health. Though the literature utilizing rat global clock gene KOs is limited, existing evidence overwhelmingly supports the theory that clock genes regulate BP. In line with studies performed in mice, Johnston et al. found that global KO of BMAL1 in Sprague-Dawley rats leads to decreased BP in both male and female animals (339). This is consistent with the BP phenotype of global BMAL1 KO in male mice (328). Unlike mice however, BMAL KO rats maintain their rhythmicity of BP. Intriguingly, male BMAL KO rats do not have rhythmicity in urinary sodium excretion, while their female counterparts maintain a significant increase in sodium excretion during the active phase. This highlights an uncoupling of sodium excretion rhythms and BP rhythms, particularly in male BMAL KO rats. These results indicate that the clock regulates CV and renal systems in a sex-specific manner, which could explain many sex differences evident in clinical outcomes (Table 1).

TABLE 1.

SEX DIFFERENCES IN CIRCADIAN CARDIOVASCULAR PHYSIOLOGY

MALES	FEMALES
Aging: Dysfunctional Clock protein leads to age-induced cardiomyopathy (290)	Aging: Protected from loss of Clock protein (290)
Salt-Sensitive Hypertension: Loss of PER1 in salt-sensitive hypertension leads to non-dipping hypertension (336)	Salt-Sensitive Hypertension: Protected from loss of PER1 in salt-sensitive hypertension(477)
Electrolyte Balance: Loss of BMAL1 in the kidney decreases blood pressure throughout the day (478)	Electrolyte Balance: Loss of BMAL1 in the kidney does not affect blood pressure likely due to better electrolyte handling (478)
Hypertension: Develop non-dipping hypertension with overactivation of RAAS following Ang II administration; associated with suppression of the baroreflex (355)	Hypertension: Develop non-dipping hypertension with the same Ang II treatment, but no impact on baroreflex and instead have disrupted diurnal regulation of heart rate (355)
Myocyte Composition: Loss of PCYT2 impairs cardiac insulin signaling, elevating membrane arachidonic acid and LCPUFA, leading to systemic hypertension and cardiac hypertrophy (475)	Myocyte Composition: Loss of Pcyt2 impairs cardiac insulin signaling, but there is no change in LCPUFA levels nor development of heart disease (475)
Cardiac Aging: Disruption of CLOCK dysregulates mRNAs and miRNAs in the PTEN-AKT signal pathways leading to age-dependent cardiac hypertrophy (289)	Cardiac Aging: Ovarian hormones protect against the development of cardiomyopathy with age, even if CLOCK is dysregulated (289)
Resilience to MI: Survivorship post-MI (day 7) is better when infarcted during wake time vs. sleep time (106)	Resilience to MI: Survivorship post-MI (day 7) is worse when infarction is during wake time vs. sleep time (106)

In line with mouse studies, rat models implicate PER1 in CV health and BP control. Of note, Zietara et al. demonstrated that global KO of PER1 in male rats on a Dahl salt-sensitive (SS) background exacerbates salt-sensitive hypertension (133). The Dahl SS rat is a translational model for salt-sensitive hypertension, as these rats become hypertensive when placed on a high-salt diet. Dahl SS rats lacking PER1 become even more hypertensive and develop more robust kidney injury. This is also associated with disrupted circadian regulation of BP and HR. Furthermore, male PER1 KO rats have high levels of circulating aldosterone and reduced sodium excretion, consistent with a maladaptive response to a high-salt diet, which drives a positive salt balance and subsequently exacerbates hypertension. Therefore, PER1 is critical to coordinating cardiac and renal systems in response to salt-sensitive hypertension, likely through alteration of endocrine systems, which is similar to results seen in mice lacking core clock genes. These global KO studies demonstrate the complexity of the clock in BP regulation but, importantly, help to identify potential mechanisms which can alleviate system-wide dysfunction in disease states. By harnessing the molecular clock, there is great potential to combat complex pathology upstream of multiple pathways. This can be achieved through the use of pharmacologic agents which target the molecular clock, including nuclear receptor subfamily 1, group D member 1 (REV-ERB) agonists; retinoic acid receptor-related orphan receptor (ROR) agonists; cryptochrome (CRY) stabilizers; and CK1 inhibitors. These agents are already being tested for the treatment of CV pathologies such as hypertension, kidney disease, and heart disease in several different preclinical animal models. Additionally, exciting research is underway testing the use of environmental/behavioral interventions such as time-restricted eating and timed exercise, as discussed in more detail further below. This is therefore an exciting and promising time for intervening with the molecular clock to attenuate disease.

4.1.2. Blood Pressure Regulation in Tissue Specific Knock Outs in Mice

Though global KO models are beneficial in identifying the role of clock genes in BP regulation, tissue-specific mechanisms are lost. This is addressed through tissue-specific clock gene KO models. The BP phenotype evident in mice with global BMAL1 KO male mice has been determined to be partly due to the role of BMAL1 in smooth muscle cells. Gong and colleagues demonstrated that loss of BMAL1 in smooth muscle cells led to disruption of time-of-day effects of vasoconstriction of renal and mesenteric arteries in response to phenylephrine and serotonin and also attenuated the circadian rhythm of BP (340). Lutshumba et al. found that these same mice also are protected against the formation of abdominal aortic aneurysms, highlighting the importance of BMAL1 in smooth muscle cell function (341). Given the significant contribution of smooth muscle cells to BP regulation, these findings are critical to fully understanding the impact of the clock on CV health.

The kidney is critical for the homeostatic regulation of BP due to its control of fluid and electrolyte balance. Kidney-specific KO studies demonstrate the role of this organ in maintaining the relationship between the molecular clock and CV function. In the kidney, mouse models have been generated to KO BMAL1 from specific segments of the nephron. This is of interest because the structure and function of the tubule vary along the nephron, as do the channels and transporters expressed in each segment. Targeting specific segments of the nephron can improve BP regulation and kidney health while minimizing risks of further disrupting electrolyte balance. Loss of BMAL1 from the renal collecting duct decreased BP without affecting the rhythmicity of BP or sodium excretion (342), with similar effects seen with loss of BMAL1 from the entire nephron (343). Intriguingly, Crislip et al. showed that male mice lacking BMAL1 in the distal nephron do not exhibit changes in the circadian rhythm of BP but are resistant to increases in BP induced by a low potassium and high sodium diet, possibly through mediation of inflammatory pathways (344). Several different kidney-specific Cre drivers have been used to KO BMAL1 and these have all demonstrated that loss of BMAL1 results in an ~7 mm Hg decrease in overall BP without an effect on the actual circadian rhythmicity of BP. These effects were consistent and reproducible whether the Cre driver was constitutive (341, 344) or inducible in adulthood (342). Douma & Costello et al. investigated the impacts of kidney-specific distal nephron KO of PER1 in male mice treated with a high salt diet and an aldosterone analog (345). This study revealed that loss of PER1 in the distal nephron leads to exacerbated salt-sensitive BP increases and aberrant sodium handling in male mice, likely due to higher levels of plasma aldosterone and endothelin-1; a limitation of this study was the use of terminal BP measures, thus assessment of circadian rhythms in BP was not provided. A further limitation of the available literature is that there do not appear to be cardiomyocyte-specific knockout studies in which they also investigate blood pressure. Thus, the direct role of the CV clock on blood pressure warrants further investigation. These studies highlight how the clock functions in a tissue-specific manner and that changes in the clock can be adaptive or maladaptive depending on the internal or external stressor introduced. Furthermore, the role of the kidney clock in BP regulation and CV function depends on the individual clock component investigated and the challenge the body is facing. Though this can, at times, be adverse in terms of CV health, the malleability of the molecular clock makes it a promising target for hypertension and CV health.

Using an adrenal zona glomerulosa-specific cyclization recombination (Cre) driver, Costello et al. found that adrenal-specific KO of BMAL1 in male mice led to a shortened period of their BP circadian rhythm, indicating disrupted circadian regulation of BP (346). This effect was observed in activity patterns as well. These results were only evident when the mice were stressed due to being placed in metabolic cages, confirmed by high levels of corticosterone. In addition to smooth muscle cells, the kidney, and adrenal gland, BMAL1 expressed in perivascular brown adipose tissue may contribute to BP regulation. Male mice lacking BMAL1 in brown adipocytes have an extreme dipper phenotype due to decreased BP during the inactive phase, likely due to disrupted angiotensinogen signaling from perivascular tissue to smooth muscle cells (347). This communication between peripheral clocks driven by BMAL1 in BP regulation is further supported with work done by Pati et al (348). This study demonstrated that male mice lacking BMAL1 in liver cells have decreased systolic BP during the inactive phase, with no changes in HR or diastolic BP. This was attributed to perivascular adipose tissue and blood vessel interactions impacting vascular function. This would suggest that it is not only the canonical CV systems that are critical to clock regulation of BP and CV health, but cross-talk between other peripheral systems plays an important role as well.

These data, in conjunction with global KO studies, provide compelling evidence that the clock functions in a tissue-specific manner to regulate BP via potential mechanisms that could be exploited to alleviate a wide variety of issues associated with poor BP regulation. An advantage of this complexity is that it may be harnessed to modulate multi-factorial pathology. For instance, agonists of molecules in the ancillary clock mechanism, such as ROR and REV-ERB, have shown promising effects on attenuation of myocardial injury, atherosclerosis, and hypertension in pre-clinical models (349, 350). Data from tissue-specific as well as global clock gene KO rodent models demonstrate that while loss of clock protein function reproducibility alters overall BP levels, it does not always affect BP rhythms. These data suggest that clock proteins likely function through non-circadian pathways in addition to their roles within the molecular clock.

4.1.3. Blood Pressure Regulation in Other Knock Outs That Cause Circadian Disruption

Clock gene KOs disrupt BP regulation, and disruption of other genes can alter clock gene function and, subsequently, BP. Knockout of PR domain-containing protein 16 (PRDM16) in vascular smooth muscle cells lowers BP during the active period, leading to blunting of rhythmicity of BP (351). This was associated with diminished vascular responsiveness to phenylephrine and changes in clock gene Npas2 expression, indicating aberrant adrenergic signaling and disrupted clock function. Staub et al. found that loss of renal tubular serum and glucocorticoid-regulated kinase SGK1 in male mice exposed to a high potassium diet blunts the increase in BP leading up to the active phase, disrupting BP rhythmicity (352). With this evidence, it is clear that the molecular clock works in conjunction with a variety of pathways in a bi-directional manner, further emphasizing the potential in harnessing the clock for health.

As summarized in FIGURE 11, global KO of any of the core clock genes results in a BP phenotype. Interestingly, these phenotypes vary from disrupting the rhythm of BP (global BMAL1 KO mice for example) to causing overall lowering of BP without a change in rhythm (kidney-specific BMAL1 KO mice, for example). These findings clearly demonstrate a role for the molecular clock components as well as their targets such as the PAR bZIP transcription factors, PRDM16 and SGK1, in the physiological regulation of BP. Below we describe our current state of understanding for how the clock mechanisms function in the pathophysiological state of hypertension.

4.1.4. Preclinical Models: Hypertension

The bi-directional relationship between BP regulatory elements and the molecular clock is perhaps most evident in disease states. Models of hypertension all have distinct impacts on the circadian regulation of BP. Furthermore, hypertension is caused by a variety of different mechanisms or even by a combination of mechanisms. The most common are hypertension induced by sensitivity to salt (salt-sensitive), overactivation of regulatory systems due to other disease states (secondary), and genetic disturbances that promote BP dysregulation (essential). Pre-clinical literature highlights that regardless of the mechanism driving hypertension, the molecular clock is disrupted, which can further exacerbate disease.

As stated, various mechanisms drive hypertension, including those secondary to some other disease state such as adrenal carcinoma driving hyperaldosteronism. There are many mechanisms of secondary hypertension, but one of particular interest is through overactivation of the renin-angiotensin aldosterone system (RAAS). Pre-clinical studies which have investigated RAAS-induced hypertension and the molecular clock utilized either a transgenic rat model in which renin is over-expressed, or pharmacological intervention through administration of Angiotensin II (Ang II), the primary RAAS signaling peptide. TGR(mREN-2)27 rats are transgenic rats which overexpress the gene for renin, which promotes secondary hypertension due to high activation of RAAS. Lemmer et al published in 1993 that TGR(mREN-2)27 male rats exhibit an inverted BP profile, meaning the highest BP in these rats is during the resting phase (353). Activity rhythms follow the same pattern as BP in TGR(mREN-2)27 rats. Dzirbíková et al replicated this phenotype, and expanded upon the characterization of this model to show that TGR(mREN-2)27 rats also had a higher amplitude of HR with an overall reduction in mean HR (354). This indicates that with this form of hypertension, there are disruptions in circadian regulation of CV parameters. This is adverse to health, and can further exacerbate pathology and development of heart and kidney disease.

Overactivation of RAAS is also induced by administration of Ang II. Ang II-induced hypertension has recently been linked to sex-dependent manifestations of CV pathology associated with changes in circadian machinery in the vasculature. Visniauskas et al found that male and female mice given Ang II for 8 weeks developed non-dipping hypertension and increased HR, but the specifics of the phenotype were quite different between the two sexes (355). This is expanded on in the next section.

Another mechanism which drives hypertension is salt-sensitivity. Salt-sensitive hypertension can be modeled through the Dahl salt-sensitive (SS) rat, or through introduction of high-salt diet in combination with an aldosterone analog. Dahl SS rats exhibit non-dipping BP phenotype on HS diet (356). They have also been shown to exhibit an increase in amplitude of mean arterial pressure (MAP). Additionally, the acrophase of MAP is delayed on high salt, while amplitude of HR is decreased (357). These are all adverse circadian CV responses to a high salt diet. Hernandez et al induced salt-sensitive hypertension in male Sprague-Dawley rats through high salt diet exposure in combination with deoxycorticosterone acetate (DOCA), an aldosterone analog (358). This led to disrupted circadian control of CV parameters, with HS/DOCA treated rats exhibiting a higher nocturnal increase in BP but a lower increase in HR during the same timeframe. Together, these data suggest that endocrine systems such as the RAAS contribute to the circadian regulation of BP which has major implications for the treatment of secondary and salt-sensitive hypertension treated with pharmacologic agents which target RAAS such as angiotensin receptor blockers (ARBs) and angiotensin converting enzyme inhibitors. It is possible that the timing of these drugs can further alleviate hypertension by harnessing this relationship, and by correcting non-dipping phenotypes.

To model hypertension rooted largely in genetic disruptions, researchers utilize the Spontaneously Hypertensive Rat (SHR). Cui et al used male Wistar-Kyoto (WKY) rats and SHR bred on the same background and found that both SHR and WKY rats have a diurnal rhythm in BP (359). However, SHR have a slower decline in BP leading into the light phase and an earlier rise leading into the dark phase, with SHR remaining hypertensive throughout the dark phase. Heart rate rhythms followed a similar pattern to that seen in BP.

These studies suggest that all forms of hypertension disrupt circadian control of BP, exacerbating pathology. This would imply that treatment of circadian regulation of BP could slow the progression of hypertension to other chronic and at times fatal disease. Though hypertension is a complex multi-factorial disease, driven by both gene-environment interactions, these pre-clinical studies highlight that regardless of the underlying mechanism, hypertension disrupts circadian regulation of CV parameters. The bi-directional nature of the relationship between circadian biology and hypertension forms a vicious cycle driving disease, one which could be broken by targeting circadian machinery.

4.1.5. Translating from preclinical animal models to humans

The direct effects of circadian mechanism genes on cardiac physiology and pathology have been examined through a variety of mutant and transgenic rodent models. Whole-body models are valuable for mimicking systemic circadian desynchrony, such as that observed with environmental misalignment caused by shift work or jet lag. Organ-specific transgenic models allow focused study of circadian misalignment in the heart or other CV tissues. Inducible knockouts add another important dimension, as they avoid the developmental confounds of lifelong circadian mutation and instead reveal the effects of clock disruption at specific developmental stages or in adulthood. Each of these approaches addresses distinct questions, but together they are all essential for building a comprehensive understanding of circadian regulation in CV biology. In addition, a key issue in translating rodent findings to humans is the incorporation of time-of-day as a key biological variable since mice and rats are nocturnal and many experimental procedures are typically performed during the rodent inactive phase. Thus, it is critically important that the time-of-day at which physiological measurements are made is reported by preclinical researchers. Primates, unlike rodents, are diurnal, yet they exhibit similar circadian rhythms in physiological functions. Limited data from non-human primates demonstrates that multiple physiological functions exhibit circadian rhythms (360). Squirrel monkeys have circadian rhythms in activity (361), the sleep/wake cycle (362)), drinking patterns (363), and diurnal rhythms in renal function (364). As in rodents, these rhythms are synchronized by both light and feeding schedules (365). Importantly, these rhythms are driven by the same molecular machinery as in rodents. Mure et al characterized the diurnal transcriptome of tissues from baboons (366). This study collected several tissues from baboons maintained in a consistent 24-hour light/dark and feeding cycle. These tissues were processed for RNA sequencing, and it was shown that baboons exhibit circadian rhythmicity in gene expression and cellular function across tissues. Importantly, the authors directly compared the rhythmicity of circadian machinery and the overall transcriptome in baboons to nocturnal C57BL/6 mice in several tissues including, the heart, aorta, and kidney. This comparison demonstrated that the peak expression levels for Bmal1 and Per1 in baboons were ~12 hours apart from those in nocturnal mice. Thus, the timing in both physiology and molecular machinery is opposite in baboons compared to nocturnal mice. Although mice are nocturnal and primates are diurnal, both species share remarkably similar circadian machinery and regulatory patterns. Overall, the key difference between these species lies in the phase of circadian functions: mice are active during the night, while primates are active during the day. Nonetheless, it appears that when this temporal discrepancy is properly accounted for, findings from one species can be meaningfully translated to the other.

5. CIRCADIAN MEDICINE. INTEGRATING CIRCADIAN BIOLOGY INTO THE MANAGEMENT OF CARDIOVASCULAR DISEASE

The emerging field of Circadian Medicine integrates circadian biology into clinical care and lifestyle interventions to optimize health. As noted above, circadian rhythms regulate critical physiological processes, including BP control, cardiac metabolism, renal function, and tissue repair (4, 229). These processes are synchronized by environmental time cues, termed Zeitgebers, such as light exposure, meal timing, and physical activity. However, modern lifestyles frequently disrupt these rhythms, increasing CVD risk. A major focus of Circadian Medicine is enhancing alignment of behaviors to the underlying circadian rhythmicity to reduce disease risk and improve outcomes. This is achieved by establishing and maintaining correctly timed Zeitgebers, to aid synchronization, which is crucial for coordinating processes like circulation, metabolism, hormone release, immune function, growth, repair, and sleep/wake cycles, optimizing the body’s functions across the 24-hour cycle (19, 218, 219, 224, 228, 367). Brief laboratory-controlled trials demonstrate that aligning sleep-wake cycles, meal timing, and exercise with endogenous rhythms improves BP control, reduces nocturnal hypertension, and enhances metabolic function (294, 301, 368). Key approaches are being developed to stabilize circadian rhythmicity using longer-term interventions such as increasing morning light exposure, maintaining a consistent sleep schedule, and ensuring meal intake aligns with metabolic rhythms (369). Circadian Medicine also includes chronotherapy, the practice of timing medications or behavioral/surgical interventions to align with circadian physiology. For example, in a murine model of pressure-overload induced cardiac hypertrophy, treatment with a short-acting angiotensin converting enzyme inhibitor (ACEi) at sleep-time significantly reduced cardiac remodeling, whereas dosing at wake time was no more effective than placebo. The key factor was timing; delivering the drug when its target was at peak expression markedly improved efficacy (367). Later, in silico analyses revealed that many protein-coding genes are rhythmically expressed in at least one tissue, and numerous essential drugs with short half-lives target the products of these rhythmic genes (128). Therefore, aligning drug delivery with body time may enhance therapeutic effectiveness. Moreover, understanding circadian influences on drug metabolism or drug effects further enables more precise treatment strategies, minimizing side effects while optimizing therapeutic benefits. For example, it is common clinical practice to recommend nighttime administration of short-acting statins to enhance cholesterol metabolism, while some antihypertensive medications have been reported to provide better BP control when taken in the evening, particularly in non-dipping BP patients (218, 370). As discussed below, by integrating circadian principles into medicine, clinicians could target circadian misalignment as a modifiable risk factor for CVD, potentially leading to more effective prevention and treatment strategies (371). The following sections will explore the three primary Zeitgebers: light exposure, meal timing, and physical activity and their applications in Circadian Medicine for cardiology (FIGURE 12, Call Out Boxes). We also discuss the broader goal of optimizing Circadian Health at multiple levels: individual, institutional and occupational, and public health (FIGURE 13). Building on this foundation, we address the often-misunderstood field of chronotherapy, and conclude by exploring emerging New Frontiers in the field.

Figure 12. Zeitgeber-based strategies in Circadian Medicine to modulate circadian phase, support cardiovascular health, and enhance rhythm amplitude with a special focus on the elderly.

This figure illustrates how the timing of key Zeitgebers such as light, eating, and exercise can be leveraged to promote circadian health across three domains. The left column focuses on circadian phase alignment in humans: morning exposure to full-spectrum light, especially blue or short wavelengths, enhances alignment and amplitude, while evening blue light delays phase and impairs sleep; restricting food intake to daytime hours helps maintain circadian phase, whereas night eating and round-the-clock feeding, as seen in shift workers, is disruptive; daytime physical activity reinforces rest-activity cycles, while nighttime exercise can misalign rhythms. The middle column highlights preclinical applications for cardiovascular disease therapy as described in the text, where aligning these behaviors with circadian timing may improve patient outcomes, recovery, and resilience. The top panel focusses on preclinical models for circadian lighting in ICUs (e.g., (28, 371), timing of eating to improve cardiac repair post-MI (e.g., (403), and timing of rest or activity to benefit cardiac outcomes post-MI and post-TAC (e.g., (493). The right column emphasizes circadian amplitude and the special case of the elderly: amplitude declines with age, but timed interventions like morning light, consistent daytime meals, and scheduled exercise can enhance rhythm strength. Exercise timing may also influence muscle mass and be applied to benefit cardiac rehabilitation outcomes in older adults (e.g., potential to enhance cardiac repair), though optimal timing has not yet been investigated. Together, these strategies form the foundation of behavioral circadian medicine interventions for cardiovascular health and disease.

Figure 13. Multilevel strategies to promote Circadian Health.

This figure illustrates the application of circadian science across three levels: individual health, institutional and occupational settings, and public health. Individual level (left column). In the top panel, clinicians are encouraged to assess patients’ daily exposure to Zeitgebers such as light, meal timing, and physical activity, to offer personalized circadian alignment strategies. The bottom panel emphasizes that aging reduces circadian amplitude, but well-timed exposure to zeitgebers can help restore rhythm strength and improve health outcomes. Institutional and Occupational settings (middle column). The top panel shows that when reasonably possible, aligning medication timing, nursing shifts, or surgical procedures with circadian biology can enhance treatment efficacy and recovery. The bottom panel addresses occupational disruptions, such as shift work and jet lag, which misalign circadian rhythms and increase CVD risk. The health of aviation personnel may also be impacted by trans-meridian travel and erratic schedules. Public health (right column). The top panel highlights the need to adjust school start times to better match adolescent circadian rhythms. At night, urban lighting design should minimize circadian disruption by favoring longer-wavelength (non-blue) lighting. The bottom panel advocates for eliminating seasonal clock changes and adopting year-round standard time to support population-level circadian alignment. Together, these strategies reveal the translational potential of circadian science across healthy systems and society.

5.1. The Light Zeitgeber

Light is the most influential Zeitgeber – indeed it is unique as it is also operative at the population level through urban lighting. Its importance for our physiology and pathophysiology was highlighted by Michael Siffre’s elegant studies (372), where he lived in isolated caves without exposure to natural light for extended periods, recording his sleep-wake cycles and physiological rhythms. He showed that although our internal clocks operate independently of external light, they recalibrate daily with light cues, aligning day-night functions. This finding is crucial because it highlights the fundamental role of light in synchronizing biological rhythms for our normal physiology and has implications for disorders related to circadian misalignment.

The key components for understanding this Zeitgeber are timing, intensity and wavelength. The timing determines if it delays the central circadian clock to a later hour, advances it to an earlier hour, or has no effect (141). Light during the beginning of the circadian night causes phase delays, while light during the end of the biological night causes phase advances. Light intensity is measured in lux. Daylight ranges from 1,000 lux on a cloudy day to 100,000 lux in full sunlight, while typical indoor lighting provides around 500 lux. The wavelength of light determines color temperature of the visible spectrum (short [e.g. blue] to long [e.g. red] wavelength light), which influences the strength of the effect of light on phase shifting and melatonin suppression. Even dim light (>8 lux) can inhibit melatonin (158); the most effective wavelength for melatonin inhibition is 440–490 nm (blue). Melanopsin, a photopigment in ipRGCs is particularly sensitive to this short-wavelength blue light (201). Although the basics of light and circadian rhythms are well-established, the specific impact of light on CV health and disease is less understood. Light is a crucial Zeitgeber regulating CV health and the foundation of CV circadian medicine, which is described in detail in the sections below.

5.1.1. Light Regulates Gene Expression in Cardiac Structure and Function.

Earlier fundamental studies hypothesized that light could drive rhythmic patterns in genes essential for cardiac structure and function. Using microarrays and bioinformatic analyses, it was discovered that light as a Zeitgeber significantly influences gene expression in the heart over the 24-hour cycle (80). Some cardiac genes cycle rhythmically throughout the day and night, while others surprisingly shift abruptly at light and dark transitions. Rhythmic patterns in cardiac proteins and their post-translational modifications mirror this gene cycling effect across the diurnal cycle (98, 99, 373). As a result, during the day the heart’s metabolic profile is focused on its contractile needs; at night, expression emphasizes repair and renewal, as has been shown in both mice and rats (including obese mice in one study), and a tau hamster misalignment study (91, 225, 228, 374). The heart exhibits different molecular profiles during the day versus the night, highlighting the need for time-of-day considerations in research, diagnosis, and treatment.

5.1.2. Effects of Disrupted Light/Dark Cycles on Cardiovascular Tissues.

A second fundamental area of research has focused on the consequences of misaligned light cycles, as seen in everyday real-life situations like shift work, or analogous diurnal disruptions through our work, travel, and social commitments. As noted further above, there is strong epidemiological data, primarily from shift workers, that circadian disruption is associated with adverse CV consequences (24, 301, 375–377). The first causal evidence of direct impacts of circadian misalignment on cardiac pathology was found in studies using the +/tau hamster, which has a 22-hour internal clock that conflicts with the 24-hour day (292). In a 24-hour light/dark environment, these misaligned hamsters have a reduced lifespan with dilated cardiomyopathy and renal disease. However, when aligned by an internal:external 22-hour environment matching their internal clock, they exhibited normal life expectancies and no cardio-renal disease (292). Subsequent studies developed the relationship between light Zeitgeber misalignment and CV pathology in a murine model of acquired pressure overload-induced cardiac hypertrophy and heart failure, transverse aortic constriction (27, 378). The evolution of cardiac remodeling under normal 24-hour light and dark conditions was compared to misalignment in a 20-hour environment. Echocardiography in non-entrained mice showed increased left ventricular end-systolic and end-diastolic dimensions and reduced contractility, while restoration of normal diurnal schedules effectively reversed the pathology (27, 378).

In humans, light:dark disruptions such as shift work significantly impact CV health. The World Health Organization classifies shift work as an occupational disruptor (379), and research links it to increased heart disease risk (380). Epidemiological studies form the primary basis of this connection. Shift workers in Sweden had an increased risk of ischemic heart disease compared with that risk in day workers (381). U.S. female nurses had an increased risk of coronary heart disease, especially those with 6 or more years of shift work (24). A meta-analysis of 34 studies of shift workers revealed an increased risk for major vascular events such as myocardial infarction (256). Studies in Germany and Japan also linked shift work to increased risk of atherosclerosis (382), and progression of hypertension (383), as compared to people who were not shift workers. Data from the UK Biobank, analyzing light exposure in 89,000 individuals, collected from 13 million hours of light sensor data, found that nighttime light exposure and disrupted light patterns were linked to a higher risk of type 2 diabetes and increased cardiometabolic mortality (384–386). Stable light-dark cycles with circadian alignment may be an effective strategy to mitigate diabetes risk, even for individuals with a genetic predisposition. In terms of mechanisms, it has been shown that shift work alters autonomic function (375) and BP patterns (387), potentially contributing to CVD. Moreover, experimental studies in humans show that circadian misalignment negatively affects endocrine, metabolic, and autonomic factors tied to CV risk (26). With millions of workers following irregular schedules, understanding their impact on health and performance is essential to minimizing risks.

5.1.3. Circadian Lighting and Cardiac Repair

A third important area of research has shown that even brief light/dark misalignment at critical times can have profound long-term consequences. This acute effect on cardiac repair has been demonstrated in mice where myocardial infarction was induced by surgically ligating the left anterior descending coronary artery. Immediately after, the mice were placed in a 10:10 hour light:dark cycle environment for 5 days, simulating misalignment by circadian disruption as might be experienced in the ICU (28). The experimental group profoundly altered the expression of cytokines and the orderly recruitment of inflammatory cells, predominantly neutrophils and macrophages. Myocardial repair was disorganized, resulting in permanent cardiac damage characterized by increased left ventricular (LV) hypertrophy, infarct thinning and expansion, and significant LV dysfunction. The mechanism appears to be disruption of the molecular circadian mechanism in the myocytes due to adversely timed light exposure (28). This finding is particularly relevant to cardiac intensive care in humans (371).

5.1.4. Optimizing Patient Care with Light/Dark Cycles

In humans, the harmful effects of blue light at night from phone, computer and TV screens are well-recognized (282). Exposure to blue light at night disrupts melatonin production, delaying circadian rhythms and affecting key physiological processes. Importantly, blue light serves an adaptive purpose as well. The crucial role of morning blue light for circadian alignment is often overlooked. On the other hand, morning bright light exposure helps maintain circadian function and is associated with good sleep outcomes (388). In a clinical setting, it appears to be warranted to optimize the light environment by increasing natural light exposure during the day and minimizing light at night. For instance, patients could be exposed to ample daylight or bright artificial light in the morning, while evening light (especially blue light) exposure ought to be minimized, particularly a few hours before bedtime. Blue light-blocking glasses may be helpful in the evening but counterproductive during the day, as daytime blue light is essential for circadian regulation. Brown or amber sunglass lenses block blue light; grey lenses let blue light pass. Implementing bioadaptive circadian lighting systems in hospitals and care settings, which mimic natural 24-hour light-dark cycles in terms of intensity, color temperature, and wavelength, is now feasible due to advances in lighting technology. Patient care may also be optimized by aligning tasks like blood draws, computer use, and medication timing with patients’ circadian rhythms. Scheduling nursing shifts, medications, and other interventions in synchrony with natural circadian cycles can enhance recovery. Circadian alignment can further be supported by promoting good sleep hygiene, such as adhering to regular sleep/wake schedules, even on weekends. This has been strongly recommended (371).

Patients with CV conditions, such as those recovering from myocardial infarction or suffering from cardiomyopathy, atherosclerosis, hypertension, arrhythmias, heart failure, or diabetes, are particularly vulnerable to circadian disruptions. As demonstrated in the above animal studies (27, 292), misalignment of circadian rhythms can impair acute and chronic cardiac healing processes by interfering with the body's natural repair mechanisms. As noted above, a brief loss of entrainment during a critical phase of an illness may result in long-term health consequences (28); ensuring proper circadian regulation is vital for optimal recovery and management of these conditions (371).

The elderly and proper diurnal light alignment deserve special consideration. A recent study of 552 community dwelling seniors revealed that over one-half had significant exposure to light during sleep as documented by wrist, actigraphy, and light measurements. This group exhibited nearly twice the rates of diabetes, obesity, and hypertension, compared to those who slept in the darkness (273).

Finally, elderly residents in long-term care often spend their days in dimly lit rooms; they also have weakened endogenous circadian rhythms. Indeed, the elderly may be less responsive to light stimulation as a Zeitgeber, compounding the inadequacy of their circadian clock (389, 390). This then leads to poor sleep quality, reduced alertness, exacerbation of metabolic disorders and cognitive decline. These findings suggest that patients, especially those with dementia, ought be exposed to abundant daily natural light; they may also benefit from bioadaptive lighting systems, or actual morning bright light systems (391) to reinforce their circadian rhythms.

Call-Out box A summarizes the key circadian medicine features of light as a cardiovascular Zeitgeber, as was discussed in detail above.

Box A. Call-Out Box: Light as a Cardiovascular Zeitgeber.

• Morning Light Exposure: Encourage patients, such as the elderly, or post-myocardial infarction, to receive bright light exposure in the morning to reinforce circadian alignment and support cardiovascular healing.

• Nighttime Light Minimization: Reduce evening and nighttime blue light exposure to preserve melatonin production and circadian rhythmicity, especially in Intensive Care Unit and long-term care.

• Clinical Settings: Implement bioadaptive lighting in hospitals to mimic natural day-night cycles. Align care activities (e.g., vitals, medications, labs) with circadian physiology to improve outcomes.

• At-Risk Populations: Patients with cardiomyopathy, heart failure, hypertension, diabetes, and dementia benefit from circadian-aligned light routines.

• Shift Workers: Advocate for circadian re-entrainment strategies and schedule adjustments to reduce risk of cardiovascular disease.

5.2. The Meal Timing Zeitgeber

Circadian rhythms play a crucial role in regulating metabolism, particularly food intake timing. During the day, there is a balance between leptin (satiety), ghrelin (hunger), and insulin/glucagon secretion, along with circadian regulators like cortisol and melatonin and daily rhythms in adiponectin and inflammatory cytokines. Daytime eating helps maintain CV health by preserving cardiac vagal modulation, stabilizing sympatho-vagal balance, reducing PAI-1 levels, and lowering BP, even if the sleep/wake cycle is misaligned relative to the central circadian clock as is typical in night work (310). Daytime eating also prevents internal circadian misalignment and disruptions in glucose regulation, which may offer CV benefits (294).

The application of time-restricted eating (TRE) involves consuming all daily calories within a restricted time window (193, 294, 392–395). While often discussed as a weight loss strategy, TRE’s significant role as a metabolic Zeitgeber is underacknowledged, even by physicians. Irregular or late-night eating disrupts metabolism, increasing the risk of metabolic syndrome—a cluster of conditions including obesity, hypertension, dyslipidemia, and insulin resistance—a major risk factor for type 2 diabetes and CVD (395–399). TRE typically aligns eating patterns with circadian rhythms and can improve glycemic regulation, supporting broader benefits for cardiometabolic health in adults with metabolic syndrome. Chellappa and colleagues have further shown that nighttime eating, which is common among night workers, results in a misalignment between central and peripheral (glucose) circadian rhythms, impaired glucose tolerance, and adverse CV changes, while restricting meals to only during the daytime prevents these adverse effects (294, 310). These findings provide the foundation for the development of a behavioral approach to managing the increased risk of diabetes and CVD observed in shift workers.

5.2.1. Improving Cardiac Repair – Microbiome and Meal Timing

Circadian interventions such as TRE have been linked to improved gut microbiome health. This is critical as the gut microbiome plays a major role in CV outcomes (400, 401). In the CV system, the microbiome significantly influences recovery after MI (402). Studies in mice with antibiotic-induced gut dysbiosis show impaired post-myocardial infarction outcomes, partly due to reduced short-chain fatty acids, which are essential for immune function and cardiac repair. Interestingly, co-housing dysbiosis mice with healthy ones restored microbiota through coprophagy, improving recovery. The same study also explored whether TRE could help restore gut health. Daytime-restricted feeding in the rodent model reveals this to be a practical alternative (403), suggesting potential benefits for human patients. Aligning food timing with circadian rhythms may not only improve microbiome composition, but also enhance cardiac repair, highlighting TRE as a promising rhythm-based therapeutic approach. While fecal microbiota transplantation shows promise for restoring gut health, clinical use remains limited by safety and logistical concerns. Future research on the utility of probiotics may offer a more practical and immediate approach to microbiome rehabilitation.

5.2.2. Promoting circadian health through time restricted eating

As noted above, the elderly may be less sensitive to light as a Zeitgeber. Animal studies have shown that the response to non-photic stimuli such as exercise or eating is not significantly impaired by aging. Indeed, these studies suggest that timed feeding schedules could effectively resynchronize circadian rhythms in older individuals, potentially reducing the risk of circadian-related metabolic disorders (e.g. (404–406). To maximize the benefits of TRE, patients could be educated on the importance of consistency in eating times and the benefits of TRE. Narrowing eating windows to the recommended 10 hours and adapting to TRE may take time. Healthcare professionals must recognize that social and work commitments may introduce shift-work-like barriers to implementing TRE. Combining TRE with lifestyle interventions such as regular physical activity, managing the quantity and composition of food at each meal, and even stress management may assist in achieving this. Circadian-aligned feeding interventions, such as probiotics tailored to match circadian rhythms or structured meal schedules, could particularly benefit elderly patients who are more vulnerable to CV damage and frequent antibiotic use. If the patient is in the hospital or long-term care, institutional scheduling may facilitate the introduction of TRE and its concepts. However, introducing calorie restriction alongside TRE may complicate the approach and potentially lower compliance. Restriction may also negatively impact functional aging through inadequate protein intake, aggravating sarcopenia in the elderly.

Call-Out box B summarizes key circadian medicine features of meal timing for cardiometabolic health, as discussed above.

Box B. Call-Out Box: Meal Timing and Cardiometabolic Health.

• Time-Restricted Eating: Recommend eating within a consistent ~10–12-hour daytime window to reduce metabolic syndrome and improve blood pressure, glucose control, and insulin sensitivity.

• Night Eating Risks: Discourage eating at night as it worsens glucose tolerance and increases risk of cardiovascular disease. This strategy may have potential in night shift workers.

• Hospital and Long-Term Care Settings: Structure meal delivery around daytime hours to enhance circadian entrainment in patients, particularly the elderly.

• Gut Microbiome and Cardiac Repair: Time-Restricted Eating may support a healthy microbiome, aiding repair post-Myocardial Infarction via improved immune responses and reduced inflammation.

• Time-Restricted Eating in Aging: Timed eating is an effective circadian cue for older adults with impaired light responsiveness; may reduce obesity, diabetes, and hypertension risk.

5.3. The Exercise Zeitgeber

Exercise is a powerful yet underappreciated Zeitgeber as detailed in several reviews (407–410). In brief, exercise modulates peripheral clocks, especially in muscles and the liver, and influences circadian rhythms. Unlike light, which primarily resets the central clock in the SCN, physical activity adjusts the phase and amplitude of peripheral clocks, promoting physiological and metabolic health. The muscle clock regulates key metabolic pathways, including glucose uptake, lipid oxidation, and protein turnover. Genes involved in these processes exhibit rhythmic expression that aligns with the body’s circadian rhythms. Disruption of this system, as demonstrated in rodent models with muscle-specific BMAL1 or CLOCK knockouts, impairs glucose metabolism, shifting energy reliance toward alternative substrates such as lipids or proteins. This can have an adverse effect on cardiometabolic health.

5.3.1. Insights on cardiovascular effects of exercise timing in humans

Human data from the UK Biobank and Look AHEAD study reveal that moderate-to-vigorous physical activity later in the day (afternoon or evening) is associated with lower all-cause and CVD mortality (411) and CV risk in individuals with obesity and type 2 diabetes (412, 413). Engaging in moderate-to-vigorous physical activity in the afternoon has been associated with improved glycemic control in adults with type 2 diabetes, potentially contributing to better CV health (414, 415). Evening aerobic training performed in the evening decreased clinical and ambulatory BP in treated hypertensive patients (416). Morning exercise, however, may elevate blood glucose and BP in diabetic populations, making it potentially less suitable for this group (417–420). These findings suggest that exercise timing could be tailored to optimize outcomes for various conditions, including chronic diseases and cardiac rehabilitation.

5.3.2. Promoting circadian health through timed exercise

Incorporating exercise as a circadian intervention requires careful consideration of individual needs and environmental contexts. Physical activity reinforces circadian rhythms best under optimal, natural light, as opposed to exercise performed in a darkened room or while watching a screen. The timing of exercise can elicit circadian-phase shifting effects (421), and may be dependent on chronotype (422). Morning exercise to advance the clock should be beneficial for individuals with delayed sleep phase disorders or those who have difficulty waking up early. Exercising later in the day by delaying the clock might be helpful for those with early sleep phase disorders or who naturally wake up early but struggle with staying awake in the evening; however, late-night exercise might disrupt sleep if it is too close to bedtime (423, 424). Personalized, time-of-day-specific exercise regimens aligned with an individual's circadian biology can maximize therapeutic potential (425–427), particularly for cardiometabolic diseases. Long-term physical activity enhances rhythmic gene expression and improves metabolic and CV outcomes, and of course, institutional bed rest, such as occurs with long-term care or hospital care, should be minimized appropriately as this increases musculoskeletal wasting and the risk of thromboembolism. Call-Out box C summarizes exercise as a circadian therapeutic tool.

Box C. Call-Out Box: Exercise as a Circadian Therapeutic Tool.

• Time-of-Day Tailoring: Afternoon/evening exercise improves glycemic control and blood pressure in diabetic and hypertensive patients; morning activity may be less effective or even harmful in these groups.

• Chronotype-Based Guidance: Use morning exercise to advance the phase in late chronotypes or those with delayed sleep phases; use afternoon/evening activity for early chronotypes.

• Hospital & Long-Term Care Considerations: Minimize bed rest. Promote light-exposed movement opportunities to preserve muscle mass and prevent thromboembolic complications.

• Phase-Shift Potential: Timed physical activity can shift peripheral clocks, restore circadian amplitude, and improve cardiovascular outcomes.

• Rehabilitation Strategy: Incorporate exercise timing into cardiac rehabilitation programs for optimal recovery.

5.4. Timed medication administration for cardiovascular disease

Chronotherapy leverages the principles of circadian biology to optimize treatments, to improve outcomes, and minimize side effects (218–220, 226, 228). Chronotherapy can include optimally timing light exposure, meals, exercise and/or medications (91, 228, 371). Ongoing research is exploring methods to accurately and quickly determine an individual's internal body time of day (i.e., circadian phase), as timing therapy to internal time would have much more promise than timing therapy based on time of day, or time of behaviors (such as waking up, or bedtime) (FIGURE 14). In terms of estimating internal circadian phase, some studies use chronotype questionnaires and biological markers such as melatonin and cortisol. More recently, investigators have been working to develop 24-hour circadian gene and protein profiles, either multiple time points over 24-hour cycles or, more recently, single time point sampling (428–431). While these approaches hold significant potential, they are not yet sufficiently advanced or standardized for routine clinical use. For now, it is much more common for clinical practice to time medication administration according to typical wake and sleep schedules, or meal schedules, or sometimes the clock on the wall (as would be more likely in a hospital setting and potentially dictated by the health-workers’ schedules). The following sections will explore key aspects of chronotherapy relevant to clinical settings.

Figure 14. Timing of therapy according to different reference times.

We consider there to be four modes of timing of therapy. First is “ad-hoc”, and can occur at any time of day, according to preference or convenience. The second is based on external clock time (e.g., taking medications at specific times of day, as is likely to occur in a hospital setting when healthcare workers may give medications according to times of their work shifts or lights schedules in the hospital). Third based on time relative to habitual behaviors (e.g., taking medications immediately before bedtime, or upon awakening, or with a specific meal, or before exercise, etc.). Fourth is based on the phase of the internal circadian clock (e.g., this would be important where medicine either targets receptors or neuro-endocrine functional variables that are strongly controlled by the circadian clock or targets an actual component of the molecular circadian clock itself). This last variation remains a challenge for the field of Circadian Medicine, especially since easy, quick, and cheap markers of the internal circadian clock time are lacking at present.

There are many examples in contemporary clinical cardiology where chronotherapy has been shown to be effective. For instance, short-acting statins are typically administered at night to optimize cholesterol metabolism (432), and studies suggest that nighttime aspirin may reduce morning platelet activity (433), potentially lowering morning myocardial infarction (MI) risk. Similarly, coronary function differs in the day versus night, and as such nitroglycerin patches are routinely applied only during the day (which also averts the development of tolerance by wearing the patch for only ~12 hours instead of 24) (434). Nocturnal hemodialysis helps regression of LV hypertrophy in people with chronic kidney disease (435). Perhaps most classically, treating OSA at night with continuous positive airway pressure therapy suppresses abnormal nocturnal sympathetic activity, lowers nighttime BP, and improves myocardial systolic function in patients with heart failure (436). Surgery for aortic valve replacement in the afternoon may be safer than in the morning (437), but there are other recent examples where the time of day of surgery does not influence outcomes or where the outcomes are worse for afternoon/evening surgeries (438–440). Common hormone therapies, like morning thyroid hormone for metabolism and nighttime melatonin for sleep, also exemplify the impact of timing. Many CV drugs directly target the products of circadian rhythmic genes and may thus benefit from a chronotherapeutic approach based on internal circadian phase rather than time of day, or time of sleep (128, 226, 435).

Animal studies have highlighted additional benefits of chronotherapy, particularly in reducing cardiac remodeling, as these models allow for examining longer-term pathophysiological outcomes on more accessible timelines than human studies. Research using the murine transverse aortic constriction (TAC) model demonstrated that administering captopril, a short-acting angiotensin-converting enzyme inhibitor (ACEi), at sleep time not only lowered BP but also protected against cardiac and vascular remodeling (367). Notably, administering the same dose during wake time provided no such benefit despite similar BP reductions (367). This reflects circadian variations in the renin-angiotensin-aldosterone system activity and gene myocardial expression, which support CV repair and renewal during sleep. Thus, these findings further suggested that the prevention of target organ damage in hypertension by drugs such as ACEi is strongly influenced by the timing of treatment.

Chronotherapy for hypertension has been a topic of intense research and debate in recent years. Initial promising findings of clinical trials in specific populations have been somewhat negated by negative outcomes from recent trials in unselected general populations and have led many to dismiss its role in hypertension management. However, this perspective is likely too simplistic—a case of the pendulum swinging too far, and reflecting an overcorrection. Thus, we review this specific example to highlight the importance of adopting a rational, targeted approach to chronotherapy in general. Call-Out box D highlights clinically relevant approaches to timing medications or procedures for CVD, as discussed in more detail below.

Box D. Call-Out Box: Timing Medications or Procedures for Cardiovascular Disease.

• Statins & Antihypertensives: Administering shorter-acting statins at night when endogenous targets peak (e.g., cholesterol synthesis) can maximize efficacy and minimize side effects. For antihypertensive see Table 2.

• Chronotherapy for Obstructive Sleep Apnea and Chronic Kidney Disease: Nighttime continuous positive airway pressure improves blood pressure and cardiac function; nocturnal dialysis reverses left ventricular hypertrophy more effectively than daytime sessions.

• Surgical Timing: Some evidence suggests improved outcomes for afternoon cardiac surgeries (i.e. aortic valve replacement) highlighting the need for individualized approaches.

• Hormone Therapies: Morning thyroid meds and nighttime melatonin support metabolic and circadian alignment.

• Clinical Implementation: When chronotype-specific biomarker testing is unavailable, use standard wake-sleep patterns and clock time to guide medication timing.

5.4.1. Chronotherapy and Hypertension

Contemporary therapeutic strategies for hypertension often aim for 24-hour control and may involve receptor targeting using multiple doses or long-acting drugs, typically administered in the morning for convenience. This approach may overlook circadian rhythms of adverse effects and expose patients to the risk of side effects with minimal benefit. Long-acting angiotensin-converting enzyme inhibitor (ACEI) may reduce glomerular filtration rate with renal impairment or cause hyperkalemia (441). In common clinical practice, administration of a calcium channel blocker at night may ameliorate the ankle swelling associated with daytime administration. Conversely, it is also clinically known that evening dosing of certain antihypertensive medications may lead to adverse effects such as nocturia, sleep disturbances, nocturnal hypotension, and an increased risk of falls in elderly patients. Such side effects may lead to the discontinuation of a long-acting drug; however, a well-timed, short-acting medication could provide sufficient therapeutic benefit while minimizing these risks. Given such findings, as well as the well-described diurnal pattern of BP, which is typically high during the day, lower during the night, and with a surge in the early morning (442–446), the practice of chronotherapy for management of hypertension has become widespread. Moreover, Ambulatory Blood Pressure Monitoring (ABPM) has emerged as a crucial tool in the diagnosis and management of hypertension. In terms of correlations with clinical outcomes, ABPM is superior to sporadic office BP measurements, and provides a comprehensive 24-hour profile of BP variations, allowing for more accurate CV risk and treatment assessment. It has been repeatedly found that an inadequate decrease in night-time BP (i.e., non-dipping BP) is associated with increased CV risk (447–450).

The first large clinical chronotherapy antihypertensive studies, MAPEC (370) and Hygia (451), both targeted this non-dipping pattern of BP and extensively used APBM for measuring BP. They demonstrated significant CV benefits from evening dosing of antihypertensive medications, which helped restore a natural dipping pattern of BP during the night, and significantly reduced the risk of major CV events such as myocardial infarction, stroke, and CV mortality. These trials highlighted how aligning medication timing with circadian rhythms can enhance BP management and improve CV outcomes. However, the findings of MAPEC and Hygia have been called into question due to several concerning issues (452). Moreover, subsequent large-scale trials, all using self-reported BPs rather than APBM, have challenged these findings. For example, Mackenzie et al. (453) found no significant difference in CV outcomes between morning and evening dosing. This was further corroborated by the recent Canadian BedMed and BedMed-Frail trials across diverse general patient populations (454, 455). Since these large-scale studies did not show significant differences in major adverse CV events between morning and evening dosing for the general population, many are now dismissing chronotherapy in hypertension management. However, there is much to be learned from these negative results.

The concept of chronotherapy in hypertension is based on several solid lines of evidence. For instance, a sub-analysis of the TIME study (456) revealed a clinically significant lower rate of non-fatal myocardial infarction events when the dosing time was aligned with chronotype, especially late chronotypes taking their medications in the evening. These results suggest that aligning medication timing with a patien’s natural BP rhythm could provide better CV protection– specifically, non-dippers and those with significant morning BP surges should benefit more from evening dosing.

In reality, managing hypertension entails complex challenges and opportunities shaped by individual patient factors and significantly influenced by circadian rhythms. 24-hour BP profiles, including BP dipping status, morning surge pattern, chronotype, and sex, interact in intricate ways, making a one-size-fits-all approach to medication timing suboptimal. Hypertension, much like fever, is a symptom that can arise from various underlying conditions. Just as penicillin is ineffective for treating fever in general but is highly effective for many bacterial infections, administering antihypertensive therapy at night should be especially beneficial for specific patient groups such as evening chronotypes, non-dippers, and those who experience a significant morning surge in BP (depending on the half-life of the medication). This approach may better mitigate target organ damage, as evidenced by the murine TAC model study by Martino et al (367) for example, managing hypertension in patients recovering from a myocardial infarction or individuals with hypertrophic cardiomyopathy or congestive heart failure. Thus, chronotherapy remains relevant in hypertension management; however, its application has become more refined. The focus has shifted from a blanket recommendation for evening dosing to a more personalized approach considering individual patient factors, chronotype, BP day/night profile, adherence patterns, shift work, and potential adverse effects. This tailored strategy allows for the potential benefits of chronotherapy while minimizing the risk of adverse effects, ultimately improving patient outcomes in hypertension management (220). In conclusion, the pendulum for the therapy of hypertension should swing back towards a more balanced view. By recognizing the diversity of hypertensive syndromes and the principles of chronotherapy, healthcare providers can offer more tailored and effective treatment strategies. This nuanced approach moves beyond the limitations of one-size-fits-all solutions, acknowledging that the timing of medication administration can significantly impact patient outcomes. This is particularly crucial for specific patient populations who may benefit most from time-based treatment strategies (Table 2).

Table 2.

Practical Applications of Chronotherapy in Hypertension Management

Domain	Key Points
Clinical Application	Chronotherapy is an emerging approach in hypertension management, aligning treatment timing with circadian rhythms for improved outcomes.
24-hour Monitoring	Ambulatory blood pressure monitoring and emerging wearable technologies provide accessible tools for full-day blood pressure profiling.
Chronotype Assessment	Chronotypes can be identified using sleep-wake patterns, diaries, questionnaires, evening alertness, meal timing, activity data, and personal devices.
Personalized Treatment	Hypertension chronotherapy should be tailored to individual characteristics, accounting for patient chronotype, adherence likelihood, and therapeutic goals.
Nighttime Dosing Strategy	Recommended for patients with evening chronotype, non-dipping blood pressure profiles, early morning blood pressure surges, or specific conditions (e.g., cardiac remodeling, recovery following myocardial infarction, hypertrophic cardiomyopathy, heart failure).
Medication Considerations	Long-acting antihypertensives may interfere with circadian physiology and pose side effect risks. Time-specific administration of short-acting drugs may improve safety and efficacy.

The Practical Application of Chronotherapy in the Management of Hypertension

Blood Pressure Profiling:

Determining BP profiles without ABPM can be challenging. Numerous devices offer 24-hour monitoring and are widely used in clinical settings. Newer technologies and advancements in promise to make 24-hour monitoring more accessible. For patients without access to ABPM, home BP measurements upon awakening may help detect morning surges. In contrast, concomitant conditions like cardiac or renal disease and sleep apnea can help identify people with a non-dipping BP profile.

Identifying Chronotypes:

Classifying patients by chronotype can help guide treatment but requires some effort. Evening chronotypes, for example, may be identified by questionnaires regarding alertness or preference for later meals, or objective assessments, such as sleep-wake cycle timing and increased evening activity. Wearable devices, personal diaries, or online questionnaires can assist in identifying chronotypes for more effective treatment.

Personalized Approach:

Incorporating chronotherapy into hypertension management requires tailoring treatment to the diverse characteristics of hypertensive conditions, considering individual patient preferences, adherence, and overall treatment efficacy.

Nighttime dosing:

This may benefit specific groups, including evening chronotypes, non-dippers, those with significant morning BP surges, patients experiencing cardiac remodeling, individuals recovering from MI, and those with hypertrophic cardiomyopathy or heart failure. Aligning medication timing with a patient’s chronotype has been shown to reduce the risk of non-fatal MI, particularly in late chronotypes who take their medication in the evening (456).

Long-acting antihypertensives:

These offer practical advantages for adherence and 24-hour BP control; however, they may obscure natural circadian BP rhythms and increase certain side effects (e.g., electrolyte disorders, renal impairment, nocturia, pedal swelling and, orthostatic hypotension) limiting therapeutic benefits. Short-acting medications, when timed according to circadian principles (chronotherapy), may still maintain effective BP control but better align drug action with body physiology and periods of cardiovascular vulnerability and may reduce adverse effects, especially in select populations

Other hypertensive and CV chronotherapies:

Recent studies highlight the importance of chronotherapy for CV health and disease and are beyond the scope of this review but the reader is directed to several excellent reviews (218, 226, 457).

In summary, selective chronotherapy represents a significant advancement in hypertension management. Considering individual patient factors and treatment timing, this personalized, time-based approach addresses target organ damage and optimizes CV care, leading to better overall outcomes. However, the overall clinical benefit of hypertension chronotherapy, particularly for long-acting agents, remains debated, and treatment should be individualized to optimize both efficacy and safety (Table 2).

6. NEW FRONTIERS IN CIRCADIAN MEDICINE

6.1. Navigating Multiple Clocks in Circadian Medicine

Circadian Medicine holds great promise for clinical cardiology, with encouraging results already emerging and many exciting avenues under active investigation. As noted above, as the field advances and incorporates circadian principles into clinical practice, there is the need to consider various key “clocks” when timing therapies to optimize outcomes: (1) ad-hoc (i.e., no specific therapeutic time window); (2) external or clock time (e.g., standard hospital medication dosing schedules shaped by shift patterns or lighting schedules); (3) behavioral time (e.g., taking medication before bed, after waking, or with meals); and (4) internal circadian phase (e.g., when a drug targets clock-regulated pathways or molecular clock components) (FIGURE 14). Even in people with regular routines, the phase angle of entrainment (the timing relationship between internal circadian signals and external behaviors) can vary widely. This variation is often driven by chronotype, with evening-types typically having a longer delay between internal markers like melatonin onset and habitual bedtime (458). Others may lack consistent schedules altogether, leading to misalignment between biological and social clocks. A common example is social jetlag, where people shift their sleep-wake schedule on the weekends compared to workdays, essentially “traveling” across time zones each week (459). Similarly shift work often forces wakefulness and sleep at biologically misaligned times and has been associated with increased CV risk, as described above. These scenarios reveal the importance of considering not just behavioral or clock time, but internal circadian timing, especially as precision medicine becomes more personalized. Although measuring internal circadian phase in clinical settings has historically been a challenge, exciting progress is being made. New methods, including molecular timetables and skin or blood- based assays are making it increasingly feasible to estimate circadian phase from a single time-point sample (429, 430, 460, 461). Clinical trials comparing behavioral, clock, and internal timing strategies may be important next steps to guide best practices. In the meantime, simple time-based strategies remain clinically useful, and with growing awareness of patient-specific rhythms, especially in shift workers or those with irregular schedules, circadian-informed care is becoming more practical. With continued research, development of rapid assessment tools, and support from healthcare systems through circadian-friendly protocols and provider education, the field of Circadian Medicine is now well positioned for the knowledge translation of circadian principles into everyday CV care. The next sections highlight emerging strategies and therapies that build on this foundation.

6.2. Drug Targeting the Circadian Clock for Cardiac Repair

Harnessing small molecules to modulate the circadian mechanism offers a novel strategy for improving cardiac repair, particularly in myocardial ischemia and reperfusion (MmI/R). MI results from blocked blood flow to the heart, causing tissue damage, maladaptive remodeling, and heart failure. While reperfusion therapies restore blood flow, they also trigger reperfusion injury, a damaging inflammatory response that worsens myocardial damage and increases infarct size, a critical predictor of poor outcomes. Addressing reperfusion injury remains a crucial challenge in cardiology. Targeting the circadian clock mechanism provides an innovative solution to this challenge.

The NLRP3 inflammasome, a key mediator of reperfusion injury, represents an attractive but complex therapeutic target. Experiments with cytokine inhibitors (462) or directly targeting the inflammasome (463–471) have shown limited potential in cell culture or animal models due to off-target effects, low specificity, or toxicity. In contrast, circadian regulators like REV-ERB offer a promising and innovative alternative, suppressing NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) production and inflammation at its source with reduced side effects.

Recent studies demonstrate the effectiveness of targeting REV-ERB with SR9009, a small molecule agonist, to mitigate reperfusion injury (107). By inhibiting NLRP3 transcription, SR9009 prevents inflammasome formation and inflammatory damage. SR9009 interacts briefly with REV-ERBα and β isoforms, activating them without disrupting circadian rhythms or causing significant side effects at therapeutic doses. In mI/R rodent models, administering SR9009 during reperfusion significantly reduced infarct size, improved cardiac repair, and preserved function compared to untreated controls (107). These benefits were attributed to decreased inflammation, lower cytokine production, and a shift in macrophage populations toward the reparative M2 phenotype, limiting the pro-inflammatory M1 response. Notably, these therapeutic advantages were absent in REV-ERB KO mice, confirming the specificity of SR9009 for REV-ERB and its critical role in cardiac protection (107).

SR9009’s efficacy demonstrates the transformative potential of circadian-based interventions in cardiology. This approach addresses inflammation at its circadian-regulated source, overcoming the challenges of NLRP3 inhibition and offering a targeted treatment strategy. Preclinical studies reveal its safety and specificity, positioning SR9009 (or derivatives) for clinical translation. With a single dose administered during reperfusion, its therapeutic approach resembles practical, one-time treatments like tissue plasminogen activator (tPA) for strokes.

Ongoing research is expanding circadian modulation to other CV conditions, including cardiac hypertrophy, valve replacement, and potentially chronic heart failure (with preserved ejection fraction [HFpEF] or with reduced ejection fraction [HFrEF]), where there are unmet therapeutic needs. Applications may also encompass the management of reperfusion injuries associated with bypass surgery or transplants. These advancements lay a strong foundation for developing circadian-based therapies to transform CV care.

6.3. Sex and Cardiovascular Circadian Medicine

Advancing circadian medicine in cardiology requires a comprehensive understanding of how sex-based differences influence CV risks, diagnoses, treatments, and outcomes. These differences are critical for developing effective and personalized therapies. Men are more susceptible to HFrEF, while women (particularly postmenopausal women) are at higher risk of HFpEF (472). These disparities stem from distinct remodeling patterns and underlying causes, such as coronary artery disease in men versus hypertension or diabetes in women (473, 474). Recognizing these sex-specific differences is crucial for refining circadian-based interventions to improve patient outcomes.

Experimental studies highlight significant sex-based differences in CV physiology (e.g. Table 1). For example, male knockout mice deficient in the enzyme phosphate cytidylyltransferase 2, ethanolamine (PCYT2) develop metabolic dysfunction along with hypertension and cardiac hypertrophy, while females with the same deficiency exhibit only metabolic dysfunction (475). Additional research highlights sex-related variations in mitochondrial function, calcium handling, and sarcoplasmic reticulum activity, which critically impact CV health (476). Circadian rhythms also influence post-MI recovery in sex-specific ways. For example, male mice recover better when MI occurs during their active (wake) phase, likely due to circadian-regulated metabolic and inflammatory gene responses (106). In contrast, female mice recover more effectively when MI occurs during their rest (sleep) phase (106). These data are consistent with the notion in humans that that hormonal regulation in pre-menopausal women may help protect against heart disease, and further suggest that the circadian mechanism plays a role.

In addition to heart disease, female subjects appear to be protected from hypertension, possibly due to the molecular clock being more adaptive. Solocinski et al. found that male global PER1 KO mice placed on a high-salt diet and administered an aldosterone analog develop non-dipping hypertension (336), while Douma et al. showed that their female counterparts do not (477). Crislip et al. expanded upon the characterization of kidney-specific (KS) BMAL1 KO mice by investigating differences in males and females (478). While male KS-BMAL1 KO mice have low BP compared to their littermate controls, female KS-BMAL1 KO mice do not have a BP phenotype. When placed on a high salt/low potassium diet, male KO mice reabsorbed more sodium while females did not, indicating these BP differences could be due to sex differences in electrolyte handling. Zhang et al. also investigated collecting duct-specific BMAL1 KO in mice and found similar effects, and the same sex difference (342). However, this resilience against certain types of CVD may not be universal in females. Visniauskas et al. found diurnal changes in CV parameters and clock gene expression after Angiotensin II-induced hypertension in both male and female mice (355). This study found that males and females both developed non-dipping hypertension with Ang II treatment, but the underlying mechanisms differed between sexes. For instance, male mice appeared to have suppression of baroreflex sensitivity while female mice did not and instead had disrupted diurnal variation in HR. This was associated with sex-specific changes in diurnal aortic gene expression, including G protein coupled estrogen receptor 1 (Gper1) and estrogen receptor 1 (Esr1). This is in contrast with previous studies which found that female mice are protected against Ang II-induced hypertension, but this was attributed to differences in methodology such as the dose of Ang II administered and housing conditions. This study highlights the relationship between secondary hypertension and the molecular clock and the importance of measuring multiple CV parameters to detect risk and pathology in males and females. There could be CV risk factors in men and women which are missed in clinical settings due to the use of only one BP measurement in the office rather than ambulatory BP monitoring in addition to measurement of other CV parameters.

Cardiac aging similarly reveals sex-dependent distinctions. Male mice with disrupted circadian rhythms experience accelerated aging, including cardiac hypertrophy and left ventricular (LV) dilation, driven by pathways such as PTEN-ATK and the circadian regulator CLOCK (289). Female mice demonstrate greater resilience, potentially due to hormonal protection and unique cardiolipin composition (290). Additionally, Alibhai et al. found that female mice lacking a functional CLOCK protein are resistant to the age-induced cardiomyopathy seen in their male counterparts (290). However, this advantage diminishes following ovariectomy, indicating the significant role of sex hormones in mediating these protective effects (290).

As summarized in Table 1, the current knowledge regarding sex differences in CV clock function are limited and underscore the need for additional research in this area. These findings highlight the need to integrate sex-specific data into circadian-focused strategies to enhance lifelong cardiac health. Historically, CV research has disproportionately focused on male physiology, overlooking critical differences in female response. Addressing these gaps is essential for improving therapeutic precision and developing circadian medicine treatments tailored to the needs of diverse patient populations..

6.4. Circadian Biomarkers and New Technologies: Advancing Precision Medicine for Cardiovascular Health

Chronotherapy holds considerable promise for patients, but its implementation in clinical practice faces challenges, including a lack of clear guidelines, inconsistent study results, and individual variability in circadian rhythms. The disorder being treated may also disrupt normal circadian patterns, further complicating treatment. There are several ways to address this.

Developing circadian biomarkers is essential for advancing precision medicine, particularly in diagnosing and managing CVD. While dim light melatonin onset (DLMO) and morning cortisol offer a general understanding of systemic circadian alignment, they are inadequate for identifying tissue-specific disruptions, including those in the heart (219). The novel development of circadian biomarkers can address this gap, enabling more precise diagnoses, improved prognoses, and treatments tailored to an individual’s circadian biology.

Transcriptomic studies using the murine heart disease model of pressure overload-induced cardiac hypertrophy (TAC) further revealed the enormous potential of uncovering hundreds of rhythmic genes linked to heart disease progression (27, 373, 378). These genes exhibit dynamic day and night patterns as the disease advances, providing valuable insights for staging the condition and monitoring therapy. Validation at the protein level further reveals their potential for diagnostic applications, such as enzyme-linked immunosorbent assay (ELISA)-based arrays that incorporate circadian insights into clinical care (98, 99, 373). Additionally, proteomic analysis of plasma, a minimally invasive medium, has revealed 24-hour rhythmic protein patterns through advanced techniques like mass spectrometry (98, 429). These findings enhance body time assessment and offer practical applications for tracking pathophysiology, optimizing chronotherapies and medical procedures, and identifying circadian misalignment.

Wearable technologies, including various smartwatches and rings, enhance the potential for real-time tracking of circadian biomarkers by continuously monitoring metrics such as HR, temperature, and BP over the 24-hour day and night cycles (479, 480). When combined with artificial intelligence (AI) these devices can analyze circadian patterns to provide personalized insights and enable clinicians to monitor patients and tailor treatments remotely (481). AI’s capability to integrate wearable data with electronic health records and genetic profiles could revolutionize chronotherapy by optimizing medication timing, adjusting schedules, and improving treatment outcomes while minimizing side effects, e.g. (482–484). Although challenges like data privacy and clinical implementation persist, AI-driven solutions are poised to become a fundamental aspect of personalized CV care.

Patient compliance is another concern, as changing medication routines can affect adherence. Excitingly, wearable devices are transforming circadian medicine, offering new solutions like continuous monitoring of physiological parameters such as HR, BP, and sleep patterns and chronotype classification, provides valuable insights into individual circadian rhythms, enabling more personalized treatment (485). Advanced biosensors in smartwatches and fitness trackers now detect arrhythmias, particularly atrial fibrillation, with increasing accuracy, enhancing their clinical utility (486).

6.5. Emerging Research and Innovations in Circadian Medicine

The field of circadian medicine is rapidly evolving, presenting exciting opportunities to improve health, prevent disease, and optimize treatment strategies. By integrating fundamental biological insights with clinical and lifestyle applications, circadian medicine offers a novel approach to enhancing well-being. This section provides an overview of emerging areas from basic mechanisms to clinical applications and practical interventions.

6.5.1. Nascent Molecular and Cellular Mechanisms

Speculative but exciting ideas are emerging at the intersection of circadian biology and cutting-edge molecular technologies. While none of these concepts have yet been experimentally validated, they represent logical extensions of current biological advances that could, in time, open new avenues for understanding circadian mechanisms and translating them into clinical cardiology. For example, epigenetic regulation is increasingly being recognized as a key mediator between environmental influences and long-term disease risk. It is intriguing to consider that circadian misalignment may similarly leave an epigenetic imprint, potentially offering new opportunities for preventative strategies to restore circadian alignment and reduce CVD burden. Likewise, though not yet explored in this context, gene-editing tools such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) (487) may eventually enable correction of inherited circadian rhythm disorders at their molecular origin. Investigating how organ-specific circadian disruption contributes to systemic dysfunction could also reveal novel therapeutic targets for complex, multi-organ diseases driven by misaligned biological clocks.

Emerging experimental platforms such as organoids (488) and induced pluripotent stem cells (iPSC) (489) may offer powerful new platforms to study circadian regulation in human-like tissues, helping to bridge the gap between bench and bedside. Beyond traditional molecular biology, there is growing interest in how magnetic fields might interact with biological clocks, an area that intersects with quantum biology and could shed light on how environmental cues like electromagnetic signals could influence circadian rhythms at the fundamental level (490, 491).

Finally, temperature is gaining attention as a non-photic Zeitgeber (e.g.,(492)). Understanding how thermal cues influence circadian timing and metabolism may lead to novel, non-pharmacological strategies to improve metabolic flexibility and manage cardiometabolic disease.

6.5.2. New Frontiers in Circadian Cardiovascular Physiology and Therapies

A critical challenge in circadian medicine involves distinguishing the specific roles of light (80) versus endogenous circadian mechanisms (90) in regulating CV function. Circadian medicine typically operates within the natural 24-hour light and dark cycle. Clarifying how light exposure interacts with internal circadian rhythms could lead to more precise therapeutic strategies. Integrating external cues, such as light, with the body’s endogenous biological clocks holds promise for optimizing CV health and enhancing treatment efficacy. A particularly novel approach in this field is the concept of “rest” as a therapeutic modality (493). Extending the morning rest period has shown significant CV benefits, particularly in conditions such as cardiac hypertrophy and myocardial infarction. Experimental studies in rodents indicate that a brief daily extension of rest reduces hemodynamic stress, relaxes myofilament contractility, and downregulates cardiac remodeling genes, contributing to the preservation of cardiac structure and function. These rest-responsive pathways are relevant to a broad spectrum of human CV conditions, suggesting that rest could become a critical element of circadian-based treatment strategies. Taken together, these findings suggest that purposeful modulaton of behavioral and environmental rhythms, such as extending rest, may represent a clinically relevant form of controlled circadian misalignment with therapeutic potential. This new frontier suggests that rest, much like diet, sleep, and exercise, could play a pivotal role in maintaining CV health and open new avenues for innovative therapies.

Matrix remodeling, a critical process in heart disease, involves the reorganization of the extracellular matrix during tissue repair and fibrosis (494). In mice it has been shown that time-restricted feeding in a model of obesity can reverse fibrosis and vascular stiffening (197, 218). Also, chronic mis-timed feeding can result in vascular stiffness and renal fibrosis (495). Understanding how circadian rhythms influence this process could improve cardiac repair and prevent maladaptive fibrosis following injury.

The intersection of circadian medicine and immunology also offers new opportunities to treat immune-mediated heart diseases, including myocardial infarction, myocarditis, and atherosclerosis (221). Aligning therapeutic interventions with the immune system’s natural rhythms could enhance treatment efficacy and improve tissue repair processes. Moreover, the gut microbiome is increasingly recognized as a significant player in CV circadian health, as described above. Evidence suggests that synchronizing microbiome rhythms with the host’s circadian clock may offer therapeutic benefits for CV conditions, including enhancing systemic inflammation control.

Circadian biology also plays a vital role in regulating autophagy (223, 496, 497), a cellular process involved in maintaining homeostasis and influencing metabolic and CV health. Given its impact on cellular renewal and stress response, autophagy pathways present as promising therapeutic targets, particularly in diseases marked by metabolic dysregulation and tissue degeneration.

6.5.3. Established Research Areas: Cardiac Metabolism, Vascular Health, and Electrophysiology

Significant progress has been made in understanding how circadian rhythms regulate cardiac metabolism, with pivotal studies from Martin Young’s group and others offering valuable insights into these mechanisms (225, 498, 499). Aligning metabolic processes with circadian cycles has shown potential to regulate cardiac function and improve or potentially mitigate CVD risk. While these findings contribute substantially to our understanding of circadian influences on CV health, they are beyond the detailed scope of this manuscript. Other established areas include the circadian basis of vascular health. Strategies targeting endothelial dysfunction and heart failure through circadian regulation of the vascular system hold promise for improving treatment outcomes (111, 500, 501). Additionally, examining electrophysiology within a circadian context is advancing knowledge on how ion channel dynamics influence arrhythmias and overall cardiac function (84, 115, 116, 502). These domains continue to build a comprehensive picture of how circadian biology shapes CV health.

6.5.4. Lifestyle and Environmental Influences on Circadian Health

Lifestyle and environmental factors significantly influence circadian function and can be strategically leveraged to align daily habits with biological rhythms, reducing disease risk. Chrono-diagnostics, which involves measuring time-dependent biomarkers, may improve early disease detection and optimize treatment timing. This is discussed in more detail in section 7 above. Refining clinical trial methodologies to evaluate chronotherapy and other time-sensitive interventions is critical to ensuring treatments align with the body's natural rhythms (128, 220, 503).

Light exposure is a well-established regulator of circadian health. However, practices such as wearing brown/yellow glasses during the day, which block the blue light needed for alertness, highlight the need for a better understanding of how light modulation affects circadian stability and overall health.

The interaction between cannabinoids, particularly Δ⁹-tetrahydrocannabinol (THC) and cannabidiol (CBD), and circadian biology remains underexplored. As cannabis use becomes more widespread, particularly among aging populations in regions where it is legalized, understanding its impact on sleep, CV health, and circadian stability is increasingly important. Despite CBD’s popularity as a sleep aid, evidence supporting its effectiveness is limited, highlighting the need for rigorous research. Similarly, the widespread use of over-the-counter melatonin and its administration, especially to children, requires careful investigation to ensure safety and efficacy.

Shift work disrupts circadian rhythms, contributing to increased health risks, including metabolic and CV disorders. Developing effective strategies to mitigate circadian misalignment in shift workers is essential for reducing these risks. Maintaining good sleep hygiene, which reinforces healthy circadian rhythms, is a critical component of CV health. Much work is being done on shift work and occupational health, which is beyond the scope of this review. In addition, much more can be done in a CV context. The reader is directed toward several excellent reviews (222, 227, 504–507).

6.5.5. Veterinary and Non-Human Circadian Applications

Circadian research extends beyond human medicine, offering significant potential to improve animal health by aligning treatment schedules with biological rhythms, as has been described in detail (508). Companion animals such as dogs and cats, which are prone to CVD, may benefit from chronotherapy that optimizes medication timing to align with natural circadian cycles. In veterinary hospital settings, regulating light and dark cycles in ICUs could enhance recovery and support the healing process in critically ill animals. Additionally, optimizing lighting environments for farmed animals may not only improve animal welfare and health outcomes but could also reduce the need for antibiotics in the food supply chain, contributing to broader public health benefits. Aquaculture is another area where circadian considerations may enhance food safety and quality. By aligning feeding schedules, light exposure, and environmental conditions with natural circadian rhythms, aquaculture practices could promote healthier growth patterns in aquatic species and improve the quality of seafood for human consumption.

6.5.6. Challenges and Opportunities in Space Exploration and Circadian Health

Looking to the future, space exploration presents a unique challenge for circadian biology. As humanity prepares for long-term space missions and potential planetary colonization, adapting to new light-dark cycles on planets with different rotational periods will be crucial. Understanding how biological clocks respond to non-24-hour day-night cycles could inform strategies to maintain circadian health in extraterrestrial environments.

7. CIRCADIAN MEDICINE AND PUBLIC HEALTH

As discussed above, the CV system follows a circadian rhythm, with BP, HR, and vascular tone cycling across the day-night period. Hormones like cortisol, melatonin, and catecholamines also exhibit circadian patterns, influencing metabolism and CV function. These rhythms reveal windows of vulnerability, such as increased risk of MI and stroke in the early morning. Many medications’ absorption, action, and metabolism fluctuate with the body’s daily cycles, and numerous CV drug targets exhibit rhythmic expression. Consequently, the treatment’s timing can significantly influence its effectiveness and potential side effects. By aligning treatments with these rhythms, practitioners of circadian medicine can optimize therapeutic outcomes and minimize risks. But, paradoxically, the medical application of our burgeoning knowledge has been limited, even peripheral, to mainstream medicine. Incorporating circadian science into medical practice requires a coordinated effort. Furthermore, the hazards of shift work or extended work hours have been extensively studied by many national occupational health and safety organizations, along with recommendations for mitigation. Thus, a potential corporate liability emerges regarding shift work demands or habitually expected long working hours and health. An excellent analogy may be found in the success of the smoking cessation campaign, which successfully combined evidence-based medicine with public health campaigns, physician advocacy, and legislative action. The key is to create a strong evidence base, educate the public and healthcare providers, advocate for supportive policies, and gradually shift cultural norms to value circadian health as a fundamental aspect of overall well-being. For example, corporate policies that reward long hours and red-eye flights should be reevaluated. Public health measures such as shift work management and environmental modifications such as exposure to natural lighting in homes, workplaces, and schools or conversely the minimizing of blue street lighting in urban areas at night are important considerations (509, 510). A detailed discussion of these broad public arenas is beyond the scope of this review.

Regular and sufficient sleep is the cornerstone of our circadian physiology and, as such, critical for our day-to-day and long-term physical and mental health. While asleep, we are protected from influences of Zeitgebers such as light, food, and muscular activity. Currently, and except for insomnia and OSA, patients are rarely interrogated about their sleep profiles, or their day/night and eating schedules. There is some recognition of the risk profile of shift work but little or no recognition that all of us experience analogous disruptions of our regular diurnal schedules - through work, travel and social commitments. In 2022, the American Heart Association formally but belatedly recognized sleep as a crucial health behavior alongside diet, exercise, and smoking cessation (511). Future guidelines are expected to acknowledge circadian biology as equally critical. Indeed, in 2024 the International Association of Circadian Health Clinics was formed with a vision to improve health through the development, assessment, and implementation of evidence based, personalized care for patients with any condition or disease of the circadian system, or associated with disruption of the circadian system.

8. SUMMARY AND CONCLUSION

Endogenous circadian rhythms regulate daily fluctuations in CV function, optimizing physiological processes by aligning CV reactivity with needs based on the anticipated daily patterns of environmental and behavioral cycles. Although controlled laboratory studies in humans demonstrate circadian rhythms of BP, autonomic nervous system activity, blood clotting, vascular tone, and metabolic function, in real life, for all of these variables there is a continuous summation of these circadian effects with effects on CV physiology from the environment, as well as ongoing behaviors and an individual’s psychological state. These circadian, psychological and environmental factors all summate in potentially nonlinear ways to confer advantages in most situations in healthy people. But the circadian effects could contribute to the morning increase in MACE in vulnerable people. Moreover, chronic circadian misalignment resulting from shift work, irregular sleep-wake cycles, and misaligned lifestyle habits is strongly associated with increased CV risk and disease progression. Circadian Medicine is an emerging field with enormous promise, for instance via optimizing light exposure, meal timing, and physical activity to restore biological alignment, or optimally timing medications or procedures at specific circadian phases that enhance treatment efficacy or reduce adverse side effects. By integrating circadian biology into CV medicine, novel strategies can be developed to prevent disease, improve patient outcomes, and enhance therapeutic precision.

CLINICAL HIGHLIGHTS.

• Circadian rhythms, governed by the body’s endogenous clock mechanism, regulate cardiovascular function by orchestrating fluctuations in critical variables like blood pressure, heart rate, vascular tone, and myocardial metabolism to align with the 24-hour day/night cycle. Circadian rhythms optimize cardiac efficiency, synchronize physiology with behavioral wake and activity, and facilitate tissue repair during rest (i.e., nighttime in humans).

• Circadian regulation is a fundamental determinant of cardiovascular health and disease risk. In vulnerable people, the internal circadian system may contribute to the well-documented morning peak in major adverse cardiovascular events, including myocardial infarction and stroke.

• Circadian misalignment caused by night shiftwork, irregular sleep-wake cycles, nighttime light exposure, or disrupted 24-hour behavioral rhythms (including eating at night) is strongly linked to increased cardiovascular disease risk. Laboratory studies demonstrate that circadian disruption impairs cardiovascular homeostasis and blood pressure regulation. Even brief misalignment during early cardiac repair after myocardial infarction can cause lasting damage.

• Circadian Medicine applies biological timing principles to health, leveraging timed physical activity, light exposure, and meals to support circadian rhythmicity. In the special case of the elderly, Circadian Medicine may have special benefits to health. Chronotherapy, i.e., optimizing medication timing or therapies to diurnal rhythms, offers a promising strategy to enhance therapy and reduce cardiovascular risk.

Abbreviations and glossary:

ABPM

ambulatory blood pressure monitoring

ACEI

angiotensin-converting enzyme inhibitor

AI

artificial intelligence

Ang II

angiotensin II

ANS

autonomic nervous system

ASPS

advanced sleep phase syndrome

ARBs

angiotensin receptor blockers

ASPS

advanced sleep phase syndrome

ATP

adenosine triphosphate

BMAL1

brain and muscle ARNT-like 1 (core circadian mechanism protein in humans. rodents, etc.)

BP

blood pressure

bpm

beats per minute

CBD

cannabidiol

CBTmin

core body temperature minimum

CCGs

clock-controlled genes

CHD

coronary heart disease

CK1

casein kinase 1 (core circadian mechanism protein in humans. rodents, etc.)

CK-MB

creatine kinase MB isoenzyme

CLOCK

circadian locomotor output cycles kaput (core circadian mechanism protein in humans. rodents, etc.)

CR

constant routine protocol

Cre

cyclization recombination

CRISPR

gene-editing technology termed Clustered Regularly Interspaced Short Palindromic Repeats

CRY1 and CRY2

cryptochrome proteins (core circadian mechanism proteins in humans. rodents, etc.)

CV

cardiovascular

CVD

cardiovascular disease

DASH

dietary approaches to stop hypertension

DBP, HLF, and TEF

direct circadian mechanism-controlled proteins. D-box binding protein (DBP), hepatic leukemia factor (HLF), thyrotrophic embryonic factor (TEF). Collectively known as PAR bZip transcription factors

DEC2 or BHLHE41

Basic Helix-Loop-Helix Family Member E41

DLMO

dim light melatonin onset

DOCA

deoxycorticosterone acetate

DSPD

delayed sleep phase disorder

ELISA

enzyme-linked immunosorbent assay

eNOS

endothelial nitric oxide synthase

Esr1

estrogen receptor 1

FD

forced desynchrony protocol

FMD

flow-mediated dilatation

GABA

gamma-aminobutyric acid

GP

glycoprotein

Gper1

G protein coupled estrogen receptor 1

HF

high-frequency band of heart rate variability

HFpEF

heart failure with preserved ejection fraction

HFrEF

heart failure with reduced ejection fraction

HR

heart rate

HRV

heart rate variability

hsCRP

high-sensitivity c-reactive protein

Hsd3b6

hydroxy-delta-5-steroid dehydrogenase, 3 beta- and steroid delta-isomerase 6

HUTT

head-up tilt table test

ICU

intensive care unit

IL-6

interleukin-6

ipRGCs

intrinsically photosensitive retinal ganglion cells

iPSC

induced pluripotent stem cell

KChIP2

Kv channel-interacting protein 2

KLF15

Krüppel-like factor 15 (a transcription factor)

KO

knockout (e.g., ko specific genes in a rodent model)

KS

kidney-specific

L:D cycle

Light:dark cycle (usually in reference to an experimentally imposed environmental schedule)

LF/HF

low-frequency to high-frequency ratio from heart rate variability analysis, a marker of sympathovagal balance

LV

left ventricular

MACE

major adverse cardiovascular events

MAP

mean arterial pressure

MI

myocardial infarction

MmI/R

myocardial ischemia and reperfusion

mRNA

messenger ribonucleic acid

MMPs

matrix metalloproteinases, e.g., mmp-2 and mmp-9

NLRP3

NACHT, LRR, and PYD domains-containing protein 3 (a protein expressed predominantly in macrophages and as a component of the inflammasome)

OSA

obstructive sleep apnea

PAI-1

plasminogen activator inhibitor-1

PAR bZip

PAR-domain basic leucine zipper (PAR bZip)

PCYT2

the enzyme phosphate cytidylyltransferase 2, ethanolamine

PER1, PER2, and PER3

period proteins (core circadian mechanism proteins in humans, rodents, etc.)

pNN50

percentage of sequential inter-beat intervals varying by more than 50msec. The proportion of pairs of successive heartbeats that differ by more than 50 milliseconds divided by the total number of heartbeats in the period of interest. This is a measure of the parasympathetic influence on the heart rate

PNS

parasympathetic nervous system

PR interval

PR interval on the electrocardiogram reflects atrioventricular conduction time

PRDM16

PR domain-containing protein 16

PTEN-ATK

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) - v-akt murine thymoma viral oncogene homolog; protein kinase B (AKT)

PVN

paraventricular nucleus of the hypothalamus

QT interval

q wave and t wave on the electrocardiogram. Time between occurrence of the Q wave and the T wave on the electrocardiogram

RAAS

renin-angiotensin aldosterone system

REM

rapid eye movement in sleep

REV-ERB

nuclear receptor subfamily 1, group D member 1 (core circadian mechanism protein in humans. rodents, etc.)

RMSSD

root mean square of successive differences

ROR

retinoic acid receptor-related orphan receptor (core circadian mechanism protein in humans. rodents, etc.)

R

R interval time between successive heartbeats on electrocardiogram

SCG

superior cervical ganglion

SCN

suprachiasmatic nucleus of the hypothalamus

SGK1

serum and glucocorticoid-regulated kinase 1 (also known as serine/threonine-protein kinase

SHR

spontaneously hypertensive rat, commonly used as a genetic animal model of primary hypertension, developed by selective breeding of Wistar rats with high blood pressure

SNS

sympathetic nervous system

SS

salt-sensitive (context: Dahl salt-sensitive (ss) rat)

TAC

transverse aortic constriction (murine model)

TEF

thyrotrophic embryonic factor

TGR (mREN-2)27

transgenic rat model used for studying hypertension and renin-angiotensin aldosterone system dysregulation. Developed by introducing the mouse Ren-2 renin gene leading to overexpression of renin. TGR refers to transgenic rat, and mREN-2 refers to the insertion of the mouse renin-2 gene, and 27 is the transgenic line

THC

Δ⁹-tetrahydrocannabinol

TIMELESS

a gene in Drosophila encoding an essential protein that regulates circadian rhythm

TNF-α

tumor necrosis factor-α

tPA

tissue plasminogen activator

TRE

time-restricted eating

TTFL

transcription-translation feedback loop

VIP

vasoactive intestinal polypeptide

WKY

Wistar-Kyoto rat, an inbred rat strain derived from Wistar rats and selectively bred for normal blood pressure. Commonly used as a control for SHR rat studies

+/tau hamster

heterozygous mutant of the tau mutation in the casein kinase 1 epsilon gene, resulting in a shortened circadian period.

BedMed and BedMed-Frail

Canadian randomized controlled clinical trials investigating whether taking antihypertensive medications at bedtime (vs. morning) improves cardiovascular outcomes, with BedMed-Frail focusing specifically on older adults in long-term care.

Biological day

the segment of the internal circadian rhythm when most people are awake. Contrast with “biological night” and contrast with “day/night pattern”.

Biological morning

the segment of the internal circadian rhythm when most people wake up and go about their waking activities. This coincides with the greatest risk for adverse cardiovascular events.

Biological night

the segment of the internal circadian rhythm when most people sleep (which can be disrupted with, for example, night shift work or jet lag). Contrast with “biological day” and contrast with “day/night pattern”.

CCM mice

cardiomyocyte-specific circadian clock mutant mice.

Chronotherapy

the practice of timing therapy (e.g., medications, light, exercise, meals, surgery) based on time of day, or phase of the internal circadian clock.

Circadian

central circadian pacemaker responsible for neural and endocrine control of circadian rhythms, including synchronizing peripheral clocks in tissues such as the heart, vasculature, and kidneys.

Circadian medicine

approach that uses timing of treatments such as medications, light exposure, meals, and exercise to align the body’s internal clock and optimize health outcomes (contrast with chronotherapy, which could include therapy tied to time of day or phase of the internal circadian clock).

Day/night pattern

diurnal, nocturnal, and nychthemeral describe patterns of behavior based on the day/night cycle. Diurnal means active in the day (e.g. humans). Nocturnal is active at night (e.g., rodents). Nychthemeral covers the full 24 hours. Circadian rhythms differ in that they are internal biological cycles of about 24 hours that persist even without external light cues. Also see “biological night” and “biological morning”.

Dipping and non-dipping blood pressure

blood pressure usually drops at night during sleep, which is a healthy pattern known as dipping. In non-dipping hypertension, this nighttime drop does not occur and is associated with an increased risk of cardiovascular disease.

E-box (or enhancer box)

a DNA response element that regulates gene expression.

Forced desynchrony Protocol

a laboratory method where participants follow a non-24-hour sleep-wake cycle (e.g., 28 hours) to uncouple the body’s internal circadian rhythm from external cues, allowing investigation of the circadian effects on physiology.

Hygia trial

a large, long-term study that found that taking blood pressure medications at bedtime, rather than in the morning, significantly reduced the risk of cardiovascular events and improved blood pressure control in hypertensive patients.

M2 macrophages

alternatively activated macrophages, characterized by anti-inflammatory functions, with roles in tissue remodeling and fibrosis.

MAPEC trial

large clinical study that showed that taking antihypertensive medications at bedtime, rather than on waking, improved blood pressure control and reduced cardiovascular risk.

P-selectin

transmembrane protein that functions as a cell adhesion molecule on the surfaces of activated endothelial cells on the surface of blood vessels and on activated blood platelets.

SR9009

a research drug developed at the Scripps Research Institute as a synthetic agonist of REV-ERB.

TIME trial

the Treatment in Morning versus Evening (TIME) trial was a large UK-based randomized clinical study that found no difference in cardiovascular outcomes between taking antihypertensive medications in the morning or evening.

TIMELESS

core clock gene in the fruit fly (Drosophila melanogaster). Also see PERIOD for humans and rodents.

Zeitgeber

an external cue, such as light, temperature, food, or activity, that helps synchronize an organism's internal circadian mechanism to the 24-hour day and night cycle. From the German term “time giver”.