Hypertension: Causes and Consequences of Circadian Rhythms in Blood Pressure

Abstract

Hypertension is extremely common, affecting nearly one in every two adults globally. Chronic hypertension is the leading modifiable risk factor for cardiovascular disease and premature mortality worldwide. Despite considerable efforts to define mechanisms that underlie hypertension, a potentially major component of the disease - the role of circadian biology - has been relatively overlooked in both preclinical models and humans. Although the presence of daily and circadian patterns have been observed from the level of the genome to the whole organism, the functional and structural impact of biological rhythms, including mechanisms such as circadian misalignment, remains relatively poorly defined. Here, we review the impact of daily rhythms and circadian systems in regulating blood pressure as well as the onset, progression, and consequences of hypertension. There is an emphasis on the impact of circadian biology in relation to vascular disease and end-organ effects that, individually or in combination, contribute to complex phenotypes such as cognitive decline and the loss of cardiac and brain health. Despite effective treatment options for some individuals, control of blood pressure remains inadequate in a substantial portion of the hypertensive population. Greater insight into circadian biology may form a foundation for novel and more widely effective molecular therapies or interventions to help in the prevention, treatment, and management of hypertension and its related pathophysiology.

Introduction

The leading cause of death in the United States across diverse racial and ethnic groups from 2000-2019 was cardiovascular disease.¹ As a leading risk factor for cardiovascular and neurovascular disease, hypertension remains a global pandemic that is associated with increased risk for a variety of clinical events,^2-4 but also clinically silent processes that include some of the most complex phenotypes (e.g., cognitive deficits and dementias) (Figure 1).^5-8 While discussion related to hypertension commonly refers to chronic changes in blood pressure (BP), acute hypertension also has important pathophysiological consequences.⁹

Figure 1.

Schematic illustrating effects of hypertension and some key components (e.g., MAP, dipping status, BP variability) on lumen diameter of large and small blood vessels, along with changes in capillaries (e.g., rarefaction). Also shown are major modifiers that interact in determining changes in BP and the vasculature. Collectively, such mechanisms contribute to a progressive loss of vascular, brain, cardiac and renal health.

Circadian mechanisms influence many cellular processes and biological systems, including the cardiovascular system. One of the most studied 24-hr rhythms is the daily fluctuation in systemic arterial BP. This review focuses on areas of health and disease that remain relatively poorly defined – daily and circadian phenotypes and mechanisms that influence the vasculature and BP under normal conditions and during hypertension. We summarize concepts related to hypertension, circadian systems, circadian or diurnal phenotypes, BP patterns in preclinical models and humans, end-organ changes, and interactions such as those at the level of the heart and brain. As part of its impact, hypertension is the leading modifiable risk factor for cerebrovascular disease and loss of brain function. Because several reviews within this Compendium emphasize aspects of cerebrovascular biology, we discuss the impact of hypertension in relation to brain health within the context of 24-hr rhythms.

Global impact of hypertension

Chronic hypertension is extremely common, affecting nearly one in every two adults based on the most recent guidelines from the American Heart Association.^2,10 Hypertension is the greatest modifiable risk factor for vascular disease and subsequent clinical outcomes or events. As a consequence, hypertension is linked with enormous negative effects in multiple cell types and organs.^2,11-13 Changes associated with hypertension include endothelial dysfunction, atherosclerosis, thrombosis, modification of the extracellular matrix, increased vascular stiffness, inward vascular remodeling, organ hypoperfusion, aortic dissection, myocardial infarction, heart failure, atrial fibrillation, and chronic renal disease.^10,13-23

In relation to the brain, hypertension is a risk factor for carotid artery and intracranial atherosclerosis, dysregulation of cerebral blood flow (CBF), and classic ischemic strokes.^24-26 Cerebral small vessel disease (SVD) - a key cause of reduced CBF, microinfarcts, microbleeds, blood-brain barrier (BBB) dysfunction, and abnormalities of the perivascular space - is also driven by hypertension.^5,14,25-29 Both large vessel disease and SVD promote the vascular component reduced brain health: loss of cognitive reserve, cognitive impairment, dementias, and neurodegeneration (Figure 1).^5,7,14,25-30 Susceptibility to develop hypertension, vascular disease, and associated end-organ and end-cellular effects are contributors to differences in relative risk and specific presentations within diverse populations.²⁹

In recent years, hypertension has been the leading cause of death and disability-adjusted life years in both men and women.^10,12 The presence of hypertension interacts or synergizes with other risk factors for cardiovascular disease including aging, genetic variation, metabolic disease (e.g., diabetes), diet (e.g., salt-intake), behavior (e.g., tobacco smoking, physical activity), and the exposome (e.g., air or noise pollution) (Figure 1).^10,12 The impact of hypertension can increase even further when combined with other comorbidities, an example being the recent waves of coronavirus disease 2019.³¹

Circadian timing system

Circadian mechanisms contribute to virtually all aspects of physiology and behavior. One of the first studies revealing the existence of circadian rhythms (i.e., ~24-hr biological rhythm generated by an organism even in the absence of external time cues) was performed by the French physicist Jean-Jacques d'Ortous de Mairan in the early 18^th century. By placing heliotrope plants in constant dim light conditions, he observed that the day/night rhythm in leaf movement was not dependent on the light/dark cycle.³² Studies in 1938 by Nathaniel Kleitman in Mammoth Cave, Kentucky showed that humans generate 24-hr rhythms in core body temperature when living on a 28-hr sleep/wake cycle, demonstrating that humans are influenced robustly by an intrinsic circadian system.³³ However, it took until the early 1970s to identify the suprachiasmatic nucleus (SCN) in the hypothalamus as the central circadian pacemaker functioning to orchestrate circadian rhythms.³⁴ The SCN projects extensively to other parts of the hypothalamus and beyond and thereby regulates many bodily functions through autonomic and neuroendocrine mechanisms.^35,36. Early genetic studies uncovered one molecular transcription-translation feedback loop (TTFL) consisting of circadian genes that enabled cell-autonomous 24-hr clock function.³⁷ Core clock genes include Brain and Muscle ARNT-Like 1 (BMAL1), Circadian Locomotor Output Cycles Kaput (CLOCK), Neuronal PAS domain protein 2 (NPAS2), three Period genes (PER1-3), and two Cryptochrome genes (CRY1-2), where the protein products of BMAL1 heterodimerize with those of CLOCK (or its paralog NPAS2) and together stimulate the transcription of the PERs and CRYs as well as clock-controlled genes (CCG). The protein products of the PER and CRY genes accumulate and when reaching a critical concentration inhibit the transcriptional activation by BMAL1-CLOCK of their own transcription (Figure 2). Once PER and CRY concentrations have dropped, the cycle can start again.³⁷ This cycle normally takes approximately 24 hrs to complete. In addition to this core loop, two nuclear receptor subfamilies, the REV-ERBs and the retinoic acid receptor-related orphan receptors (RORs), function as critical components of the circadian clock.³⁷ The 2017 Nobel Prize for Physiology or Medicine recognized the significance of the discovery of the fundamental TTFL mechanism underlying circadian rhythm generation. In the 1990s, the model of the circadian system changed dramatically when it became clear that the circadian system is not merely composed of one central circadian pacemaker in the SCN but that virtually every organ, tissue, and cell contains the same molecular machinery and enables cell-autonomous circadian rhythm generation to persist in vitro.³⁸ Virtually all cell types, including fibroblasts, adipose, vascular and cardiac cells, tick along at their inherent ~24-hr rate in vitro under constant conditions, resulting in an endogenous circadian rhythm in cell function .^39-41 Therefore, the circadian system is composed of the central circadian pacemaker in the SCN and ‘peripheral oscillators’ in organs, tissues, and cells throughout the body, including those involved in cardiovascular regulation.⁴² The SCN synchronizes the peripheral oscillators and thus, conceptually, acts as the conductor of the various musicians in a mechanistic orchestra. In addition to the TTFL consisting of clock genes that sustain cell-autonomous oscillations, clock genes regulate so-called clock-controlled genes (CCG’s) that then regulate cell metabolism and function. These act as the ‘hands of the clock’ and thereby drive 24-hr rhythms in cell, tissue, organ, and organismal function. The extent of circadian control is large. For example, within an individual tissue or organ, up to 16% of all genes exhibit 24-hr rhythmicity.⁴³

Figure 2.

Schematic illustrating effects and interactions of circadian/diurnal effects, environmental and behavioral variables, and genetics on what collectively influences BP during health (left side) and disease (e.g., hypertension) (right side). The dotted line on the right panel indicates a morning surge of greater magnitude. ANS = autonomic nervous system, including sympathetic nerves. For other details and abbreviations, see the main text.

Daily versus circadian rhythms

Daily rhythms in BP are the combined result of both endogenous circadian mechanisms, as well as 24-hr environmental and behavioral variations, such as the sleep/wake, rest/activity, fasting/eating, dark/light, and postural, mental stress, and environmental cycles (Figure 2).⁴² Behaviors can have substantial effects on biological variables, e.g., heart rate increases acutely with exercise. In disease states, 24-hr variations in medication use and disease-specific effects (e.g., sleep disorders) can further modify the daily BP rhythm. Especially in humans, the timing of behavioral activities (e.g., exercise, eating) is often based on work, school, and social schedules, as well as volitional control, instead of only being driven by a circadian system. Therefore, daily rhythms can differ depending on the timing of various behavioral activities, even if their endogenous circadian timing system remains unchanged. Therefore, measuring daily rhythms during everyday life under ambulatory conditions, i.e., including sleep/wake, rest/activity, and fasting/eating cycles are unable to quantify the relative contribution of the circadian timing system. To measure endogenous circadian rhythms independent of such ‘evoked’ or ‘masking’ effects by environmental and behavioral variations, specialized circadian protocols are required. The Constant Routine protocol⁴⁴ and the Forced Desynchrony protocol⁴⁵ are gold-standards designed to isolate the influence of the circadian timing system from such masking effects by behavioral and environmental cycles. In a Constant Routine protocol, individuals maintain continuous wakefulness, physical inactivity, and constant body posture while remaining in constant room temperature and dim light conditions (typically <10 lux) for at least 24 hr (typically at least 30 hr to allow for dissipation of transients at the start and end) to prevent any effects of light on the circadian system. Furthermore, participants eat identical snacks at uniform intervals (e.g., every hr). Hereby, any 24-hr rhythmicity in environmental or behavioral influence is removed, enabling uncovering endogenous circadian control of physiological function, including BP.⁴⁶ In a Forced Desynchrony protocol, a highly-controlled version of the protocol developed by Nathaniel Kleitman (see above), participants live on a non-24 hr sleep/wake cycle that is outside of the range of entrainment of the central circadian clock, e.g., 20 or 28 hr, and while maintaining constant room temperature and dim light conditions. All behaviors associated with the sleep/wake cycle are also carefully scheduled to the same imposed cycle length, including the eating/fasting, rest/activity, and postural cycles. Because the central circadian clock cannot entrain to these extreme sleep/wake cycles under dim light, this protocol allows careful dissociation between effects on physiology, including BP, of the central circadian clock and behavioral influences.⁴⁶

The influence of the circadian system and of environmental/behavioral factors may not simply summate but may interact in non-additive and complex ways. The Forced Desynchrony protocol has been used to determine interaction between behavioral and environmental factors with the circadian cycle, e.g., testing whether the effect of standardized exercise,^47,48 passive head-up tilt,⁴⁹ or mental stress,⁵⁰ has a different effect on cardiovascular markers, including BP, dependent on the endogenous circadian phase of exposure.

In healthy humans under regular living conditions, systemic arterial BP varies across the sleep/wake cycle, exhibiting a peak during the waking day and lower values during nighttime sleep. The magnitude of reduction in BP that normally occurs during nocturnal sleep and physical inactivity is approximately 10-20%.^51-53 Individuals that exhibit such a reduction in BP are classified as BP ‘dippers’.^52,54-56 A reduction in BP of less than 10% is termed a ‘non-dipper’ and individuals that exhibit an increase in BP at night are termed ‘reverse-dippers’ (Figure 2).^54-57 The nighttime reduction in BP is typically followed by a rise in BP during the early morning hours and after waking, a change often described as the ‘morning surge’ in BP (Figure 2).^{51,52,54,58-61} As discussed below, and depending upon the magnitude, the morning surge in BP might be viewed as a period of acute hypertension and associated cardiovascular risk.

Changes in the daily rhythms in BP (e.g., non-dipping, reverse-dipping, morning surge) have been linked to an increased risk for serious adverse cardiovascular and cerebrovascular events. Thus, a better understanding of underlying circadian, behavioral, or environmental mechanisms as well as the consequences of BP variation in normotensive and hypertensive individuals may help in the prevention, treatment, or management of hypertension and its related pathophysiology. The effects of shift work, circadian misalignment, social jet lag, and genetic variants/mutations that affect daily and circadian patterns provide further evidence of the importance of circadian systems and their disruption in relation to BP regulation and cardiovascular disease.

Shift work, circadian misalignment, and social jet lag

Shift work that is associated with circadian misalignment has been associated with increased BP and hypertension, and with reduced BP dipping, increased frequency of non-dipping, and increased risk for cardiovascular events.^62-64 The magnitude of these changes can be greater in Black individuals relative to White individuals.⁶² These associations could not be fully explained by factors such as socioeconomic status, lifestyle, or workload. Studies comparing the same individual in night work and day work, can prevent confounding by the healthy worker survivor effect and systematic, unadjusted inter-individual factors between night workers and day workers. Such a study in a real-life shift work condition has shown increased BP during and following 12-hr night work compared to day work.⁶⁵ Shift work is typically associated with misalignment between the behavioral sleep/wake cycle (and other associated behaviors) vs. the central circadian clock. This raises the question whether such circadian misalignment may be causal in the increased risk for elevated BP or if other differences between day and night shifts explain the increased BP. Highly controlled laboratory studies can further control for other potential confounding factors, such as environmental temperature and lighting, physical activity, the duration of sleep opportunity, body posture, diet, etc. Using a Forced Desynchrony protocol, it was shown that a gradual 12-hr inversion of the behavioral cycle in dim light (i.e., sleeping during the day and awake and eating during the night), caused circadian misalignment and resulted in an increase in average waking MAP by ~3 mmHg as compared to when the same participants were normally aligned.⁶⁶ Another highly-controlled study was a randomized, crossover protocol to study the effect of simulated night work compared to day work after carefully inverting all timed behaviors by 12 hrs during a transitional ‘slam shift’ day that more closely simulated shift work conditions. In this study, circadian misalignment during simulated night work increased average 24-hr SBP and DBP by 3.0 and 1.5 mmHg, respectively, as compared to circadian alignment during simulated day work.⁶⁷ This increase was primarily caused by an increase in the sleep-associated BP, and caused a blunting of the BP dip. Even in real-life chronic night workers, the experimentally-induced circadian misalignment resulted in an increase in 24-hr average SBP and DBP by 1.4 mmHg and 0.8 mmHg.⁶⁸ A recent randomized crossover trial participants’ morning BP surge was significantly higher while experiencing social jet lag (i.e., later midpoint of bedtime on weekend compared to weekday) as compared to that where the sleep/wake cycle was kept constant.⁶⁹ This was further associated with augmented carotid-femoral pulse wave velocity, suggesting a potential involvement of increased central arterial stiffness. The effect of mutations and variants of core clock genes and other genes that influence the daily BP rhythm are discussed below, grouped by end-organ function. Together, these results support the notion that circadian disruption increases BP and the risk for hypertension.

The magnitude of change in BP can vary depending on the population studied and the experimental conditions. In some cases, the noted changes in BP are relatively small, raising questions regarding their potential physiological significance. In this context, it is important to note that substantial data now suggest that modest, but population-wide differences in BP, are linked with meaningful changes (gains or losses) in cardiovascular disease and events. For example, a 5 mmHg reduction in SBP lowered the risk of developing major cardiovascular events by 10%.⁷⁰ In a study that compared ethnicities, a modest population-wide reduction in SBP of only 1 mmHg prevented a substantial numbers of cardiovascular events.⁷¹ This estimated benefit was greater for Black people compared to White people.⁷¹

Blood pressure patterns in humans

Changes in BP across the day and night have been a frequently studied cardiovascular variable. This includes diurnal patterns in BP that vary depending on inter- and intra-individual differences in health status, ethnicity, and genetics, along with behavioral/environmental factors, and circadian misalignment (Figure 2).^42,61,72 Such variables can be associated with alterations in the normal amplitude or timing of the diurnal pattern in BP (Figure 2).

In normotensive humans, one study directly examined the hypothesis that an endogenous circadian mechanism influences BP.⁴⁶ Those experiments showed that the endogenous circadian system influenced BP, thus causing a BP rhythm that persisted even in the absence of 24-hr sleep/wake, rest/activity, postural changes, fasting/eating, and dark/light cycles. Across three complementary gold-standard circadian protocols (a Constant Routine protocol and two independent Forced Desynchrony protocols), the endogenous circadian rhythm in SBP and DBP consistently showed the highest levels at a circadian phase equivalent to about 9 pm and the lowest levels occurring in the morning.⁴⁶ Mechanistically, this BP rhythm appeared to be unrelated to changes in a number of factors involved in BP regulation, i.e., urinary and plasma catecholamines, vagal modulation, urinary excretion rate, plasma cortisol, or heart rate, each of which expressed a clear endogenous circadian rhythm, but with different timing than that in BP.⁴⁶ These findings suggest that the morning surge in BP is unlikely to be due to an endogenous circadian mechanism that influences BP.⁴⁶ It should be noted that these studies were performed in healthy, non-hypertensive, unmedicated, young adults. A Forced Desynchrony protocol with an imposed 10 hr and 40 min sleep/wake cycle also showed an endogenous circadian rhythm in BP with a peak around 4-6 pm.⁷³ A small-scale preliminary study using a similar short-cycle Forced Desynchrony protocol suggested that the endogenous circadian control of BP differs between individuals with and without obstructive sleep apnea, with almost inverted circadian rhythmicity.⁷⁴ Further studies are needed to test the existence, timing, and amplitude in different populations, depending on BP status, comorbidities, age, sex, race and ethnicity.

Based on recordings of SBP, slightly more than half of individuals with hypertension were dippers at baseline, depending on the exact definition used.^75,76 Approximately one-fourth to one-third were non-dippers, with less than 10% being reverse dippers.^75,76 The percentage of dippers declined with age, while the percentage of non-dippers and reverse dippers increased with age.⁷⁶ In the general population, up to 20% of individuals may exhibit isolated nocturnal hypertension.⁷⁷ Loss of BP dipping was also seen with chronic kidney disease.⁷⁷ Nocturnal hypertension was the strongest predictor of cardiovascular events (compared to 24-hr or daytime BP), with reverse dippers being the individuals with the worse prognosis or greatest risk.⁷⁷ Nighttime BP was a stronger predictor of cardiovascular events in women than in men, even though the absolute risk associated with increased BP was lower in women.⁷⁸ Recent guidelines for diagnosis and management of hypertension encourage increased use of 24-hour ABPM or home BP monitoring, including in both the morning and evening.^55,79

In healthy humans, the influence of the sympathetic nervous system on vascular resistance and peripheral blood flow is higher in the morning than in the afternoon or evening, an influence that may contribute to daily increases in BP in the morning.⁸⁰ Individuals with Turner syndrome (females with only one X chromosome) exhibit early onset hypertension, perhaps due to vascular disease or increased activation of sympathetic nerves.⁸¹ In relation to BP, most girls with Turner syndrome are non-dippers.⁸¹. In young and middle-aged women, the risk of aortic dissection increases approximately 100-fold with Turner syndrome.⁸¹

Mechanisms that account for variations in the dipping pattern in BP have yet to be completely defined, but are thought to be multifactorial, potentially including changes in activity of the autonomic nervous system (both sympathetic and parasympathetic activity), baroreflex sensitivity, endothelial cell function, sodium sensitivity, aortic stiffness, and activity of the RAAS (Figure 2).^{52,60,62,80,82-87} The absence or presence of non-dipping can also vary with ethnicity, being relatively higher in Black than White people.^53,62,82,88 In such a case, these variations may reflect ethnic differences in salt-sensitivity and endothelial dysfunction.^62,82 Male and female Black people may have the greatest prevalence of non-dipping BP relative to other ethnic groups.⁶¹

Increasing evidence suggests that immune-related mechanisms are an important contributor to the pathophysiology of hypertension.⁸⁹ Despite these advances, relatively little is known regarding the relationship between circadian mechanisms and hypertension.^89,90 Plasma levels of interleukin-18 and -37 are both increased in subjects with hypertension, but only levels of IL-18 were linked to non-dipping of BP.^91,92 Individuals with systemic lupus erythematosus exhibited significant increases in BP and several markers of immune activation. These subjects also displayed less BP dipping and higher levels of nocturnal hypertension.⁹³ Thus, although studies in this area are limited, contributions of immune-related mechanisms to changes in BP and dipping patterns may be present during some forms of hypertension.

End-organ effects of blood pressure profiles

Morning BP surge and strokes.

Ischemic and hemorrhagic strokes, as well as myocardial infarctions, have in increased incidence between 0600 and 1400 hours.^{27,51,83,94,95} Aortic dissection and rupture of aortic aneurysms also occur more commonly in the morning hours.¹⁶ This timing has been linked and may be causally related to the morning surge in BP.^27,52 The magnitude of the morning surge can vary substantially among individuals with hypertension, particularly in the elderly.^52,94 Those with a greater morning surge had a significantly higher incidence of ischemic stroke (independent of 24-hr BP values), suggesting that the rapid increase in BP in the morning has a greater relative impact.^52,94 The magnitude of this period of acute hypertension can be substantial. In one study, a difference in SBP of approximately 70 mmHg was observed.⁹⁴ Early morning values for SBP were also positively correlated with the presence of cerebral microbleeds.⁹⁶

Mechanisms underlying the morning BP surge.

Some insight has been gained in relation to the physiological basis of the morning surge. For example, the diurnal rhythm in BP of healthy normotensive males was compared between a 24-hr ambulatory recording and a 24-hr recording during a 6° head-down tilt during bedrest in the same individuals to study the influence of physical activity and body posture. The sleep-related BP decrease was not statistically different between both conditions, indicating that the day/night difference was not entirely explained by the rhythm in physical activity or posture, but by other factors such as the sleep/wake, fasting/eating, or endogenous circadian cycle. On the other hand, this comparison showed that the morning surge in BP was primarily caused by physical activity and/or assuming an upright body posture.⁹⁷ It has been suggested that multiple mechanisms that increase BP may converge or synchronize, resulting in large increases in BP. These episodes of acute hypertension may underlie the greater risk for cardiovascular events.⁵² Even with ABPM, it may be difficult to fully quantify the extent of the morning surge. In some studies, peak levels of SBP have been linked more closely to stroke risk than levels of MAP.⁵² Increased stiffness of aorta and other large conduit arteries are thought to increase the risk for a larger morning surge and increased BP variability.⁵²

Clinical relevance of nighttime BP.

Other data raise the possibility that associations with cardiovascular events are independent of daytime BP and changes in morning BP.^{54,58,61,83,98} These data suggest BP at night was relatively more predictive of all cause death and cardiovascular death compared to other times of the day.^53,79,99,100 For example, nighttime SBP measured with ABPM was approximately 6-times more predictive of death than SBP obtained using standard clinical measurements.⁹⁹ Higher nighttime SBP or attenuated nocturnal dipping has also been associated with the presence of cerebral microbleeds (a feature of SVD) in hypertensive humans.¹⁰¹ Overall, the presence of hypertension at night may be more harmful than hypertension during daylight hrs.^52,58,79,88

Circadian disruption and atherosclerosis.

Changes in BP patterns associated with end-organ effects include atherosclerosis, increase vascular stiffness, left ventricular hypertrophy, cardiac events, and all-cause mortality.⁷⁷ The genetic combination of Bmal1 and ApoE deficiency resulted in increased formation of atherosclerosis through mechanisms that may involve oxidative stress, formation of proinflammatory cytokines (e.g., IL-6, IL1ß), and enhanced NF-κB signaling.¹⁰² Disruption of the circadian pattern using a model of jet-lag in ApoE deficient mice also increased formation of atherosclerotic lesions.¹⁰²

In relation to humans with mild to moderate hypertension, 24 hr SBP was associated with an index of carotid artery atherosclerosis (carotid intima-media thickness).¹⁰³ Based on a meta-analysis of multiple studies that included normotensive and hypertensive individuals,¹⁰⁴ carotid atherosclerosis was greater in non-dipping individuals compared to individuals who exhibited normal BP dipping. Findings such as these suggest that the most effective approach to limit the onset or progression of atherosclerosis may require effective control of BP during both day and night.¹⁰⁴

Common mechanisms underlying non-dipping and morning BP surge.

Because a lack of dipping in BP at night may contribute to higher levels of end-organ damage,^52,60,88 BP monitoring at night has been proposed as a new therapeutic priority.⁸⁸ In another study of hypertensive subjects, about 25% were non-dippers, with an even higher percentage in Black individuals.⁸⁸ Some of these same mechanisms likely contribute to both the morning surge in BP and changes in BP dipping. These include the activity of the sympathetic nervous system and the RAAS, endothelial dysfunction, platelet activation, thrombus formation, changes in vascular stiffness and circulating vasoconstrictors, such as Ang II.^{16,52,60,83,105}

Pregnancy and BP dipping.

Hypertensive disorders of pregnancy have been associated with dementia, particularly vascular dementia.¹⁰⁶ A strong association has been reported between attenuated dipping of BP at night during the second trimester and subsequent development of preeclampsia in the third trimester.¹⁰⁷

Blood pressure patterns in preclinical models

The purpose of the following sections is to highlight key differences in BP phenotypes and patterns, the influence of genetics on BP, and quantification of BP in commonly used preclinical models of BP regulation and hypertension (e.g., mice and rats).

Nocturnal vs. diurnal rhythms.

Mice and rats are the most commonly studied non-human species in cardiovascular research.^20,108 Because these rodents are nocturnal, the 24-hr pattern of BP is reversed compared to that in humans, such that their BP is highest during the dark (active) phase and lowest during the light (inactive) phase (Figure 3).^{58,60,109,110 111}

Figure 3.

A conceptual figure illustrating key differences in timing of BP changes during the day and night in humans versus nocturnal rodents (e.g., mice, rats). In the lower Disease panels, the dotted line is included for comparison and indicates the BP pattern seen under healthy conditions in the two upper panels.

Food timing.

The timing of food intake can also influence BP rhythms in mice, an influence that is independent of expression of the clock gene Bmal1, and may be mediated primarily by acute effects of food intake behavior on BP.¹¹⁰

Circadian genetic rodent models.

Multiple mouse models with alterations in core clock genes have been developed, providing direct evidence regarding the functional importance of clock genes in the regulation of BP and select end-organ effects.⁶⁰ Most models in this area have used global (i.e., whole-body) genetic deficiency, but cell-specific genetic manipulations have been used in more recent work. Every clock gene deficient or clock gene mutation model has exhibited a BP phenotype. This includes mice deficient in expression of Bmal1, Clock, Cry1, Cry2, Per1 and Per2.⁶⁰ For example, global deficiency in Bmal1 (Bmal1^−/−) resulted in reduced BP and loss of the diurnal BP rhythm present in wild-type mice.^59,60 Cell-specific deletion of Bmal1 in smooth muscle also reduced BP, while deletion of Bmal1 in cardiomyocytes had no effect on BP.^59,112 Compared to controls, mice with endothelial-specific knockdown of Bmal1 had lower BP during portions of the dark period.¹¹³ In Per1^−/− mice, baseline MAP was similar to controls during the day, but the increase in MAP during the dark phase was greater relative to Per1^+/+ mice.¹¹⁴ When treated with a high-salt diet and an aldosterone analog, Per1^−/− mice exhibited an attenuated dipping phenotype during the light period, suggesting an important influence of Per1 in a model of salt-sensitive hypertension.¹¹⁴

BP measurement techniques.

Mice and rats are also the most commonly used models in hypertension-related research.^20,108 There are several issues related to studies of hypertension in these species. Quantification of BP is scientifically more rigorous when obtained using radiotelemetry to obtain 24-hr recordings in awake, freely-moving individuals with normal activity. While this approach has some limitations, it provides a more complete data set while minimizing restraint, handling, and overall stress.^23,115,116 For these reasons, the use of radiotelemetry is considered the gold standard for the field.^{61,83,115,116}

The primary alternative to radiotelemetry in rodents is tail-cuff plethysmography to record BP. While use of this method is common, it has limitations that include the need for handling, proximity of an investigator, and physical restraint, the latter exerting a substantial effect on BP.¹¹⁶ Tail-cuff plethysmography is less accurate, less reproducible, and detecting modest increases in BP can be more difficult.¹¹⁶ Using this method only during the light period has been the most common approach. Reasons for relying on tail-cuff plethysmography likely include cost, ease of measurement, and greater through-put potential. However, data obtained can sometimes support an inaccurate conclusion (see below). Just as in humans with ABPM, round the clock monitoring in preclinical models provides greater insight into BP phenotypes, including potential daily and circadian rhythms.

Hypertension models.

Sprague-Dawley, Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) all exhibit daily variations in BP. The acrophase for SBP and DBP occurred during the dark period (between 0130 and 0300 hrs).^51,117 In contrast, a renin transgenic rat [TGR(mRen-2)27] exhibited reverse dipping, with peak values for BP during the light period (between 1100 and 1200 hrs).^51,117 While both SHR and TGR(mRen-2)27 are hypertensive, reverse dipping is only seen in the latter strain, supporting the concept that the RAAS plays a role in reverse dipping.^51,117 Although there are many models of hypertension,²⁰ we comment on a few examples that may have relevance human hypertension.

Endothelium-derived NO has pleiotropic roles in the circulation, including regulation of vascular tone and blood pressure. The primary molecular target of NO is guanylate cyclase, which produces cGMP-dependent signaling upon activation.^17,118 Under basal conditions, guanylate cyclase exhibits daily rhythms in activity in rat aorta,¹¹⁹ with maximal production of cGMP during the light/rest period.¹¹⁹ Both male and female mice genetically deficient in endothelial NO synthase (Nos3 ^−/−) are hypertensive during the day and night.^120,121 The magnitude of hypertension is similar to that observed in humans with loss-of-function genetic variants in NOS3.¹²²

Some mouse models exhibit hypertension limited to the active/dark phase. For example, BP in heterozygous Nos3 ^+/− mice was increased at night,¹²⁰ but not in the morning compared to wild type, and measured with tail-cuff plethysmography.¹²³ Other examples of nocturnal hypertension include: (a) mice expressing a human dominant negative mutation in peroxisome proliferator-activated receptor-gamma (Pparg) in smooth muscle, ¹²⁴ (b) transient receptor potential vanilloid type 1 deficient mice (Trpv1^−/−) fed a Western diet or a high-salt diet,^125,126; or (c) mice fed a high fructose diet.¹²⁷ In contrast to nocturnal hypertension, increases in BP were present during the light/rest phase in activin receptor-like kinase 1 (Alk1^+/−) deficient mice.¹²⁸ This change is similar to the non-dipper phenotype in humans (Figure 3). Mechanisms that underlie this phenotype in Alk1^+/− mice may involve activation of the central RAAS and the sympathetic nervous system.¹²⁸

Chronic systemic infusion of Ang II has been a very commonly used model of hypertension.¹⁰⁸ Although infusion of Ang II increased BP during the night and day in both male and female rats and male mice, the night/day difference disappeared.^109,129 During constant dark conditions, 24-hr average BP was higher in Per2 ^−/− mice compared to wild type, due to non-dipping during the subjective light/inactive phase.¹³⁰ Associated with this phenotype, Ang II produced a greater increase in wall thickness of the aorta in Per2 ^−/− mice.¹³⁰ Thus, chronic infusion of Ang II in Per2 ^−/− mice caused non-dipping hypertension and what appeared to be greater hypertrophy in the aorta.

Increased dietary salt intake is a risk factor for human hypertension, in salt-sensitive individuals or ethnicities.^10,131,132 One of the most commonly used preclinical models is the C57BL/6 mouse. A recent Scientific Statement from the American Heart Association concluded the C57BL/6 mouse is not salt-sensitive.²⁰ C57BL/6 mice have often been described as being salt-resistant in relation to BP.^133-135 In contrast, recent studies using male C57BL/6 mice fed a high-salt diet found that BP was significantly increased, but only during the dark phase. This may help explain why previous studies had not observed an increase in BP, if measurements were only taken during the light phase. The BP increase with high salt intake was unrelated to physical activity and was greater in C57BL/6J compared to C57BL/6N mice.¹³³ When tail-cuff plethysmography was used, a difference in BP between light and dark periods was observed, but effects of high-salt on BP was not detected, even when recordings were made during the dark phase,¹³³ revealing an example of an apparently inaccurate conclusion with this technique. Other studies have reported similar findings.¹³⁴ Nocturnal increases in BP with high-salt consumption appear to be due to increased activity of the sympathetic nervous system.¹³⁴

In summary, these findings strongly suggest that for experimental studies or clinical trials, obtaining measurements of BP at only one time of day may result in incomplete or possibly misinterpreted data. This can be an issue in hypertensive humans or when studying species that are naturally nocturnal.^133,136 Considering daily and circadian influences on absolute BP levels, BP variability, and changes in BP dipping, both diagnosis and optimal management of hypertension may likely involve similar issues.¹³⁷ Like the use of chronic ABPM in humans, the use of 24-hour radiotelemetry in rodents has been underutilized.^55,60,79

Heart disease and cardiac dementia

Hypertension is a risk factor for coronary endothelial dysfunction, atherosclerosis, increased vascular and cardiac wall thickening, thrombosis, myocardial infarction, heart failure, and atrial fibrillation.^{10,13-16,18-23,89,138,139} These major cardiac diseases may be influenced by circadian mechanisms.^15,98,140

Myocardial infarction.

In relation to daily rhythms, myocardial infarction occurs more commonly between 0600 and 1200-1400 hours, compared to other portions of the day.^{27,51,83,94,95} Ischemic heart disease is associated with variants in circadian clock genes and triggers for myocardial infarction (e.g., endothelial dysfunction, platelet activation, thrombosis, and changes in factors such as plasma cortisol).^95,105 Both endothelial cell dysfunction and increased risk for myocardial infarction have been associated with non-dipping of BP.¹⁰⁵ Nocturnally elevated BP was positively associated with cardiac events, particularly in women.⁷⁸ Some data suggest BP levels at night and loss of BP dipping may be a stronger predictor of myocardial infarction compared to daytime or 24-hr BP.^83,105

Heart failure.

Cardiac physiology and contractility exhibit diurnal rhythms that can be partly or entirely due to circadian mechanisms.¹⁴¹ Multiple lines of evidence suggest alterations in the circadian biology contribute to the pathogenesis of heart failure.¹⁴¹ Human hearts express clock genes, several of which display daily rhythms in expression.¹⁴² Altered patterns of expression of circadian clock components occur in human hearts with cardiomyopathy.¹⁴³ In failing human hearts, changes in clock gene expression and the daily pattern of expression of several ion channels suggests potential links to cardiac arrhythmia’s and sudden death.¹⁴⁴ Heart failure has been linked to abnormal diurnal patterns in BP,^140,145 including a high incidence of non-dipping BP.^82,146 For example, a relatively small percentage of individuals with heart failure exhibited a normal dipping pattern. In contrast, almost 60% were non-dippers and 20% were reverse dippers.¹⁴⁶ During heart failure, select changes in clock gene expression and circadian rhythms remained intact, despite differences in BP dipping.¹⁴⁶ These latter changes were associated with reduced nocturnal levels of melatonin but increased levels of cortisol.¹⁴⁶ In heart failure in humans, it has been suggested that the central circadian clock may be functioning at reduced output, while peripheral clocks may be intact.¹⁴⁶ Additional findings based on the study of a zebrafish (Danio rerio) model of heart failure (another diurnal vertebrate) were consistent with the concept of preserved peripheral clocks during heart failure.¹⁴⁶

Atrial fibrillation.

Atrial fibrillation is one of the most common sustained arrhythmias. It is a well-established risk factor for clinical ischemic strokes as well as microinfarcts due to microemboli.^15,147 Thus, many atrial fibrillation-induced strokes may be clinically silent.¹⁴⁷ Peripheral endothelial dysfunction can be present in a high percentage of individuals with atrial fibrillation.^15,148 Available data suggest the lifetime risk for atrial fibrillation is greater in men than in women, and greater in White individuals, compared to Black, Hispanic, Asian or Native American individuals.¹⁵ In relation to diurnal patterns, spontaneous episodes of atrial fibrillation are more common in the afternoon or evening, the later portions of the light period.^149,150 Although the mechanistic basis is unclear, reverse dipping of BP has been linked to increased risk of developing atrial fibrillation.¹⁵¹

Cardiac-brain links.

Based largely on associations, multiple studies have suggested there are links between cardiac function, heart disease, CBF, and brain health (including Alzheimer disease and vascular dementia).¹⁵² Associations between heart disease and white matter abnormalities have also been described.¹⁵³ For each of these forms of cardiac disease, it has been speculated that reductions in cardiac output,^15,154-159 or reductions in cardiac output and BP,^145,156,160 result in decreased CBF (Figure 2). The latter effect is often assumed to occur and contribute to a progressive decline in cognitive function. For example, a relatively large study reported that myocardial infarction was associated with cognitive decline and dementia.¹⁶¹ While no link with cognitive dysfunction was detected at the time of the myocardial infarction, more rapid reductions in global cognition, executive function, and memory were detected in the years past the event.¹⁶¹ The decline in cognition due to myocardial infarction was estimated to be equivalent to 6-13 years of normal aging.¹⁶¹ Treatment of atrial fibrillation in patients (most with hypertension) using catheter ablation increased global and hippocampal CBF and cognitive function for up to one year.^154,162 The mechanism that underlies these treatment-induced effects has not been defined. To our knowledge, it is not known if brain endothelial dysfunction is present during atrial fibrillation.

Abnormalities in daily rhythms in BP have been linked with cognitive decline in non-heart failure patients.¹⁴⁵ In subjects with heart failure, reverse dipping was associated with greater cognitive deficits (loss of memory and executive function) compared to non-dippers or dippers.¹⁴⁵ In addition to reduced BP and CBF, increases in cerebral venous pressure (with venous microbleeds and disruption of the BBB) were implicated in the potential pathogenesis of cognitive impairment.¹⁴⁵ Based on direct measurements, increases in venular pressure have been shown to produce disruption of the BBB.^163,164

Persons who were genetically predisposed to atrial fibrillation had a modest risk for overall dementia, a strong risk for vascular dementia, but no significant risk for Alzheimer’s disease or frontotemporal dementia.¹⁵⁶ Proposed mechanisms that might underlie these associations include the presence of stroke (including silent stroke), reduced cardiac output and BP, intracerebral atherosclerosis, or SVD.¹⁵⁶

As noted, it has been suggested that reductions in cardiac output reduce CBF. Under normal conditions, CBF does not passively follow changes in cardiac output. There are many examples in the literature where cardiac output changes substantially without significant changes in CBF. Thus, assuming BP is relatively normal, reductions in CBF in the face of heart disease with reduced cardiac output, likely involve more complex mechanisms. A decrease in CBF under these conditions may require simultaneous cerebrovascular disease (e.g., intracranial atherosclerosis, endothelial dysfunction, loss of autoregulatory capacity).^157,158,160 Some have noted it is unlikely that reduced cardiac output alone can explain decreases in CBF during heart failure.¹⁶⁰

Vascular biology

BMAL1 is an essential clock gene that contributes to circadian patterns in mammals.^{42,98,105,165} Gene variants in BMAL1 or CRY1 are associated with hypertension in humans,^166,167 while variants in several clock genes (including BMAL1) are linked with a lack of BP dipping in adults with hypertension.¹⁶⁸

Vascular expression of clock genes.

Time-dependent changes in expression of clock-related genes (e.g., Clock, Bmal1, Per1, Per2, Cry1) have been observed in blood vessels including aorta, mesenteric and cerebral arteries, and the vena cava.^58,169-173 Such findings are consistent with the concept that clock-controlled genes or clock-dependent signaling pathways may exert notable effects on circadian-related phenotypes (Figure 2).

In addition to well-described clock genes, genes that influence clock gene expression may exert notable effects on circadian-related phenotypes. For example, mRNA expression of Pparg in aorta exhibited more than a 20-fold change during the light/dark cycle.¹⁷⁴ Other lines of evidence, including interactions of PPARg with the Bmal1 promoter, support the concept that Bmal1 is a PPARg target gene.¹⁷⁴ PPARg has varied roles in vascular biology including an impact on myogenic tone, NO-signaling and vascular remodeling.^174-177

Genetic knockdown of PPARg in endothelium or smooth muscle disrupted daily rhythm changes in BP, with reductions in the maximal variations in MAP over a 24-hour cycle.¹⁷⁴ Such results suggest daily and possibly circadian rhythms in BP are regulated in part by vascular cells through PPARg-dependent mechanisms.¹⁷⁴ Consistent with such observations, chronic treatment with agonists for PPARg produce small reductions in arterial BP in humans.^178,179 As noted above, small (but population-wide reductions) in BP are linked with meaningful gains in the prevention of cardiovascular disease.

Myogenic tone.

Small arteries and arterioles commonly develop myogenic tone when pressurized.^180-182 Considering the impact of lumen diameter on vascular resistance, changes in myogenic tone have substantial effects on blood flow through the vessel as well as BP transmission downstream during acute or chronic increases in BP (Figure 1).^180,182

Isolated small arteries exhibit a circadian rhythm in the development of myogenic tone.^112,181,183 In cremaster, mesenteric, or cerebral arteries, this pattern in myogenic tone was eliminated in mice expressing a global Clock^Δ19/Δ19 mutation or knockdown of Bmal1 specifically in smooth muscle.^183,184,112 In wild type mice, the peak in myogenic tone was observed at the end of the active/dark phase, with the lowest level near the end of the inactive/light phase.¹⁸³ Differences in myogenic tone of resistance vessels may influence time-of-day-dependent changes in BP including the morning surge in BP.^52,94,183 While these phenotypes suggest circadian influences on blood flow through myogenic mechanisms, studies directly linking these variables in vivo are lacking. Experts argue that at least six time points are needed for circadian analysis,¹⁸¹ which may have contributed to the limited number of studies in this and related areas.

Rho kinase (ROCK) is an important contributor to development of myogenic tone.^185,186 BMAL1 activates the Rock2 promoter in a time-of-day-dependent manner in cultured cells, an effect that is reduced in BMAL1-deficient cells.¹¹² BP and its day/night rhythm was reduced in mice lacking Bmal1 in smooth muscle.¹¹² In addition, an increase in the wall-to-lumen ratio of mesenteric arterioles was present in this Bmal1 deficient model,¹¹² suggesting vascular remodeling may be present.

Both Rock1 and Rock2 exhibit time-of-day-dependent changes in expression in mesenteric arteries.¹⁶⁹ Contraction of the aorta to Ang II exhibited a diurnal pattern in control mice.¹⁶⁹ A daily rhythm in expression of ROCK2 was seen in aorta from normal mice.¹⁸⁷ ROCK2 was also essential for differences in vascular tone in animals maintained under traditional dark-light cycles.¹⁸⁷ Other lines of evidence suggest ROCK2 is key for generating a daily rhythm of phosphorylation of myosin light chain and myofilament Ca²⁺ sensitivity, which play a key role in contraction of vascular muscle.¹⁸⁷

Endothelium-dependent vasodilation.

Endothelial cells mediate endothelium-dependent vasodilation to diverse stimuli, while also influencing thromboresistance, vascular structure, and responses to therapeutics.^188-190 Many of these effects are mediated by eNOS-derived NO.^17,191 Normally, endothelium-dependent vasodilation varies depending on the time of day, with enhanced responses at the beginning of the active/dark phase.^171,192-194 Endothelial function is impaired in aorta from mice genetically deficient in Bmal1 or in Clock mutant mice.¹⁹² Expression and phosphorylation of eNOS and Akt1 (which increases eNOS activity) was also impaired in the Bmal1 knockout.¹⁹² The model also exhibits oxidative stress, and loss of endothelial function could be reversed by scavenging superoxide.¹⁹⁵ Mice with a genetic mutation in Per2 (Per2^Brdm1) had no change in eNOS expression, but impaired endothelial function in aorta.¹⁹³ In hypertensive humans, endothelium-dependent vasodilation was impaired in non-dippers compared to individuals that exhibit dipping.¹⁹⁶ Thus, the presence or absence of dipping has been linked to eNOS function in human hypertension.

Vascular mechanics.

Hypertension is associated with increases in stiffness (reduced distensibility) of aorta and large conduit arteries in preclinical models and humans.^22,197,198 Increases in aortic stiffness, often measured indirectly through changes in pulse wave velocity (PWV), are a predictor of cardiovascular mortality and cognitive deficits.^22,199 Changes in aortic stiffness and the subsequent impact on blood flow waveforms in the carotid artery are positively linked with cognitive deficits during aging.²⁰⁰ Individuals with the largest changes in carotid hemodynamics exhibited cognitive changes equivalent to almost two years of normal aging.²⁰⁰ In relation to diurnal changes in vascular mechanics, healthy volunteers exhibit nocturnal dipping of SBP and DBP as well as aortic PWV.²⁰¹ These changes did not occur in subjects with chronic kidney disease.²⁰¹ In individuals with hypertension, carotid-femoral PWV progressive increased from morning to evening with the highest values at 2100 hours.¹⁹ This was in contrast to changes in peripheral endothelial function which were lowest in the early morning in the same subjects.¹⁹

Circadian influences in the context of brain health

Hypertension, particularly during mid-life, is a leading risk factor for cognitive decline and some forms of neurodegeneration.^{5,7,24,26,202} The risk for hypertension-associated cognitive dysfunction may be even greater in Black and Hispanic individuals compared to White subjects.²⁰³ Interestingly, attention to this relationship has revealed that the link between hypertension and cognition may begin earlier in life than previously appreciated, even in childhood.²⁰³ High BP in young adults has been linked with young-onset dementia.²⁰³ Compared to White subjects, hypertension had an earlier age of onset and greater severity in Black individuals, along with a faster decline in global cognition and memory.²⁸ Such data support the concept of differences in hypertension-associated reductions in brain health related to ethnicity.

Even in normotensive individuals, loss of the normal rhythm in BP is associated with adverse health.⁶¹ Disruption of daily rhythms have been associated with loss of brain health.^30,204 The presence of reverse dipping increases the likelihood for a diagnosis of Alzheimer disease almost 2-fold.⁶¹ Individuals with elevated nocturnal BP are at a greater risk for dementia.⁶¹ Another common form of neurodegenerative disorder is Parkinson disease.^30,205 Changes within the autonomic nervous system and daily rhythms are common in such individuals.^30,84 In a recent study, the majority of subjects with Parkinson disease exhibited either loss of nocturnal dipping or reverse dipping, independent of medication use.^30,84,205 Such observations raise questions regarding the relative impact of disruption of daily cycles in BP, cognitive deficits, and progression of Parkinson disease.^30,84

Disruption of circadian cycles also occur in preclinical models and humans with Alzheimer disease.^30,206 The changes in circadian patterns in Alzheimer disease can be similar to those that occur with normal aging, but are often more severe and with greater impact, and are related to neuroanatomical changes in the SCN.^30,206-208 Global deletion of Bmal1 in rodent models reduces daily oscillations of ß-amyloid in brain interstitial fluid, increasing deposition of the peptide.²⁰⁹ All major dementias have a vascular component, with the most common presentation being mixed dementia (vascular dementia combined with neurodegeneration).^14,210,211 Whether circadian dysfunction specifically influences features of mixed dementia is unknown.

In relation to hypertension, a meta-analysis evaluated associations between ABPM, cognitive function, and dementia.²¹² Compared to a non-dipping pattern, normal BP dipping was associated with a lower risk of cognitive dysfunction or dementia.²¹² The greatest association with cognitive dysfunction was seen with reverse dipping, where up to a six-fold higher risk of cognitive decline was observed compared to dippers. The extent of nighttime dipping in BP was a strong predictor of cardiovascular events including strokes. Such relationships raise the question of whether restoring nocturnal dipping of BP in individuals with hypertension should be a therapeutic goal, one that might have substantial beneficial effects in relation to the rate of cognitive decline or progression to dementia.²¹²

A growing proportion of the aging population are individuals age 90 and above (the oldest-old).²¹³ The majority of subjects within this demographic have a history of hypertension. Based on ABPM, cognitively normal subjects had significantly greater nocturnal dipping of BP compared to individuals with cognitive dysfunction. Subjects exhibiting cognitive impairment across different cognitive domains and white matter abnormalities, were more likely to be reverse dippers compared to controls.²¹³. In this context, the presence of cerebral microbleeds was also greater in reverse dippers.²¹³ More clearly defining the temporal relationship between loss of normal dipping and the emergence of reverse dipping with cognitive decline may be insightful. Potential mechanisms have been suggested to include abnormalities in endothelial function, autoregulation, activity of the sympathetic nervous system and the RAAS, along with BP variability, vascular stiffness, sodium intake, and renal disease.²¹³

Cerebral blood flow

The brain has very limited energy reserves.^182,214 As a consequence, it requires an adequate blood supply of oxygen, glucose, and other nutrients under basal conditions and during increased cellular demand. Thus, normal brain function relies on well regulated, temporal control of global and regional CBF.^180,182,214

Under normal conditions, CBF gradually declines with increasing age in both men and women.^188,215 This decline can be accelerated in the presence of hypertension^26,216-218 or conditions such as heart failure.¹⁵⁹ Based primarily on temporal relationships, many studies have suggested that reductions in CBF begin prior to the onset of cognitive decline, but are accelerated in the presence of hypertension (Figure 1).^{159,216,218,219} For example, in a population based study with an almost seven year follow-up, there was an association between reductions in CBF and subsequent cognitive deficits.²²⁰ Lower CBF at baseline was associated with an increased rate of cognitive decline, particularly for memory and executive function, and a higher risk of dementia. These associations were greatest in individuals with higher levels of BP.²²⁰

Despite its fundamental importance, few studies have addressed the question of whether global or regional CBF normally exhibit a daily rhythm. Some studies comparing morning-evening values of CBF or changes in CBF during sleep exist,^194,221,222 but these generally lack sufficient time points to test for the presence of a daily rhythm.

In awake normal rats, CBF was found to be somewhat higher beginning a few hrs before and into the dark period, with lower values during the light period.^223,224 These changes were not related to differences in physical activity. Aframian et al continually measured cerebral blood volume (CBV)(which can be an index of CBF) in freely moving mice. ¹¹¹ They reported increased CBV during the awake state and decreased CBV during the sleep state. The exception were periods of rapid eye movement during sleep, when CBV increased significantly.¹¹¹

In humans, continual measurements of blood flow velocity in the middle cerebral artery in research participants studied in a modified Constant Routine protocol provided evidence for the presence of an endogenous circadian rhythm in baseline CBF with the time of circadian minimum at around noon..²²⁵. A recent study quantified CBF every 3 hrs over a 24-hr period using MRI in humans, describing a significant diurnal pattern with an acrophase at 2100 hours.²²⁶ This pattern of CBF was not observed (and thus was disrupted) in subjects with bipolar disorder.²²⁶ The differences between the timing of the peak was likely related to the fact that the participants in the study by Carlucci et al. participants were undergoing a sleep/wake, fasting/eating, and dark/light cycle (as is true in most non-circadian protocols) while the study by Conroy et al. used a modified Constant Routine protocol. Neurovascular coupling of CBF in the visual cortex has also been studied, but was not significantly altered between the hrs of 0800 and 1800.²²⁷

Data addressing potential changes in daily rhythms in CBF during hypertension is even more limited. In awake Sprague-Dawley and TGR(mRen-2)27 rats, measurements of CBF in the cerebrum suggest higher values occurred during the dark compared to the light period.²²⁸ Across three days of recording, no differences were detected in the pattern of CBF between strains. In other words, no change was detected in this model of hypertension. The acrophase in CBF was at a similar time within the dark period in both strains, despite the peak in BP occurring in different phases of the day (see above).²²⁸ In humans, a higher relative morning surge (increase in SBP) was associated with lower regional CBF to gray matter in older adults treated for hypertension.²²⁹

Blood flow to the SCN

Electrical lesions of the SCN eliminate the circadian rhythmicity in BP.^105,230 Despite its importance as the central circadian clock in the mammalian brain,^231-234 very little is known regarding regulation of blood flow to the SCN. The SCN is located within the anterior hypothalamus above the optic chiasma on each side of the third ventricle. This is a paired nucleus, containing about 50,000 neurons.^30,231,235

In humans with primary hypertension, levels of key neuropeptides (e.g., vasopressin) in the SCN are reduced by >50% compared to controls.²³⁶ Is blood flow to the SCN regulated in a unique manner and does it change during hypertension? Resting blood flow to the SCN in awake rats is in the same range as a number of other brain nuclei, including other hypothalamic nuclei.^237-239 Baseline CBF to the SCN and the ratio between local CBF and glucose utilization were very similar in SHR and WKY, despite MAP being approximately 50 mmHg higher in SHR.²³⁷ Such findings suggest steady state autoregulation of CBF is quite effective in the SCN, similar to many other brain regions.^182,237

Considering changes in Ca²⁺ dynamics, neural activity, and transcription that occur in the SCN during a circadian cycle,^232-234,240 significant changes in local CBF might be predicted to occur due to neurovascular coupling or other mechanisms. Defining to what extent local CBF or BBB function potentially change over a 24-hr cycle could be very revealing with potential implications for the mechanistic basis by which the SCN functions. Based on changes in local blood flow, activity of the human SCN was highest at noon and lowest at 0600 hrs.²⁴¹ In humans using blood-O₂ level-dependent (BOLD) functional MRI, exposure to several colors of light caused a reduction (not activation) in the BOLD-signal, suggesting reversed neurovascular coupling within the SCN.²⁴² Other data support this possibility.²⁴¹ While these findings are thought-provoking, the methodology used to image this small nuclei has been criticized,²⁴³ emphasizing that additional work on regulation of CBF to the SCN is needed. Rapid changes in eNOS expression have been seen during severe hypoxia within SCN endothelial cells, consistent with a potential influence of eNOS on local CBF.²⁴⁴ Single-cell RNA-sequencing demonstrated that all cell types within the SCN, including endothelial cells, exhibit circadian rhythmicity of multiple clock genes (e.g., Bmal1, Per2, Cry1, Nr1d1, Nr1d2, Dbp).²⁴⁵

Brain barriers

Blood-brain barrier.

The BBB consists of cellular, structural and functional features critical for maintenance of brain homeostasis. Endothelial cells are the cornerstone of the BBB, with adjacent cells anchored to each other by junctional complexes (tight and adherens junctions). Due to endothelial expression of the MFSH2a gene, the BBB normally has low levels of transcytosis. So the brain can function despite the presence of a BBB, essential molecules or proteins cross the BBB and enter the CSF via an array of molecular transporters (e.g., efflux and solute carriers) and receptors ^246-249 BBB function is facilitated by support cells, particularly perivascular astrocytes and capillary pericytes.²⁴⁷

In relation to disease, a progressive decline in BBB function occurs normally with aging,²⁴⁷ a change that can be accelerated by the presence of hypertension.²⁵⁰ This functional decline is believed to contribute to cognitive deficits, vascular dementia, and Alzheimer disease.²⁴⁷ Based on limited data, changes in daily rhythms in BP and BP variability have been associated with loss of BBB integrity in humans.²⁵¹ Loss of BBB integrity during atrial fibrillation or heart failure may contribute to the increased risk of dementia that is often associated with cardiac disease.²⁴⁹ Reductions in CBF, neurovascular coupling, and an increase in silent or covert strokes have been observed in individuals with atrial fibrillation.²⁴⁹

Although expression of junctional proteins can exhibit a daily rhythm in the retina, relatively little is known regarding potential changes in these proteins at the BBB.²⁵² A recent study suggested there is less BBB efflux during the dark period, when mice are more active, and greater efflux during the light period.²⁵³ BBB efflux transporter expression and function exhibited a diurnal pattern that was inhibited by endothelial-specific knockdown of Bmal1.²⁵³ Thus, endothelial clock genes appear to play a role in BBB efflux gene expression.²⁵³ In mice deficient in Bmal1 within the CNS, there is increased brain water content and permeability of the BBB.²⁵⁴ Mechanisms underlying these changes have not been defined, but could potentially be linked to pericyte dysfunction and its impact on the BBB.²⁵⁵

Hypertension is associated with impairment of the BBB.^24,250,256 In models of Ang II-dependent hypertension and SHR, increased permeability of the BBB has been observed.^24,256-259 This effect was increased in a model of aging.²⁶⁰ Mechanisms that have been implicated in these changes include activation of endothelial AT1 receptors, oxidative stress, perivascular macrophages, reduced expression of claudin-5, and increased endothelial transcytosis.^24,257-259 To what extent absolute levels of BP versus changes in diurnal dipping patterns influence loss of BBB integrity during hypertension is unclear.

Blood-CSF-barrier.

The choroid plexus, located in each of the brain ventricles, has multiple functions. It is the site of the BCSFB and the primary site of formation of CSF, thus playing a major role in brain fluid balance.^261,262 Unlike the BBB, the BCSFB is formed by tight junctions between epithelial cells of the choroid plexus.^261-263 The choroid plexus is a key site of ion and molecular transport (e.g., glucose, amino acids) and a gateway for surveillance and movement of immune cells and molecules into and out of the CSF.²⁶³ A variety of resident immune cells are normally present in the choroid plexus including border-associated macrophages.^261,264 Several of these functions, including formation of CSF, molecular exchange and cellular surveillance, are likely facilitated by the very high levels of blood flow that normally perfuse the choroid plexus.²⁶¹ Levels of blood flow to the choroid plexus can be approximately 10-fold greater than global CBF.^261,265-268

The choroid plexus is a major structure in relation to daily rhythms.³⁰ CSF production, composition, and distribution vary throughout the day, with some variables exhibiting a daily rhythm.^261,262 The translatome and secretome of the choroid plexus change with a diurnal pattern, with higher levels of translation during the dark phase.²⁶² The BCSFB exhibits higher levels of permeability during the light phase.²⁶² It has been suggested that the choroid plexus expresses the strongest circadian clock within the CNS.²⁶⁹

In rats and humans, intracranial pressure and CSF production (due to increased transporter activity in the CP) are increased during the dark phase.²⁷⁰ CSF may relay diurnal signals within brain and perhaps to the SCN via diffusible molecules including vasopressin and other neuropeptides.^261,269 Choroid plexus function has also been linked to cognition.²⁷¹

Function of the choroid plexus and the BCSFB are altered during hypertension. For example, transport of water across the choroid plexus was reduced by about one-third in SHR compared to normotensive controls.²⁷² Despite an increase in MAP of 70 mmHg, cortical CBF was not altered,²⁷² suggesting autoregulation was quite effective in this model. SHR exhibit increased volume of the lateral ventricles and brain water accumulation, but no increase in ICP, CSF production, or CSF outflow resistance relative to normotensive controls.²⁷³

Based on limited studies, It has been suggested that the choroid plexus and BCSFB may be more susceptible to hypertension than the BBB.²⁷² In SHR, changes in the proteome of the CSF supports the concept that the BCSFB is disrupted.²⁷⁴ In the choroid plexus from hypertensive humans, local deposition of immune complexes occurs in the walls of the choroid plexus.²⁷⁵ Permeability and water content of the choroid plexus was greater than other major brain regions and significantly greater in SHR compared to WKY.²⁷⁶ In another study, blood flow to the choroid plexus and CSF formation were reportedly higher in SHR.²⁷⁶ In relation to the central RAAS and potential local control, administration of Ang I or Ang II in the circulation or the CSF reduce blood flow to choroid plexus without significantly altering CBF.²⁷⁷ Whether there is a diurnal pattern in blood flow to choroid plexus normally or during hypertension is unknown.

Summary and future directions

The number of individuals with hypertension continues to increase globally. Despite available treatments, control of BP remains inadequate for a substantial fraction of affected individuals (e.g., partial, but insufficient, reduction in BP, resistant hypertension). Thus, major gaps in our understanding of mechanisms that initiate, promote, or sustain hypertension remain. The current review summarized current insight into concepts and mechanisms related to what appears to be highly relevant, but understudied, areas of investigation – the impact and interaction of factors that influence circadian or diurnal control of absolute levels of BP, changes in BP during the day and night, and associated clinical characteristics or events.

Because changes in the daily rhythms in BP have been linked to an increased risk for serious cardiovascular and neurovascular events, the understanding of the underlying circadian, behavioral, and environmental mechanisms and consequences of BP variation in normotensive, pre-hypertensive, and hypertensive individuals may help in prevention, treatment, and management of hypertension and related pathophysiology. The effects of shift work, circadian misalignment, social jet lag, and circadian genetic variants/mutations provide further evidence of the importance of the circadian system and its disruption in BP regulation and cardiovascular disease.

Other unresolved issues include the following. Although some effort has been made toward evaluating the effectiveness of timing of anti-hypertensive therapy, the results have been inconsistent, leaving a potentially important issue unclear. Monitoring of BP at night has been proposed as a new therapeutic priority. Such an effort could benefit from greater insight into the clinical impact and underlying mechanisms of nocturnal hypertension. Incorporation of more relevant cell-specific models may be insightful, along with greater use of large animal models of hypertension (e.g., pigs). The study of hypertension has been limited at times by a focus on predominantly male subjects. Greater effort is needed to determine to what extent underlying mechanisms and therapeutic targets are sex-specific? Lastly, single-cell RNA-sequencing has demonstrated that diverse cell types exhibit circadian rhythmicity of clock genes and CCGs. More effort is needed to better define the single cell transcriptome and proteome in relevant preclinical models and human hypertension.

Box 1. Glossary of Key Terms.

ABPM

Ambulatory BP monitoring for non-invasive measurements of systemic arterial BP for 24 hours or more. Data include values during the day and night, a 24-hour average, BP variability, and BP dipping patterns.

Acrophase

The time point during a cycle when a variable peaks.

Circadian misalignment

Mistiming of circadian cycles relative to environmental (environmental misalignment), behavioral (behavioral misalignment), and or other circadian (intrinsic misalignment) cycles.

Circadian period

The cycle length of a circadian rhythm.

Circadian phase

The timing of a circadian rhythm, described as a particular time in the cycle such as the peak relative to another timeframe, such as local clock-time or the behavioral sleep/wake cycle.

Circadian rhythm

A biological rhythm that: (1) has a period of approximately 24 hrs; (2) is generated by a self-sustained circadian oscillator, and thus persists in the absence of changes in the environment and behavioral changes, such as dark/light, environmental temperate, sleep/wake, and fasting/eating cycles; and (3) can be shifted by Zeitgebers (time cues); e.g., to synchronize the organism to Earth’s 24-h night/day cycle.

Clock-controlled genes (CCGs)

Genes that are directly regulated by core clock genes and function as ‘hands of the clock’, i.e., transmitting the circadian molecular clock signal to influence cell transcription and function.

Core clock genes

Genes that constitute the transcription-translation feedback loops (TTFL) that enable cell-autonomous circadian generation.

Daily, day/night, or nycthemeral rhythm

A rhythm with a period of about 24 hrs, where the rhythm may be due to a combination of endogenous circadian influences and acute influences (also called ‘masking’ or ‘evoked’ effects) of environmental and/or behavioral factors, such as the dark/light, sleep/wake, rest/activity, and fasting/eating cycles.

Dipper

An individual who exhibits a typical decrease of blood pressure during nighttime sleep as compared to the daytime, typically defined as a decrease of 10-20%. This pattern of dipping is generally considered healthy and has been associated with decreased cardiovascular risk compared to other patterns.

Diurnal

Used to describe the timing of the main activity occurring during the day, i.e., a diurnal species is one that is primarily active during the day, the light phase. Sometimes it is used interchangeably with the word ‘daily’ to describe an observed 24-hr rhythm caused by combined circadian and environmental/behavioral cycles (see above).

Extreme dipper

An individual who exhibits an exaggerated decrease of blood pressure during nighttime sleep as compared to the daytime, typically defined as a decrease of >20%.

Nocturnal

Used to describe the timing of the main activity occurring at night, i.e., a nocturnal species is one that is primarily active at night, during the dark phase.

Nondipper

An individual who exhibits a reduced dipping of blood pressure during nighttime sleep as compared to the daytime, typically defined as a decrease of <10%.

Peripheral circadian oscillator

A circadian oscillator that resides outside of the suprachiasmatic nucleus.

Reversed dipper

An individual who exhibits an increase of blood pressure during nighttime sleep as compared to the daytime.

Small vessel disease

Structural or functional microvascular changes that promote hypoperfusion, increased vascular permeability, microinfarcts, microbleeds, changes in the extracellular matrix, and the perivascular space.

Suprachiasmatic nucleus (SCN)

The central circadian pacemaker, located in the anterior hypothalamus on top of the optic chiasm in mammals.

Nonstandard Abbreviations and Acronyms

Ang II

angiotensin II

BBB

blood-brain barrier

BCSFB

blood-CSF barrier

BP

blood pressure

CBF

cerebral blood flow

CSF

cerebrospinal fluid

DBP

diastolic blood pressure

CCG

clock-control genes

eNOS

endothelial nitric oxide synthase

MAP

mean arterial pressure

NO

nitric oxide

PPARg

peroxisome proliferator-activated receptor γ

RAAS

renin-angiotensin-aldosterone system

SVD

small vessel disease

SBP

systolic blood pressure