Effects of seasonal factors on brain function: Systematic review and future perspectives

Summary

The empirical evidence for the effects of changing seasons on humans and other species has long been a subject of public and scientific curiosity. However, supporting evidence is predominantly behavioral, warranting further quantitative/mechanistic validation. Three putative mechanisms proposed here include (1) variability in photoperiod and temperature, (2) gravitational effects from varying Earth-celestial body distances, including lunar phases and eclipses, and (3) changes in geomagnetism. This systematic review (PubMed, Web of Science, and Scopus; until September’24) aims to identify studies on the effects of seasonality, gravity, and geomagnetism on brain function, establishing a baseline for the proposed hypotheses. Distinct search queries were tailored to capture the relevant literature. Behavioral and observational data support the hypotheses, with limited EEG/MRI evidence indicating potential neural correlates, although most research is cross-sectional and preliminary. Future work should employ well-powered, longitudinal, and hypothesis-driven designs to clarify the observed effects, thereby confirming or challenging competing views.

Graphical abstract

Highlights

• •We did a systematic review of seasons/gravity/geomagnetism affecting brain function
• •We found evidence across both animal and human literature supporting the premise
• •Evidence is, however, primarily behavioral, with inadequate mechanistic insights
• •Future studies must be well powered, longitudinal, and mechanistically grounded

Environmental health; Physiology; Neuroscience

Introduction

Hippocrates in the 5th century said, “it is chiefly the changes of the seasons which produce diseases.” These natural seasonal rhythms have been empirically shown to affect the life and behavior of animals and humans.^1,2 Some of the factors that have been hypothesized to underlie these biological effects are (1) the seasonal changes in length of day and temperature,^3,4 (2) Earth’s relative distances from (and hence alignment with) celestial bodies and the resultant changes in gravity experienced on Earth (this includes concomitant changes in lunar phases⁵ and axis alignments such as eclipses⁶), and (3) Earth’s magnetic field⁷ (also referred to as geomagnetism). While the effect of the length of day on brain function has been studied using functional magnetic resonance imaging (fMRI) and neuroimaging,⁸ the purported effects of other factors have been hypothesized mainly based on behavioral (and some electroencephalogram [EEG]) studies. Seasonal changes, lunar phases, and eclipses induce changes in the gravitational field experienced on Earth as well as the Earth’s magnetic field, which in turn have been shown to influence animal biology.

For instance, the European Robin migrates from Northern Europe to the tip of Spain every year, navigating thousands of miles.^9,10 To determine the basis of their navigational system, Henrik Mouritsen and team observed the activities of these birds in an experimental setup. They tested the hypothesis that the birds may be using the Earth’s magnetic field to determine the north-south direction. The birds were caged (aluminum Faraday cages), and the direction of the magnetic field inside the cage was systematically varied.^11,12,13 The Faraday cages attenuated the time-dependent electromagnetic interferences but not the Earth’s static magnetic field. It was observed that the movement of the birds inside the cage always aligned with the magnetic field. Attempting to explain this observation directly based on the Earth’s magnetic field is problematic since it is too weak to be detected by the brain based on the electric currents induced in the neurons due to the change in Earth’s magnetic field.¹⁴ An alternative explanation is based on the phenomenon of quantum entanglement.^15,16,17 It is believed that light acts as the stimulator for the compass in the brain that helps them navigate such long distances accurately. The incidence of light on the eye of the bird is the entrance of a photon into the eye.¹⁵ This photon, on entering the retina, creates an entangled pair of electrons. These entangled pairs dictate the state of each other despite them being separated in space. It is expected that they remain in the same state near the equator and the opposite near the poles. The weak magnetic field of the Earth and slight variation in the magnetic field during the traversal journey likely influences the state of these entangled electron pairs that triggers the internal compass. The main point of this example is to show that biological mechanisms that can detect very weak magnetic field changes potentially exist in the animal world. This is important because any seasonal effects on human brain function have thus far been mainly attributed to the entrainment of neural rhythms with light exposure and hence the changes in length of day across seasons. However, it is important to recognize the fact that other mechanisms such as geomagnetism may also play a part in seasonal variations in brain function. In this article, we review the literature on observed seasonal effects on brain function, including speculations in the literature about the underlying mechanisms. We then propose hypotheses that can be tested using neuroimaging about the possible mechanistic roles of the factors discussed above.

Results

In the following sections, evidence from a systematic review have been presented. There are three broad sections in this systematic review: (1) seasonality, (2) gravity, and (3) geomagnetism. Our aim was to identify the effects of seasonality, gravity, and geomagnetism on brain function.

Seasonality

Seasonality is the outcome of the variations in the environment that affects life on Earth. Changing seasons result in adaptation, hibernation, camouflaging,¹⁸ and/or migration in the living world across species. Living organisms, including humans, rely on these changes for proper physiological and behavioral functioning^1,2 that is responsible for maintaining body temperature, hormones, and the sleep/wake cycle. Before we dig into the literature in section named “systematic review: seasonality and MRI” and beyond, it is important to understand certain concepts as described next (sections named “rhythms of the brain” to “seasonality of mental disorders: seasonal affective disorder”).

Rhythms of the brain

The central nervous system is capable of generating various rhythms. The neurons fire in a specific pattern, which is responsible for generating oscillations with frequencies ranging from 0.02 to 600 Hz.¹⁹ The human brain comprises several biological rhythm generators (or “clocks” as they are referred to in the literature) that account for various rhythms such as the circadian rhythm, the infradian rhythm, and the ultradian rhythm, which operate at different time scales. Circadian rhythm means “about a day” in Latin. It refers to the whole-day-long sleep-wake cycle (24 h long) that is synchronized between the brain and the environment through receptors. In plants, this rhythm is responsible for photosynthesis. In animals, seasonality influences their brain size,²⁰ hormone secretion²¹ that regulates behavioral and physiological changes, reproductive patterns,²² etc. The rhythm that repeats itself more than once in a day is termed as the ultradian rhythm (such as our heart rate). On the other hand, the rhythm that stretches over a span of as long as a month, like the menstrual cycle, is termed as the infradian cycle. All these biological rhythms—circadian, ultradian, and infradian—are influenced directly or indirectly by the brain.²³

Rhythmicity and hormones

One study reported on the secretory patterns of prolactin in male and female dogs. The levels were seen to be higher during the 3 months of summer with longer daylight, thereby suggesting its circannual rhythmicity. There was an ultradian difference between the two genders over that time span.²¹ Likewise, the summer carp (Cyprinus Carpio) also demonstrates higher pituitary prolactin levels.²⁴ A similar circannual periodicity was observed in serum cortisol levels in healthy horses with highs during spring and plasma adrenocorticotropic hormone (ACTH) and alpha-melanocyte-stimulating hormone (α-MSH) levels being high during the fall/autumn.²⁵ Neurogenesis in the chemosensory epithelia of red-backed Salamanders exhibited seasonal patterns of higher nuclei count during the month of May than others.²⁶ A distinct effect of seasons has been observed in mammals^27,28; examples include the change in brain size as observed in the common shrew, with a decrease in brain mass by 10%–26% from summer to winter (more prominent in females than males) and a regrowth of 9%–16% during spring (similar for both sexes).²⁰ Apart from animal studies concerning seasonal plasticity and hippocampal volume,^28,29,30,31 a cross-sectional 3 T (3T) MRI-based human study also exhibited preliminary evidence for the association between smaller hippocampal volume (that is identified with depression) and shorter photoperiod.³² Previously, it was believed that only plants and animals relied on the intensity and length of daylight for proper physiological functioning. Nonetheless, present-day research shows that humans too are equally sensitive to light, and it tends to phase shift the circadian rhythm or affect melatonin secretion, the hormone responsible for regulating the sleep-wake cycle. Humans’ circadian rhythm can be phase shifted with simulated dawn/twilight signals.³³ A retrospective study assessed data from four different states of Australia comprising 67.9% females over a time period of 3 years. They observed higher levels of cortisol in the population during March to August than during September to January, and these changes increased with greater distance from the equator (lowest or no change closer to the equator).³⁴

Rhythmicity and clock genes

The pacemaker of the circadian rhythm is associated with the suprachiasmatic nucleus (SCN) located in the hypothalamus. The molecular mechanism of these rhythms is explained by phenotype-driven genetic analysis.³⁵ The genes that are responsible for the expression and working of this circadian rhythm are termed as the clock genes that are expressed in many brain regions and body parts including the SCN.³⁶ To maintain the circadian rhythm, these clock genes generate oscillations in the SCN, and the peripheral oscillators are governed by the transcriptional-translational autoregulatory feedback loop network. The genes that are identified by researchers involved in the conduct of these networks are Clock, Bmal1 (brain and muscle Arnt-like protein-1), Period (Per1, Per2, and Per3), Cryptochrome (Cry1, Cry2), and a dozen more.^35,37 Per3 is the most robust oscillating gene (i.e., the gene’s expression is rhythmic), controlling the circadian rhythm in humans and animals.

Specifically in humans, circadian rhythms are found to have an association with diurnal preference, mental disorders, and non-visual responses to light, along with an effect on brain and cognition due to sleep loss and/or circadian misalignment.³⁸ Per3 forms the nexus between sleep and mood regulation that leads to the outcomes of seasonal changes.³⁹ The clock genes’ prevalence in different brain structures implicated in neuropsychiatric disorders makes it plausible for the development of mental illness due to alteration in these rhythms. Most neuropsychiatric disorders are associated with a sleep disorder. Sleep is the amalgamation of two processes: the circadian process that dictates the time of sleep and a homeostatic process that dictates the requirement of one.⁴⁰ Sleep deprivation is related to disorders such as depression,⁴¹ schizophrenia,⁴² attention-deficit hyperactivity disorder,⁴³ and bipolar disorder.⁴⁴ Some scientists speculate the connection between sleep and neuropsychiatric disorders (especially depression)⁴⁵ to be a result of the disrupted circadian rhythm.

Seasonality of mental disorders—seasonal affective disorder

Since the diagnosis of seasonal affective disorder (SAD) in the year 1984,⁴⁶ a special type of recurrent seasonal depression that transpires mostly in the winter months, research has found a link between mood, sleep disorders, circadian rhythms, and seasonality. These individuals encounter increased melatonin (secreted by the pineal gland that induces sleep) secretion and decreased serotonin (a neurotransmitter responsible for mood regulation) secretion that hamper the circadian rhythm. The purported dependency stems from an inability to lower serotonin transporter binding levels during the winter months by these individuals.⁴⁷ The SAD estimates depend hugely on the latitude, with prevalence being 1.4% to 9.7% in Northern America, 1.3%–3.0% in Europe, and 0%–0.9% in Asia. However, a striking observation shows the rareness in these numbers in the native people of Lapps, Finland⁴⁸ or the Icelanders of Canada.^49,50,51 The reason behind such an observation can be due to genetic adaptations.^48,49 Additionally, it can be elucidated that sunlight has a large effect on brain function, and consequently its magnitude varies geographically across latitudes. The environmental conditions in the tropical region, regions near to the equatorial belt, and those toward the poles vary extensively, and the acclimatizing capability of people living there too depends on their span of inhabitance.

A less severe version of SAD is the subsyndromal-SAD (s-SAD), also known as “winter blues.” The Seasonal Pattern Assessment Questionnaire (SPAQ)⁴⁶ estimated around 14.3% of the US population experiences s-SAD.⁵² However, as expected, the counts tend to be higher for self-reported assessments than clinical assessments.⁵³ Exposure to natural outdoor light reduces the prevalence of SAD,⁵⁴ thereby opening up the possibility of treatment for both seasonal and non-seasonal neuropsychiatric disorders.^55,56,57,58 Likewise, this light treatment regime has been extended to treat non-seasonal major depressive disorder and bipolar illness, showing remarkable effectiveness.^59,60 Accordingly, photoperiod is found to play a vital role in healthy individuals as well as persons suffering from various psychiatric disorders due to its strong behavioral implications. A study reported the mood scores of 250 post-menopausal women four times during an year, who were residents of Boston area (aged 43–72 years). They showed higher depression-dejection, anger-hostility, and tension-anxiety scores during fall/autumn and lower scores during spring.⁶¹ This is likely because even though fall/autumn and spring have the same average photoperiods, the length of day is typically decreasing during the fall and increasing during the spring. Another similar study discerned, via the SPAQ⁴⁶ (140 elderly subjects, mean age = 79 years, 90% women), that the fluctuations were very nominal.⁶² Nonetheless, the prevalence of SAD and s-SAD diminishes with increased age (over 60 years); however, it is still persistent to a significant extent in elderly people.⁶³

The suffering of women of reproductive age are 2–4 times more than men of the same age due to SAD between the autumnal and vernal equinox in the northern United States.^52,63,64 Shorter days (between December and February) showed worse individual symptom scores of anxiety and marginal depression in women, whereas men showed no correlation with day length, rather they showed monthly variations with high mean score during October and low scores during March and July.⁶⁴ Considering the onset of gender differences after puberty⁶⁵ and transcending to minimal differences in old age,⁶⁶ the susceptibility of women to SAD can be attributed to their hormonal cycle linked to their reproductive status. The SPAQ used in the epidemiological study of SAD among medical students of China showed reverse findings with respect to the months of distress. SAD and s-SAD were more prevalent in summer than winter, most likely due to temperature and long daylight hours, which is in contrast to the Western studies.⁶⁷ These antithetical results signify the role played by geography, monthly ambient temperature, and length of day, in addition to seasonality. The prevalence of SAD based on the season (winter or summer) in various geographical locations emphasizes the fact that sunlight has an effect on brain function. The length of day and its consequent exposure varies in different parts of the world. Additionally, the inhabitance history signifies how comfortable the population has become over time through habitation and genetic adaptations. While studying the effects of seasonality on brain function, these variations are an important aspect to note. For example, the length-of-day variation in higher latitudes such as the United States is more prolonged than tropical countries. Thus, it can be speculated that the effects of seasonality would be more pronounced in those regions.

Systematic review: Seasonality and MRI

Search strategy: empirical observations of the relationship between seasonal/circadian rhythms and mental health state or disorders have led to basic scientific research investigating the neural basis of such observations.⁶⁸ Till date, there have been very few studies that took advantage of MRI to investigate these hypotheses. A comprehensive PubMed, WOS, and Scopus search with the following keywords “(((“seasonality”) OR (“weekly variation”) OR (“monthly variation”) OR (“seasonal variation”) OR (“Circadian activity”)) AND ((“brain activity”) OR (“brain analysis”) OR (“brain template”) OR (“sleep”) OR (“brain rhythm”) OR (“network connectivity”) OR (“brain network”)) AND ((“fMRI”) OR (“functional MRI”) or (“functional Magnetic Resonance Imaging”) OR (“functional Magnetic Resonance”) OR ((“magnetic resonance imaging”)))) NOT (Review)” returned 34 (6, 8, and 20 from Pubmed, WOS, and Scopus, respectively) articles, of which 23 were unique and eight were relevant. A manual search returned four more articles. One among them was based on magnetic resonance spectroscopy (MRS) data. The list of papers and the flowchart of the review are presented in Table 1 and Figure 1, respectively.

Table 1.

Studies focusing on the effects of seasonality using MRI or MRS

Study	Modality	Year of publication
Reproducibility and temporal structure in weekly resting-state fMRI over a period of 3.5 years69	3T Philips Achieva scanner	2015
Hippocampal activity mediates the relationship between circadian activity rhythms and memory in older adults70	3T Siemens Skyra MRI scanner	2015
Photoperiod is associated with hippocampal volume in a large community sample32	3T Trio TIM whole-body scanner	2015
Bright-light intervention induces a dose-dependent increase in striatal response to risk in healthy volunteers71	3T Siemens Trio scanner	2016
Seasonality in human cognitive brain responses8	3T head-only scanner (Magnetom Allegra; Siemens)	2016
Uncovering a “sensitive window” of multisensory and motor neuroplasticity in the cerebrum and cerebellum of male and female starlings72	7T horizontal MR system	2021
Unraveling the role of thyroid hormones in seasonal neuroplasticity in European starlings (Sturnus vulgaris)73	7T horizontal MR system	2022
Seasonal effect—an overlooked factor in neuroimaging research74	–	2023
The influence of season on glutamate and GABA levels in the healthy human brain investigated by magnetic resonance spectroscopy imaging75	3T Siemens Prisma scanner	2023
Seasonal variations of functional connectivity of human brains76	–	2023
Diurnal variation of brain activity in the human suprachiasmatic nucleus77	3T Siemens Prisma	2024
Circadian rhythms tied to changes in brain morphology in a densely sampled male78	3T Siemens Prisma	2024

Figure 1.

PRISMA flowchart concerned with the effects of seasonality studied using MRI or MRS

Exclusion criteria: studies that did not focus on explicit seasonal effects were excluded. Specific neuropsychiatric illness, brain injury, and migraine studies were also not considered.

Evidence: eleven relevant studies that are mentioned in Table 1 used MRI and one used MRS to investigate the influence of seasonality on brain function. This was examined through diverse lenses, with some studies targeting circadian rhythm dynamics and others exploring the impact of light exposure. To explore the circadian activity rhythm, one of the groups utilized actigraphy to record the sleep-wake pattern of older adults for 10 days followed by the acquisition of fMRI data during an associative memory task.⁷⁰ The task revealed that better memory performance is related to consistent circadian activity rhythm irrespective of sleep duration.⁷⁹ However, in BD (bipolar disease) patients, their impaired activity rhythm as well as sleep inefficiency result in a decline in their working memory brain response.⁷⁹ Impaired circadian activity rhythm impacts mood and behavior,^80,81 likely explaining the seasonal variability of limbic activity. In addition, regions belonging to the sensorimotor network have also been shown to vary according to seasons likely, given their role in processing exposure to light.⁷⁴

Expanding on this seasonal dependency of brain function, a cross-sectional study by Meyer and colleagues⁸ demonstrated how cognitive brain responses vary with time of year. In their study, a group of 28 healthy young adults were retained in an environment devoid of seasonal cues for 4.5 days, and their sleep pattern over the prior 3 weeks was tracked using an actigraph. During fMRI sessions, participants completed sustained attention and working memory tasks, which revealed seasonal variation in performance. The sustained attention task had maximum and minimum responses around summer and winter solstice, whereas working memory showed maximum and minimum responses around fall/autumn and spring/vernal equinox, respectively. However, it is important to note that, in this study the participants were in an artificial setting, isolated from natural environmental factors. In contrast, Xu and colleagues analyzed resting-state fMRI (rs-fMRI) data from 410 individuals in a more ecologically valid context, drawn from the Human Connectome Project.⁷⁶ Their findings revealed that brain functional connectivity, low-frequency fluctuations, and the topological properties of brain networks showed significant seasonal effects, with fall/autumn exhibiting the strongest connectivity and activity. Choe et al. reproduced this result using data derived from a longitudinal rs-fMRI study involving a healthy man scanned over a span of 3.5 years.⁶⁹

As outlined in the preceding section named “seasonality of mental disorders—seasonal affective disorder,” individuals suffering from SAD showed reduced depressive symptoms when exposed to light.^59,60 Building on this, investigation on healthy participants have also revealed similar associations between brain function and light exposure using fMRI.⁷¹ The study was conducted during the winter months of 11/2010–2/2011 and 11/2011–2/2012 (daylight availability the least) with 32 healthy individuals. fMRI data obtained before and after light exposure indicated a dose-dependent increase in BOLD fMRI activation in the ventral striatum and the head of caudate nucleus, proportional to the amount of sunlight exposure. This increased activity has also been observed in SCN due to exposure to not only sunlight but also ambient light, marking the importance of light per se in regulating our internal biological rhythm.⁷⁷

In addition to functional changes, structural alterations have also been linked to seasonal light variation. Specifically, individuals exposed to shorter photoperiods were found to have reduced hippocampal volume, a feature commonly linked to depressive symptoms.³² Extending beyond seasonal effects, daily (diurnal) fluctuations have also been shown to influence brain morphology. Specifically, reductions in total brain volume, gray matter volume, and cortical thickness were observed from morning to evening, suggesting that structural brain metrics are sensitive to time-of-day variations.⁷⁸ Interestingly, similar patterns are observed in animal models: even in European starlings, the photosensitive period (short day length) is associated with effects on the brain. Specific changes are observed in the song control regions, the cerebellum, and visual/auditory pathways, accompanied by a decline in thyroid hormone levels.^72,73

While the aforementioned fMRI studies highlight the effects of seasonality on brain function, MRS offers a complementary perspective by examining the brain’s chemical environment. A 3T MRS cross-sectional study from 2023 on 159 individuals (79 female, mean age ±SD = 25.4 ± 5.3 years, ranging from 18 to 50 years)⁷⁵ found that the neurotransmitter levels of glutamate and GABA did not fluctuate during the year but Glx/tCr did (ratio of glutamate plus glutamine, Glx to total creatine, tCr) in the hippocampus. It is important to note that neurometabolic responses are individual-specific, influenced by genetics, metabolism, and environmental factors. Therefore, a longitudinal study would be more appropriate to better capture these variations. Additionally, the reliability and accuracy of 3T MRS in detecting GABA remains a subject of ongoing debate.

Beyond studies involving healthy participants, research in individuals with psychiatric disorders has shown that the season of birth may influence risk, for example in multiple sclerosis (MS).^82,83 Individuals born in the spring months (primarily March, April, and May in the northern hemisphere) appear to have a slightly elevated risk of developing MS.^84,85,86,87 In the southern hemisphere too, a higher risk of MS was observed for individuals born in the southern spring month of November.⁸² This is likely due to reduced sunlight exposure during pregnancy, thereby leading to vitamin D deficiency in the child before birth. This likely compromises their immune system,⁸⁶ making them susceptible to acquiring MS. This purported mechanism is based on our understanding of the relationships between sunlight exposure and immune health. For example, MS relapses are more likely to occur during spring in both the hemispheres.⁸⁸ Further, studies have shown that geographic regions with lower levels of sunlight exposure may also pose comparable risks.⁸² Specifically, relapse timing varies with latitude,⁸⁹ given that the further away from the equator, the less the sunlight exposure.

Similar to MS, seasonal patterns have been observed in other neurological and psychiatric disorders. For example, an increase in admissions of patients with schizophrenia was reported during July and August in England and Wales,⁹⁰ between April and July in Japan,⁹¹ and between January and June in Austria.⁹² Parkinson disease patients have also shown seasonal variation in their hypoactive dopaminergic systems, further suggesting that environmental factors, such as light exposure and seasonal hormonal fluctuations, or other unknown/unidentified parameters, may influence symptom expression and disease dynamics.⁹³

Nonetheless, existing studies are observational in nature, and the role of seasonality in psychiatric and neurological disorders warrants further investigation through rigorous longitudinal neuroimaging studies to unravel the underlying mechanisms.⁹⁴ Taken together, current findings suggest that seasonality exerts a rhythmic influence on brain function across both healthy populations and individuals with psychiatric conditions. This effect appears to be shaped by an interplay of geographical location, environmental cues (such as light exposure), and intrinsic biological rhythms—particularly hormonal fluctuations—that modulate neural and immune system activity in a season-dependent manner. A deeper understanding of these interactions could offer new insights into vulnerability windows, inform preventive strategies, and contribute to more personalized approaches in the diagnosis and treatment of neuropsychiatric disorders.

Systematic review: Seasonality and EEG

Search strategy: while the number of studies with MRI meeting our criteria is small, investigations on the effect of seasonal factors on brain function have been relatively more numerous using other modalities. Observations similar to MRI studies have been made, especially with EEG, strengthening the case that these effects are modality independent. A search was performed with the following query for seasonality and its effect on brain function involving EEG: “(((“seasonality”) OR (“weekly variation”) OR (“monthly variation”) OR (“seasonal variation”) OR (“Circadian activity”)) AND ((“EEG”) OR (“Electroencephalogram”) OR (“electroencephalography”))) NOT (Review).” A manual search returned a few more articles (3) that were included as well. Search results were manually examined by reading the paper title, abstract, and in some cases the full paper to identify relevant content. The search returned 34, 54, and 28 articles with PubMed, WOS, and Scopus, respectively, of which 50 were not relevant, or did not include the acquisition of EEG data, but rather used the term “EEG” for a different research question unrelated to our hypothesis (after removing duplicates). Two more studies were found from a manual search. After reading the title and abstract, followed by the full text of the remaining papers, 18 (16 + 2) studies were found to be relevant that passed the inclusion criterion and are presented in Table 2 along with the PRISMA flowchart in Figure 2, which are discussed next.

Table 2.

Studies related to the effects of seasonality using EEG

Study	EEG characteristic; human (H)/animal (A)	Year of publication
Electroencephalographic studies in toads95	Sleep EEG; A (toads)	1966
Ultradian periodic, diurnal, and annual rhythms in the electroencephalogram96	Beta and alpha EEG; H	1984
P300 seasonal variation97	P300; H	1991
Seasonal variation of the human circadian rhythms (2) sleep EEG98	Sleep EEG; H	1991
Seasonality in human sleep99	Sleep EEG; H	1992
Season, gender, and P300100	P300; H	1994
Sex differences in seasonal variations in P300101	P300; H	1998
Circadian and seasonal variability of resting frontal EEG asymmetry102	EEG asymmetry; H	2009
Photoperiod alters duration and intensity of non-rapid eye movement sleep following immune challenge in Siberian hamsters (Phodopus sungorus)103	Sleep EEG photoperiod variation; A (hamsters)	2012
Should it matter when we record time of year and time of day as factors influencing frontal EEG asymmetry?104	EEG asymmetry; H	2012
Seasonal variation of spontaneous blink rate and beta EEG activity105	Beta activity; H	2018
Seasonal variation in sleep homeostasis in migratory geese: a rebound of NREM sleep following sleep deprivation in summer but not in winter106	Sleep EEG; A (barnacle geese)	2021
Roles of cardiovascular autonomic regulation and sleep patterns in high blood pressure induced by mild cold exposure in rats107	Sleep EEG temperature variation; A (rats)	2021
EEG responses to mood induction interact with seasonality and age108	Alpha EEG; H	2022
Seasonal variation in rest-activity patterns in barnacle geese: are measurements of activity a good indicator of sleep-wake patterns?109	Sleep EEG; A (barnacle geese)	2022
EEG correlates of emotional memory and seasonal symptoms110	EEG power; H	2023
Emotional bias among individuals at risk for seasonal affective disorder—an EEG study during remission in summer111	Alpha EEG; H	2023
Seasonal variation in sleep time: jackdaws sleep when it is dark, but do they really need it?112	Sleep EEG; A (jackdaws)	2024

Figure 2.

PRISMA flowchart concerned with the effects of seasonality studied using EEG

Exclusion criteria: data spanning a month or lower was deemed insufficient to study the effect of seasonality, thereby excluding such studies. Participants with specific ailments such as epilepsy and headache were excluded. EEG studies on sleep without seasonality component were also excluded from this review.

Evidence: researchers have investigated the seasonality of brain function using various EEG characteristics, including sleep EEG, beta and alpha power, event-related potentials (P300), and EEG asymmetry. Below, we discuss each of these themes. Sleep EEG patterns exhibited seasonal variation,⁹⁸ with humans experiencing a phase shift of 1–1.5 h during the winter months.⁹⁹ In the animal world, sleep homeostasis is found to be flexible and season-dependent,¹⁰⁶ which can be characterized using sleep EEG (e.g., see this sleep EEG study in barnacle geese,^106,109 as well as in jackdaws¹¹² and toads⁹⁵). Sleep EEG has also been used to investigate the seasonality of brain function due to variations in temperature¹⁰⁷ and photoperiods.¹⁰³

Beta and alpha EEG activity represent important markers of brain function during task engagements and resting state, respectively, and have been shown to be a marker of seasonality of brain function. For example, a seasonal study on blink rate (an indirect marker of dopamine) showed increased blink rate during summer accompanied by a corresponding increase in EEG beta activity.¹⁰⁵ These increases align with longer photoperiods, when individuals are typically more alert. This increased beta EEG activity that plateaued during the summer months gradually decreases by the end of fall.⁹⁶ Similarly, alpha EEG activity reaches its annual maximum during summer months.⁹⁶ Individuals with high seasonality scores based on the SPAQ, i.e., those whose behavior are more vulnerable to seasonal differences, exhibit elevated alpha power during rest, which diminishes during induced sad mood.¹⁰⁸ Follow-up studies suggest that changes in EEG power in individuals with higher seasonality scores were specific to certain brain regions, time windows, frequency bands,¹¹⁰ and experimental context.¹¹¹

Event-related potentials, particularly the P300 (P3) component linked to attention and decision-making, also exhibit seasonal variation. In young adults, P3 amplitudes in response to auditory stimuli were significantly higher during the spring and summer compared to the fall and winter.^97,100 This seasonal fluctuation in P3 amplitude was more pronounced in women, suggesting heightened sensitivity to seasonal effects in female participants.¹⁰⁰ Further emphasizing sex-specific responses, women have been shown to exhibit greater seasonal sensitivity in cognitive performance, particularly in P300 responses.¹⁰¹ These sex-specific effects of seasonality on EEG P300 components is supported by previous research, which suggest that seasonal affective symptoms such as low mood and anhedonia in winter, and increased fatigue during extended daylight in summer, have been more commonly reported among women than men.¹¹³

Frontal EEG asymmetry, often considered a trait marker for depression, has been shown to vary with time of year (TOY) and time of day (TOD).¹⁰² However, in a replication study, Velo and colleagues found that it was not TOY or TOD alone that influenced frontal EEG asymmetry, but rather the amount of time an individual had been awake prior to measurement acted as the key modulator.¹⁰⁴ This suggests that frontal EEG asymmetry is sensitive to the amount of light exposure. Given that frontal EEG asymmetry is a trait marker of depression, as well as modulated by light exposure, it may be an important EEG biomarker available for understanding the interplay between light exposure, seasonality, and mood disorders.

Gravity

The relative alignment of the Sun, Moon, and Earth manifests itself as lunar phases and eclipses. When the Sun, Moon, and Earth are aligned, but their planes of rotation are different, we get the extremities of new and full moons. When their rotational planes are aligned, we get solar and lunar eclipses on new and full moon days, respectively. Many empirical observations have been made regarding the correlation between the alignment of these celestial bodies (such as lunar phase and eclipse) on the one hand and human brain function and mental health on the other. Since the net gravitational force experienced by humans depends on this alignment, such empirical observations have been attributed to gravity.

Systematic review: Lunar phase, eclipses, and MRI

Search strategy: studies employing MRI to investigate the effects of gravity on brain function, in terms of changes in the gravitational field induced by changing lunar phases and solar/lunar eclipse events, were investigated using the following search query: “((((“solar eclipse”) OR (“lunar phase”) OR (“lunar eclipse”) OR (“lunar rhythm”) OR (“lunar effect”) OR (“lunar cycle”)) AND ((“fMRI”) OR (“functional MRI”) OR (“functional Magnetic Resonance Imaging”) OR (“functional Magnetic Resonance”) OR ((“magnetic resonance imaging”))) NOT (Review).” This returned three articles in total (two PubMed, one WOS, and one Scopus) after removing duplicates, of which only one was relevant as presented in Figure 3. The term “eclipse” was not used in isolation since the studies based on solar or lunar eclipses would use the word “eclipse” in conjunction with “solar” or “lunar” and not use it in isolation.

Figure 3.

PRISMA flowchart concerned with the effects of lunar phase or eclipses on the brain studied using MRI

Evidence: in the one relevant paper found, 15 participants throughout the four lunar phases (new moon, first quarter, full moon, and third quarter) between January 27, 2019 and February 24, 2019 were scanned to observe structural changes. Ventricular, brain hemispheric, hippocampal, and lentiform volumes along with the thalamus diameter were found to be the lowest during new moon (p < 0.05).¹¹⁴ This suggests an inverse correlation between gravitational pull and volume. These results should be interpreted with caution due to the small sample size.

Systematic review: Lunar phase, eclipses, and EEG

Search strategy: studies focusing specifically on EEG to observe the effects of gravity were identified with the following search query: “(((“solar eclipse”) OR (“lunar phase”) OR (“lunar eclipse”) OR (“lunar rhythm”) OR (“lunar effect”) OR (“lunar cycle”)) AND ((“EEG”) OR (“electroencephalogram”) OR (“electroencephalography”))) NOT (Review).” The query returned 25 articles (5, 9, and 11 from PubMed, WOS, and Scopus, respectively), of which 14 were unique and eight were relevant (one from manual search), which are included here. The selected studies were concerned with the effect of lunar phases or eclipses on brain function (flowchart depiction in Figure 4) and are presented in Table 3.

Figure 4.

PRISMA flowchart concerned with the effects of lunar phase or eclipses on the brain measured using EEG

Table 3.

Studies related to the effects of lunar phase or eclipses on the brain measured using EEG

Study	Conclusion	Study Type Human (H)/Animal (A)	Year of Publication
Clinical and electroencephalographic observation on epilepsy during solar eclipse 1980115	refutation	H	1981
Convulsive threshold in humans and rats and magnetic field changes: observations during total solar eclipse116	reduction in the convulsive threshold	H & rats	1981
Evidence that the lunar cycle influences human sleep117	reduced delta activity around the full moon	H	2013
Lunar cycle effects on sleep and the file drawer problem118	refutation	H	2014
Bad sleep? Don’t blame the moon! A population-based study119	no significant differences across moon phasesa	H	2015
Seasonal variation in sleep homeostasis in migratory geese: a rebound of NREM sleep following sleep deprivation in summer but not in winter106	decreased NREM sleep during full moon days	A (geese)	2021
Identifying seizure risk factors: A comparison of sleep, weather, and temporal features using a Bayesian forecast120	seizure risk factor	H	2021
Revisiting an ancient legend: Influence of the lunar cycle on occurrence of first-ever unprovoked seizures121	seizure refutation	H	2022

^afull, waxing/wanning, and new moon.

Exclusion criteria: review or irrelevant studies not presenting explicit EEG data or that did not investigate the relationship between brain function and gravity (solar or lunar phases) were excluded.

Evidence: stories around eclipses have been quite fascinating folklore for ages and have been linked to lunar phases. The scientific relevance behind it is still under exploration. Two main themes emerge from our survey: reduction in sleep and associated EEG changes during full moon as compared to new moon, as well as changes in behavior and corresponding EEG changes in psychiatric populations. Below, we elaborate on these themes.

Recently, experiments proclaimed the dependency of sleep on lunar phases.¹¹⁷ They created a strict environment that continuously measured hormone levels and EEG activity during NREM sleep. Various biases were managed by not informing the participants about certain aspects of the study. The study found a reduction in EEG delta activity around the full moon. However, with the failure to replicate these EEG findings in a similar study, Cordi et al. provided an alternative explanation that discussed publication bias, i.e., the file drawer problem, where not all results are published, especially those with negative outcomes.¹¹⁸ In line with these findings, Haba et al. reported no significant differences in sleep EEG spectral parameters across full, waxing/waning, and new moon phases in a large population-based study.¹¹⁹ The reduction in delta activity refers to a decrease in deep sleep and an increase in light sleep, which might be caused by the increased luminosity during full moon days or perhaps through the additive gravitational effect of both the Sun and Moon experienced on Earth. In the animal world, geese showed a decrease in NREM sleep during full moon days compared to new moon days.¹⁰⁶ The flexibility in sleep thereby links with behavioral and physiological adaptations that are necessary for its survival. Nonetheless, the link between circalunar rhythm and sleep deprivation, which tends to lead to a plethora of psychiatric disturbances in humans, requires empirical validation.¹²²

Numerous researchers and health professionals have studied the impact of lunar phases (full moon or new moon) on behavioral and physiological changes in humans, especially in psychiatric populations.^123,124 Among the eight participants analyzed using the Bayesian seizure forecasting model, a significant relationship between lunar phase and seizure likelihood was observed in only one case.¹²⁰ Similarly, Wang et al. found no significant difference in epileptiform abnormalities among individuals experiencing their first seizure episode across the four lunar phases in a relatively larger sample.¹²¹ On the other hand, Keshavan et al. demonstrated that there was a significant reduction in the convulsive thresholds of both humans and rats at the time of solar eclipse.¹¹⁶ They attribute the underlying mechanism to geomagnetism rather than gravity. This is discussed later in section named “Systematic review: Geomagnetism and EEG.” Despite numerous studies claiming to disprove a link between lunar phases/eclipses and brain function,^115,119,121 their conclusions are often undermined by flawed methodologies, such as failing to control for essential variables like gender, age, chronotype (morningness-eveningness), and light exposure, in addition to lack of comprehensive analysis.

Systematic review: Lunar phase, eclipses, and mental illness

Search strategy: upon reading the papers from our systematic literature search relating to EEG, we noticed that hospital admissions were reportedly affected by eclipses and lunar phases. We thus saw the need for performing a systematic review on hospital admissions related to mental illnesses and their association with lunar and solar rhythms. We used the following search term: “(((“solar eclipse”) OR (“lunar phase”) OR (“lunar eclipse”) OR (“lunar rhythm”) OR (“lunar effect”) OR (“lunar cycle”)) AND ((“mental illness”) OR (“mental patient”) OR (“hospital admission”) OR (“psychiatric”) OR (“hospitalization”) OR (“hospitalizations”) OR (“brain activity”))) NOT (Review).” For additional references, a manual search was performed. The search results were manually examined by reading the paper title, abstract, and in some cases the full paper to identify relevant content. Inclusion criteria were studies that identified mental illness, brain activity, and psychiatric disorders relating to either lunar or solar eclipses and lunar phases. Figure 5 shows the PRISMA flow chart for inclusion and selection of studies. PubMed, WOS, and Scopus returned 42 (41 peer-reviewed, one preprint), 34 (31 peer-reviewed, one Brief Communication, one Letter, one Proceeding paper), and 46 articles, respectively, of which 67 were unique. After including the manual search results (28 papers), a total of 66 (38 + 28) studies were found to be relevant and are listed in Table 4.

Figure 5.

PRISMA flowchart concerned with the effects of lunar phases or eclipses related to hospital admissions

Table 4.

Studies documenting the association between lunar phases or eclipses with hospital admissions

Study	Year of publication
(1968–1999)
Lunar effect on mental illness: The relationship of moon phase to psychiatric emergencies125	1968
Homicides and the lunar cycle: Toward a theory of lunar influence on human emotional disturbance126	1972
The questionable relationship between homicides and the lunar cycle127	1974
A psychosocial study of 292 schizophrenic patients treated in a psychiatric hospital128	1977
Lunar cycles and emergency room visits129	1978
Human aggression and the lunar synodic cycle130	1978
The lunar effect: Biological tides and human emotions131	1978
The moon and madness: A comprehensive perspective124	1980
Behavioral and tetratogenic effects of solar eclipse132	1980
Convulsive threshold in humans and rats and magnetic field changes: observations during total solar eclipse116	1981
Effects of total solar eclipse on mental patients: A clinicobiochemical correlation133	1981
Factors associated with the seclusion of psychiatric patients134	1983
Full moon and crime135	1984
Much ado about the full moon: A meta-analysis of lunar lunacy research136	1985
Lunar phase and acting-out behavior137	1986
Exploring environmental cycles in psychiatric patients138	1989
Trauma and the full moon: A waning theory139	1989
Parasuicide and the lunar cycle140	1991
Geophysical variables and behavior: LXXII. Barometric pressure, lunar cycle, and traffic accidents141	1993
Lunar phases and psychiatric hospital admissions142	1994
Lunar rhythms of the meal and alcohol intake of humans143	1995
Frequency of contact with community-based psychiatric services and the lunar cycle: a 10-year case-register study144	1997
Lunar cycle and consultations for anxiety and depression in general practice145	1997
Lunar cycles and violent behavior146	1998
Lunar phase and psychiatric illness in goa147	1999
Lunar cycles and presentations to a community assessment and treatment (crisis) team123	1999
(2000–2014)
Lunacy revisited367	2000
Relationship of the lunar cycle and the presentation of individuals with psychiatric problems to an accident and emergency department: A case-control study170	2001
Solar eclipse and suicide185	2002
The yearly distribution of suicide and parasuicide151	2002
Self-inflicted burns: A sporadic phenomenon176	2004
Anticipation of total solar eclipse and suicide incidence186	2004
Influence of lunar phases on suicide: The end of a myth? A population-based study158	2005
Meteorologic factors and subjective sleep continuity: A preliminary evaluation182	2005
No effect of lunar cycle on psychiatric admissions or emergency evaluations160	2006
A link between lunar phase and medically unexplained stroke symptoms: an unearthly influence?179	2008
Association of environmental factors with the onset of status epilepticus149	2008
Moonstruck? The effect of the lunar cycle on seizures159	2008
Admission to intensive care for parasuicide by self-poisoning: variation by time cycles, climate, and the lunar cycle169	2008
The dark side of the moon156	2009
Relationship between lunar phases and serious crimes of battery: A population-based study157	2009
Emergency psychiatric condition, mental illness behavior, and lunar cycles: Is there a real or an imaginary association?162	2010
Lunar phase cycle and psychiatric hospital emergency visits, inpatient admissions and aggressive behavior148	2011
Impact of seasonal and lunar cycles on psychological symptoms in the ED: An empirical investigation of widely spread beliefs163	2013
Aneurysmal subarachnoid hemorrhage: Relationship to solar activity in the United States, 1988–2010184	2014
Pediatric psychiatric emergency department visits during a full moon166	2014
Lunar cycle effect on patient visit to psychiatry hospital emergency room: Studying the ‘Transylvanian effect’ in an Islamic society171	2014
Association between lunar phase and sleep characteristics181	2014
(2015–present)
mOONSTROKE: Lunar patterns of stroke occurrence combined with circadian and seasonal rhythmicity: A hospital-based study178	2015
Psychiatric presentations during all 4 phases of the lunar cycle165	2017
The association between lunar phase and intracranial aneurysm rupture: Myth or reality? Own data and systematic review175	2017
No association of moon phase with stroke occurrence180	2018
Exploring the potential psychiatric implications of astronomical phenomena164	2019
Is it the moon? Effects of the lunar cycle on psychiatric admissions, discharges, and length of stay161	2019
Lunar cycle and psychiatric hospital admissions for schizophrenia: new findings from Henan province, China153	2020
Occurrence of behavioral changes and its management in persons with mental illness due to lunar effects152	2020
Does lunar synodic cycle affect the rates of psychiatric hospitalizations and sentinel events?150	2021
Relationship of the lunar cycle and seasonality with stroke174	2021
Decoupling of global brain activity and cerebrospinal fluid flow in Parkinson disease cognitive decline155	2021
The influence of moon phases on the frequency of admissions to a psychiatric hospital172	2022
Revisiting an ancient legend: Influence of the lunar cycle on occurrence of first-ever unprovoked seizures121	2022
Don’t blame it on the Moon168	2023
Is there an association between the lunar phases and hospital admission for different episode types in bipolar disorder? A retrospective study in northern China167	2023
The effects of lunar phases and zodiac signs on recurrent youth suicide attempts: Experience of University Hospital173	2023
Analysis of patient presentations to the emergency department due to anxiety associated with the lunar cycle and seasonality183	2024
Subarachnoid hemorrhage incidence pattern analysis with circular statistics177	2024

Exclusion criteria: studies that primarily investigated the relationship between lunar phases and other physiological effects on the human body (nose bleeding, renal colic, gastrointestinal hemorrhage, cardiac, and so forth) unrelated to the brain were excluded.

Evidence: Parmeshwaran et al.¹⁴⁷ studied the effects of full moon, new moon, and other lunar phases on mental health in patients with non-affective psychosis, depression, and mania. The results showed a significant relationship between mental health and lunar phase, with a greater number of new patients admitted with non-affective psychosis on full moon days, more so during visible lunar eclipse days (specifically the ones visible in the study location: Goa, India). Similar observations were made by other researchers on increased admission of patients with mental illness,^138,148 status epilepticus (SE),¹⁴⁹ occurrences of sentinel events that necessitate chemical restraints in psychiatric patients,¹⁵⁰ and parasuicide cases.^140,151 The number of admissions with the onset of status epilepticus was minimal 3 days prior to new moon, but the numbers increased significantly 3 days after new moon.¹⁴⁹ With psychiatric conditions, individuals showed mild behavioral changes specially during new moon and full moon days,¹²⁴ which could be controlled with medication.¹⁵² Categorically, schizophrenic patients were seen to have aggravated symptoms during specific lunar phases,¹²⁸ particularly during the first quarter and full moon.¹⁵³

Many mechanisms have been proposed to explain the link between brain dysfunction in psychiatric disorders and lunar phases/eclipses. One such hypothesis relies on the effect of gravity on the cerebrospinal fluid (CSF). Accordingly, the gravitational effect experienced by a human on Earth is the strongest during the new and full moon days when the Sun, Earth, and Moon align in a straight line. This gravitational effect is the reason behind the formation of tides on Earth that have a 6-h cycle. While variations in gravity due to lunar phases (having a 15-day cycle) are primarily caused by the changing angle subtended between the Earth, Sun, and Moon, the variations in gravity during the 6-h tidal cycle (two high tides and two low tides occur in a day) are caused by the changing distance between the ocean and the Moon as the Earth rotates. To begin hypothesizing how gravity could possibly affect human biology, a useful analogy to understand the concept is to compare ocean tides with “biological tides” that are considered to occur in the fluid-containing compartments of the human body, including the CSF in the ventricles of the central nervous system.¹³¹ In the neuroscience literature, the cyclical CSF flows, along with other hormonal shifts, is found to be correlated with emotional disturbances, leading to increased homicides during new and full moon days at perigee.¹²⁶ CSF flow has been found to modulate brain function and vice versa.^154,155 During sleep, the brain consolidates memory, and the CSF drains out metabolic wastes. There exists a coupling between neural and hemodynamic rhythms and the CSF rhythm during non-rapid eye movement (NREM) sleep.¹⁵⁴ There is a decrease in this coupling in patients suffering from Parkinson disease with mild cognitive impairment.¹⁵⁵ The potential mechanism by which variations in gravity modify brain function can potentially be explained with the interlink between CSF flow and brain function. These are speculations that are in their nascent stage and require further investigation to test the hypothesis.

The interplay between science, myth, and societal influences on the effects of lunar phases on homicides (violent behavior) and/or heightened psychological symptoms is fascinating. On one hand, some studies report a positive correlation between lunar phases (full moon) and violent behavior,^130,135,156 and on the other, some studies have refuted these claims,¹²⁷ with more recent research employing advanced statistical methods to challenge these findings.¹⁵⁷ For example, a population-based study in Bavaria, Germany found no association between lunar phases (full, new, and the interphases of moon) and serious crimes such as aggravated assaults,¹⁵⁷ but a small trend in non-violent suicides during new moon in men under 40 years of age was observed.¹⁵⁸ One reason for these discrepancies is the varying definitions of lunar phases used in research, with studies considering different time windows (12, 24, 48, or 72 h) to determine statistical significance. Other sources of variation such as varying sample sizes as well as studies performed at different locations, where luminosity based on latitude¹⁵⁹ can be significantly different, can impact the results.

The debate around the effects of eclipses or lunar phases on mental illnesses has yet to see a definitive conclusion. There are articles that question these inferences of any correlation between lunar phase and brain function based on admission of patients during these times.^{123,125,129,134,137,142,144,145,146,160,161,162,163,164,165,166,167,168,169,170,171,172} The association between lunar phases and human behavior was specifically questioned after a meta-analysis¹³⁶ did not find a statistically significant association of lunar rhythm with nutrient intake,¹⁴³ parasuicide cases,¹⁴⁰ first unprovoked seizures,¹²¹ or behavioral discrepancy.¹⁴¹ Additionally, researchers noted no correlation between lunar phases and recurrent suicide attempts,¹⁷³ ischemic stroke occurrence,¹⁷⁴ intracranial aneurysm rupture,¹⁷⁵ self-inflicted burns,¹⁷⁶ subarachnoid hemorrhage and timing of the same,¹⁷⁷ or major trauma during full moon.¹³⁹ The collection of findings present conflicting viewpoints.

On one hand, some studies direct researchers to believe that there is a significant relation between lunar phases and stroke incidence,^178,179 and on the other hand, there are rebuttals to the same.¹⁸⁰ Full moon nights are the most illuminated, potentially affecting sleep patterns. A retrospective study found reduced sleep efficiency in women on full moon days.¹⁸¹ On the contrary, another study reported no strong association between ambient light and lunar phase (lunar illumination) with participant’s sleep duration.¹⁸²

Both the associations and refutations mentioned in this section were conducted 10–55 years ago, and scientific question remains unresolved. This question has not been mechanistically probed in the recent past, even though cutting-edge technologies and new analysis techniques have emerged since. Although we observed conflicting outcomes, it is important to note that many of the studies lacked rigor, did not probe directed questions, and their methodology was questionable. For instance, all psychiatric conditions cannot be combined together under a single umbrella; emergency visits of each hospital division with specific medical history need to be analyzed separately. A recent study with mostly female young adults (18–44 years) analyzed anxiety-related emergency visits.¹⁸³ An increase in visits was observed during summer and the waning gibbous phase, while frequent hospitalizations were noted in winter, primarily among young male patients with psychiatric diagnoses. This example illustrates the value of targeted analysis and stratification by demographic and clinical factors, which can uncover nuanced patterns that broad, undifferentiated studies may miss.

In addition to lunar phases, the effects of a total solar eclipse have also been studied. The prolactin hormone, which has a causative link with behavioral changes, was increased during and just after a solar eclipse.¹³³ The reduction of convulsive threshold in humans as well as in rats undergoing electroconvulsive therapy was observed during a total solar eclipse.¹¹⁶ Similarly, a study on rodents showed fetal deaths in the case of pregnant mothers exposed to light during a solar eclipse.¹³² On the contrary, one study reported trends but not statistically significant association between lunar phase and solar eclipse on the one hand, with the behaviors of patients with psychiatric disorders on the other hand.¹⁶⁴ Rosenbaum et al. performed statistical analysis of the nationwide inpatient sample (NIS) from 1988 to 2010 and showed lower incidence of aneurysmal subarachnoid hemorrhage during periods of relative maximum solar activity (solar flux and sunspots).¹⁸⁴ However, others have not been able to replicate this result.¹⁷⁷ Voracek et al. reported that media attention and anticipation surrounding a total solar eclipse in Austria (1999) led to reduced suicide rates before the eclipse, although there was no decrease in numbers on the day of the event.¹⁸⁵ Moreover, the same group found a similar trend in Romania but not in Latvia (which was not in the path of totality for the same total solar eclipse of 1999).¹⁸⁶ In view of the evidence presented here, lunar phases and eclipses demonstrate the possibility of influencing brain health. However, objective measures and controlled brain experiments are required to identify the neural basis of such effects.

Systematic review: Gravity and brain function

From the literature discussed so far, it is apparent that gravity is a possible mechanism through which the Sun and Moon could potentially influence human biology on Earth, which become pronounced during extremum points such as new moon, full moon, and eclipses. Space travel by Yuri Gagarin in 1961 followed by others exposed humans to hetero gravity, sparking scientific interest in the effects of changes in gravity on human physiology.^187,188 Humans are conditioned to the 1G environment (gravitational force experienced by us on the surface of the Earth). Any change in it imposes several changes in body physiology. Increased space travel, both for investigation and commercial reasons, has increased research interest in the effects of various gravitational conditions on the brain (especially cognitive functioning). The 0G condition is understood as weightlessness, and the force stronger or weaker than the 1G terrestrial gravity value is termed as hypergravity or microgravity, respectively. To simulate altered gravitational conditions for research purposes, various models are employed. Parabolic flights recreate alternating gravitational loads—typically transitioning from 1G to 1.8G (during pull-up), to 0G (microgravity), back to 1.8G (pull-out), and finally returning to 1G.¹⁸⁹ Ground-based analogs include head-down tilt (HDT) in humans (typically at −6°or −15°), tail-suspension models in rodents to simulate microgravity, and short-arm human centrifugation to simulate hypergravity.

The years following the first human space flight in 1961 have seen efforts to identify the effects of such flights on the brain after the astronauts complained of nausea and disorientation during weightlessness. The frontal lobe, the control center of the brain, became the primary focus to study space adaptation.¹⁹⁰ Like humans,¹⁹¹ rats exhibited motion sickness under hypergravity conditions. Studies found a decrease in motion sickness in rats (using pica behavior as an index) with an amygdalar lesion and an increase in the same with hippocampal lesion.¹⁹² Amygdala is responsible for developing and getting accustomed to motion sickness caused due to prolonged exposure to hypergravity in rats.¹⁹³ The hypothesis of changed brain function and efficiency during hypergravity could be due to reduced brain energy metabolism as observed in rats.¹⁹⁴ Investigating further, exposure of rats to hypergravity conditions (1G, 2G, and 4G) via centrifugation led to increased Fos protein expression, a neuronal activity marker, within the hypothalamic-pituitary-adrenal axis,¹⁹⁵ indicating activation of stress-related neuroendocrine pathways.

EEG studies have explored the neural effects of altered gravitational conditions during parabolic flights, which involve both hyper- and hypogravity. Schneider et al. reported alterations in both EEG beta and alpha activity during hypo- and hypergravity conditions.^{189,196,197,198} To gain a more comprehensive understanding of brain function during altered gravity, multimodal neuroimaging approaches offer additional insights. A combined analysis using EEG, low-resolution electromagnetic tomography (LORETA), and functional near-infrared spectroscopy (fNIRS) revealed focal changes in electrocortical activity in Brodmann Areas 6 and 9 (frontal lobe) during weightlessness in a parabolic flight.¹⁹⁹ This is hypothesized to be due to psychological stress.²⁰⁰ In contrast, others have shown global changes in cortical activity using EEG and transcranial Doppler ultrasonography at 0G.²⁰¹ Although these findings offer important insights into the neural effects of relatively large changes in gravity, the present review centers on more subtle variations in gravitational influence caused by seasonal variations. Accordingly, these results are not discussed in further detail. Rather, we focus on a systematic review of the effects of relatively smaller changes in gravity on brain function.

Search strategy: to explore studies investigating the effects of smaller changes of gravity on brain function, we conducted the following search: “(((“brain activity” OR “brain function” OR “brain cortical activity” OR “brain cortical function” OR “brain functional activity”) AND ((“microgravity” OR “gravitational field” OR (“weightlessness”)) NOT (“hypergravity”)))) NOT (Review)”. The 1G gravitational force is toward the center of the earth, while the lunar and solar gravitational fields exerted on an object is away from the center of the earth. Therefore, seasonal variations in the gravitational field experienced by an object on the surface of the earth are always less than 1G. Therefore, we have used the term “microgravity” in the search. The search returned 47, 46, and 91 articles from PubMed, WOS, and Scopus, respectively, of which 102 were unique. The search results were manually examined by reading the paper title and abstract followed by the full paper to identify relevant content. A manual search returned five additional relevant articles. These 47 papers (42 + 5 in total) are presented in Table 5 along with the PRISMA flowchart in Figure 6.

Table 5.

Studies related to brain function under various gravity conditions

Study	Duration/Condition	Method/Model	Year
Clinorotation-induced weightlessness influences the cytoskeleton of glial cells in culture202	30 min microgravity	cell level (rats); glial cells	2002
Proteomic analysis of mice hippocampus in simulated microgravity environment203	microgravity	cell level (mice)	2006
Proteomic analysis of mouse hypothalamus under simulated microgravity204	microgravity	cell level (mice)	2008
Differential expression of specific cellular defense proteins in rat hypothalamus under simulated microgravity induced conditions: Comparative proteomics205	microgravity	cell level (rats)	2014
Transcriptomic analysis of embryonic mouse hypothalamic N38 cells exposed to high-energy protons and/or simulated microgravity206	microgravity	cell level (mice)	2024
Changes of brain response induced by simulated weightlessness207	(−15° and 45°)	HDT and HUT (H)	1992
The effect of head-down tilt on brain potentials related to visual attention208	(−10° and 20°)	HDT and HUT (H)	1995
Dynamic change of ERPs related to selective attention to signals from left and right visual field during head-down tilt209	(−10° and 20°)	HDT and HUT (H)	1998
Temporal and spatial features of slow positive potential related to visual selective response during head-down-tilt210	(−10° and 20°)	HDT and HUT (H)	1998
Effect of simulated weightlessness on the response characteristics of human brain211	(−10 to 15)∘)	HDBR (H)	1989
Brain microstructure and brain function changes in space headache by head-down-tilted bed rest212	40 min (−10°)	HDBR (H)	2023
Influence of body position on cortical pain-related somatosensory processing: An ERP study213	90 min (−6°)	HDBR (H)	2011
Altered baseline brain activity with 72 h of simulated microgravity: Initial evidence from resting-state fMRI214	03 days (−6°)	HDBR (H)	2012
Altered regional homogeneity with short-term simulated microgravity and its relationship with changed performance in mental transformation219	03 days (−6°)	HDBR (H)	2013
The time course of altered brain activity during 7-day simulated microgravity221	07 days (−6°)	HDBR (H)	2015
Default network connectivity decodes brain states with simulated microgravity220	07 days (−6°)	HDBR (H)	2016
Objects mental rotation under 7 days simulated weightlessness condition: An ERP study222	07 days (−6°)	HDBR (H)	2017
Executive function on 16 days of bed rest in young healthy men223	16 days (−6°)	HDBR (H)	2009
Effect of simulated microgravity on human brain gray matter and white matter: Evidence from MRI227	30 days (−6°)	HDBR (H)	2015
Structural brain changes following long-term 6° head-down tilt bed rest as an analog for spaceflight226	40+ days (−6°)	HDBR (H)	2015
Decreasing ventromedial prefrontal cortex deactivation in risky decision-making after simulated microgravity: Effects of −6° head-down tilt bed rest229	45 days (−6°)	HDBR (H)	2014
Effects of prolonged head-down bed rest on working memory228	45 days (−6°)	HDBR (H)	2015
Decision making after 50 days of simulated weightlessness224	60 days (−6°)	HDBR (H)	2009
Head-down tilt position, but not the duration of bed rest, affects resting state electrocortical activity225	60 days (−6°)	HDBR (H)	2021
Effects of a spaceflight analog environment on brain connectivity and behavior244	70 days (−6°)	HDBR (H)	2016
Increased brain activation for dual tasking with 70 days of head-down bed rest231	70 days (−6°)	HDBR (H)	2016
Change of cortical foot activation following 70 days of head-down bed rest232	70 days (−6°)	HDBR (H)	2018
Vestibular brain changes within 70 days of head-down bed rest241	70 days (−6°)	HDBR (H)	2018
Simulated weightlessness procedure, head-down bed rest has reversible effects on the metabolism of rhesus macaque230	42 days (−6°)	HDBR (rhesus monkey)	2024
Touch down: The effect of artificial touch cues on orientation in microgravity246	ISS	spaceflight	2006
Human cerebral autoregulation before, during and after spaceflight248	1 and 2 weeks	spaceflight	2007
Effect of actual long-term spaceflight on BDNF, TrkB, p75, BAX, and BCL-XL genes expression in mouse brain regions240	1 month	spaceflight	2015
Cortical reorganization in an astronauts brain after long-duration spaceflight243	169 days	spaceflight	2016
Brain structural plasticity with spaceflight237	2 weeks and ISS mission of 6 months	spaceflight	2016
Brain tissue-volume changes in cosmonauts238	long duration for 209 days; short duration for 9 days	spaceflight	2018
Prolonged microgravity affects human brain structure and function235	long duration for 162.7 ± 21.8 days; short duration for 14.7 ± 1.6 days	spaceflight	2019
Intracranial effects of microgravity: A prospective longitudinal MRI study234	long duration	spaceflight	2020
Optic nerve length before and after spaceflight239	microgravity at the ISS; microgravity	spaceflight	2021
Brain connectometry changes in space travelers after long-duration spaceflight233	6 months	spaceflight	2022
Brain potential responses involved in decision-making in weightlessness245	6 months	spaceflight	2022
Brain and behavioral evidence for reweighting of vestibular inputs with long-duration spaceflight242	6 months	spaceflight	2022
Mice display learning and behavioral deficits after a 30-day spaceflight on Bion-M1 satellite218	30 days	spaceflight (mice)	2022
Changes in working memory brain activity and task-based connectivity after long-duration spaceflight247	6 months	spaceflight	2023
Impacts of spaceflight experience on human brain structure236	long duration for 6/12 months; short duration for 2 weeks	spaceflight	2023
Activation of HIF-1α and its downstream targets in rat hippocampus after long-term simulated microgravity exposure215	28 days (30°);	tail suspension (rats)	2017
Early changes to the extracellular space in the hippocampus under simulated microgravity conditions216	day 3 and 7	tail suspension (rats)	2022
Effects of short-term simulated microgravity on changes in extracellular space structure and substance diffusion and clearance217	3, 7, and 14 days	tail suspension (rats)	2024

Note: the studies are listed in groups based on the Method/Model of research. Method/Model in order: Cell Level; combination of Head Down Tilt (HDT) and Head Up Tilt (HUT); Head Down Bed Rest (HDBR); Spaceflight (some to the International Space Station [ISS]; Tail suspension). (H) are human studies.

Figure 6.

PRISMA flowchart concerned with the effects of different gravity conditions on brain function

Exclusion criteria: studies focusing on the following topics were excluded: traumatic brain injury; hypergravity; artificial gravity greater than 1G; parabolic flight experiencing 1.8G (hypergravity); head-down bed rest (HDBR) with elevated carbon dioxide concentration mimicking space environment; effects of space radiation; potential of microgravity to cure depression; and weightlessness due to dry immersion (which emphasizes the effects of support withdrawal).

Microgravity/zero gravity/weightlessness: cellular mechanobiology informs us on how crucial it is to understand the effects of changes in gravitational forces, be on earth or in space. Glial cells (C6 line derived from rat brain tumor) that are fundamental to brain function (supporting the functioning of the central nervous system) experience cytoskeleton disruption after 30 min of exposure to microgravity.²⁰² The effect weakens with prolonged period of exposure (20 h). Similarly, mouse hypothalamic cell line (N38) exhibited transcriptomic changes under microgravity, including alterations in genes UCN2 and UGT1A5, which are associated with brain function.²⁰⁶ Such research at the cellular level is sparse, but exploring further with varied simulated hypogravity conditions is informative.

Another widely used research model is to study rats, mice, and other mammals with similar genetic expression to humans. A proteomic analysis showed an alteration in the antioxidant system in the hypothalamus,²⁰⁵ pointing to oxidative imbalance in mice under simulated microgravity.²⁰⁴ Similar analysis exhibited loss of proteins in the hippocampus too.²⁰³ Increased oxidative stress was also observed in rat models under long-duration simulated microgravity environment.²¹⁵ However, short-duration exposure experiments identified an increase in the volume fraction of the hippocampal extracellular space, where interstitial fluid drainage was initially expedited (3 days) but slowed after 7 to 14 days.^216,217 Whether poor drainage directly or indirectly causes oxidative stress leading to protein loss or vice versa remains unclear. However, it is well studied that poor drainage hinders effective brain function, leading to cognitive decline as observed in mice after a 30-day spaceflight on Bion-M1 satellite with a gradual and incomplete rebound.²¹⁸

Further investigating the effects of weightlessness, HDT, also denoted as HDBR by some researchers, is an effective Earth-based analog for simulating both short- and long-duration microgravity exposure. Wei et al. in the early to late 1990s extensively investigated the effects of simulated weightlessness by varying HDT between −10° and −15°, comparing it with head-up tilt (HUT) at 20°and 45°.^207,211 Investigations of somatosensory evoked potentials with simulated weightlessness by (−10° to 15°) HDT indicated changes in brain function.^207,211 Their studies revealed changes in EEG power spectra, slow positive potentials, and event-related potentials (ERPs), notably highlighting increased positive potentials that reflect inhibitory processes in the brain, suggesting a reduced capacity for sensory processing and attentional regulation under simulated microgravity.^208,209,210 These findings highlight the importance of careful and thorough study of the effects that have an impact on brain function and cognitive abilities.²⁰⁷

As mentioned previously, researchers studied HDBR condition with a 6°(as well as 10°or 15°) downward head inclination for a varied set of timelines. The setup mimics space environment (weightlessness) well devoid of cosmic radiation, fluctuating temperature, and elevated CO2 levels. An fMRI investigation showed a decrease in the amplitude of the low-frequency fluctuations (ALFF) in the left thalamus during microgravity after 3 days of −6° HDBR.²¹⁴ A follow-up study found change in the mental transformation ability, along with an increase in regional homogeneity (ReHo) in bilateral medial frontal gyrus and left superior frontal gyrus in the same microgravity analog.²¹⁹ Studies on 7 days of HDBR observed an increase in rs-fMRI functional connectivity in the default mode network²²⁰ and emphasized that the brain has the capability to self-adjust and adapt to the unique environment of microgravity,²²¹ yet the reaction time to mental rotation task does increase.²²²

Durations as low as 40 or 90 min of HDBR reveals key outcomes; for example, during microgravity subjects exhibit reduced pain perception²¹² and altered pain regulation capabilities.²¹³ It is important not only in the context of altered gravity conditions in space but also how pain is perceived in patients confined in bed and what implications a change in gravity can have on Earth. While a short-term (16 days) HDBR study did not find any alteration in executive function,²²³ research on 60 days of HDBR highlights a reduction in executive function mediated by a lack of physical activity.²²⁴ Subsequent studies on prolonged HDBR confirmed the effects of physical activity/exercise on physical activity.²²⁵ Exercise might mitigate fluid shifts in weightlessness conditions. In particular, morphological changes such as alteration in CSF homeostasis²²⁶ and gray matter changes in specific regions of the brain responsible for performance, locomotion, learning, memory, and coordination²²⁷ are apparent in prolonged HDBR studies (40+ days and 30 days, respectively). These changes might have an impact on the decrease in reaction times during working memory task²²⁸ or a decrease in activations in higher level cognitive functions²²⁹ probably due to neurotransmitter imbalance as seen in Rhesus macaque monkeys.²³⁰ However, dual tasks such as foot tapping while performing HDBR for 70 days shows increased connectivity in the cerebellum, fusiform gyrus, hippocampus, and middle occipital gyrus, implying an increase in neural control to compensate for the absence of somatosensory inputs and mitigate the effects of HDT.^231,232 Despite an array of HDBR studies, it is difficult to identify the neural basis due to lack of cohesive studies.

Having said that, spaceflight studies are another way to observe structural and functional changes. However, there are certain constraints in obtaining the data. Despite the difficulty of using MRI to observe an astronaut’s brain in a real-world situation, many studies acquire pre- and post-flight data. These studies on cosmonauts aboard the International Space Station observed microstructural changes in white matter pathways largely within the corpus callosum, arcuate fasciculus, corticospinal, corticostriatal, and cerebellar tracts.²³³ Furthermore, increased pituitary deformation, altered CSF hydrodynamics, and an increase in brain and fluid volume²³⁴ were also observed, along with significant changes in total ventricular volume contingent to mission duration.²³⁵ A more detailed study correlating these changes with time spent in space reported that the right lateral and third ventricles expand significantly within the first 6 months, followed by a slower rate of expansion over extended durations.²³⁶ Apart from expanded ventricles, a decrease in gray matter volume was also reported in the temporal and frontal poles around the orbit²³⁷ and orbitofrontal and temporopolar regions.²³⁸ Yet another structural modification, clinically termed as the spaceflight-associated neuro-ocular syndrome (SANS), was reported, whereby the optic nerve length increases along with displacement of the optic chiasm and the nerve head.²³⁹ These are mostly due to certain physiological parameters (age and weight) and long-term microgravity exposure (around 6 months) that causes a pressure difference between the brain, the ventricles, and the eye. In contrast, a spaceflight as low as a month is enough to cause gene dysregulation that poses a risk to the BCL-XL gene responsible for behavioral abnormalities in the long-duration spaceflights.²⁴⁰

Microgravity is also considered to cause adaptive neural changes in the vestibular system as seen in both 70 days −6°HDBR²⁴¹ and 6-month-long spaceflight.²⁴² Astronauts on returning to Earth demonstrate compromised mobility and balance due to alterations in vestibular signaling and sensory reweighting that occurs to acclimatize them in space environmental conditions. Post-flight rs-fMRI (9 days post-entry, after a mission of 169 days) showed reduced connectivity within the right insula and posterior cingulate cortex,²⁴³ and a similar outcome was observed with 70 days HDBR.²⁴⁴ Other studies have also shown alterations in brain activity, connectivity, and associated behavior²⁴⁵ due to spaceflight-induced microgravity.^243,244,246 Connectivity alterations in sensory, cognitive, and motor regions may reflect a mix of neural disruption and compensatory adaptation.²⁴⁷

The hypothesized mechanism of brain activity changes in microgravity are based on altered fluid flow (blood and CSF) induced by changes in gravity. It is easier for the heart to pump blood to the brain during microgravity, which leads to the puffed faces seen in astronauts while in space. On Earth, exercise, physical exertion, and importantly inversion therapy (a posture that increases the flow of blood to the brain with the head in a position lower than the heart) cause an increase in cerebral blood flow. The brain maintains stability in such situations by a physiological process called cerebral autoregulation. While cerebral autoregulation is impaired during spaceflight, it is preserved upon return to Earth, and short-duration spaceflights may even enhance it.²⁴⁸ Stable cerebral blood flow is essential for optimal brain function. Although astronauts embarking on spaceflights are mostly exposed to microgravity, they experience periods of hypergravity during launch and re-entry. Spaceflight studies presented here must be viewed in that light.

Effects of terrestrial gravity alterations on brain function

Distinct from these simulations of externally induced gravity changes, there are natural variations in the earth’s gravitational field that must be taken into account. The Earth’s gravitational field oscillates cyclically. Researchers have fit these data to a sine wave with a period of 5.9 years.^249,250 The purported mechanism underlying this oscillation is thought to be the cyclical nature of the relative positions of other celestial bodies in the solar system with respect to the Earth.²⁵¹ The Earth is not a perfect sphere; it exhibits an equatorial bulge due to its rotation. As a result, the maximum distance from the Earth’s center to its surface occurs not at the summit of Mount Everest but along the equator. This geometric difference, combined with the effects of centrifugal force from Earth’s rotation, means that the gravitational acceleration is actually slightly weaker at the equator than at the poles. In general, Earth’s gravitational field is strongest near the surface and decreases with increasing altitude or depth, governed by the planet’s mass distribution and density gradients. While small variations in local gravitational acceleration can occur due to atmospheric mass changes or seismic events, these effects are extremely subtle and not perceptible to humans. Solar eclipses may produce minor gravity anomalies through large-scale atmospheric redistribution.²⁵² High-magnitude earthquakes can shift Earth’s mass and slightly alter regional gravity fields²⁵³ as observed by satellite gravimetry missions such as GRACE.²⁵⁴ Other phenomena such as solar flares may indirectly influence gravity via atmospheric coupling, though empirical evidence for this remains limited and speculative.

Search strategy: thereby, we performed a systematic review on the effects of terrestrial gravity changes on brain function to highlight any studies, if any. The following search query was implemented: “(“brain activity” OR “brain function” OR “brain cortical activity” OR “brain cortical function” OR “brain functional activity”) AND ((“earthquake” OR “Seismic activity” OR “terrestrial gravity fluctuations” OR “geological fault”) NOT (“PTSD”) NOT (“hypergravity”) NOT (“microgravity”) NOT (Review).” It returned 50 articles (nine from PubMed, 16 from WOS, and 25 from Scopus), of which none of the studies were relevant as depicted in Figure 7.

Figure 7.

PRISMA flowchart concerned with the effects of terrestrial gravity alterations on brain function

Evidence: unlike artificially introduced hypo- or hypergravity, the variation in the gravitational effects introduced by changes in the alignment of the Earth, Moon, Sun, and other cosmic bodies is quite small. Nevertheless, research on artificially introduced hypo- and hypergravity is important to appreciate the fact that changes in gravity, in-principle, affect brain physiology. It remains to be seen whether small changes in gravitational effects experienced by humans due to seasonal/cyclical factors are enough to change brain function so as to affect behavior and clinical presentation. Future controlled experiments should probe this question.

Geomagnetism

Geomagnetism, i.e., the earth’s magnetic field, is known to vary across space and time. Given that the changes in magnetic field may affect brain function, this is a potential factor impacting the seasonality of brain function. The Earth’s magnetic field is a result of a giant bar magnet (comprising of molten iron) passing through the central longitudinal axis, thereby creating a dipole magnetic field as shown in Figure 8. This magnetic field intensity is neither geographically constant across the globe nor constant over time at a given location, as seen in Figure 9. For instance, it has a positive variation over the Asian subcontinent, and maximum and minimum intensities are observed around the poles and equator, respectively. The variation in the geomagnetic field is due to an interplay of complex factors: (1) the gradual drift of the magnetic poles, (2) periodic reversal of the polarity of the north and south poles, and (3) disturbances caused by solar winds and flares that perturb the magnetosphere, termed as geomagnetic activity. According to The Russell-McPherron (R-M) effect, these geomagnetic activities are seasonal, with peaks occurring around the equinoxes (spring and fall/autumn).^255,256

Figure 8.

The Earth’s magnetic field with its field lines

The flares originating from the sun interferes with these field lines, disrupting the geomagnetic field.

Figure 9.

The isodynamic chart of Earth’s magnetic field

Left: map of magnetic field intensity contour for 2020, where we can observe the gradual decrease in intensity from the poles to the equator. Right: map of predicted annual rate of change of intensity from 2020 to 2025. (Reproduced from http://www.geomag.bgs.ac.uk/education/earthmag.html).

The migratory nature of birds

In the year 1822, evidence surfaced for the migration of birds. About 2,000 species of birds migrate several hundreds to thousands of miles every year following the changing patterns of the seasons. However, the mystery is how these birds travel such great distances without the help of a compass. With the availability of technology at our disposal, such as radiotelemetry²⁵⁷ and GPS trackers,^258,259 it is possible to document the path traversed by these birds, and so the effects of external interventions can be determined. The European robin is widely used to investigate such behavior. It was observed that the path of these birds was influenced by the wavelength of light (disruption of migratory path in artificially produced 633 nm red light) and magnetic pulse (0.5T for 4 ms),²⁶⁰ as well as magnetic field intensity along with the inclination angle,²⁶¹ and interference by artificially produced radiofrequency signals in the MHz range.⁹

The hypotheses behind the migratory nature

For decades, an array of hypotheses have been proposed to explain the magnetoreceptive property seen in the animal world. The three most popular models are the optical one,²⁶² chemical one,²⁶³ and the ones involving permanent magnets.^264,265 Walcott and colleagues inspected the tissue of the Harderian gland (located between the eye and eye socket) of homing pigeons to discover it being magnetic and containing magnetite.²⁶⁶ This drove the hypothesis by many physicists and biologists about the presence of a compass in the bird’s eye.²⁶⁷ This compass was thought to be utilized by the birds to align themselves with the Earth’s magnetic field. However, the strength of the Earth’s magnetic field is too weak (about 0.05 mT) to explain the birds’ sensitivity to it within the framework of classical physics. Alternatively, the physics behind this theory could be explained using the concept of quantum entanglement.²⁶⁸

According to this concept, two electrons created at the same time from the same quantum process are entangled and remain so even after they are separated by huge distances. The quantum state of one particle is hypothesized to affect the state of the other. It is believed that light entering the eyes of these migrating birds triggers two electrons present in its eye due to photopigments such as cryptochromes. These electrons are entangled and have opposite spins, and thus align in opposite directions due to the Earth’s magnetic field, forming an electron pair following the EPR (Einstein-Podolsky-Rosen) paradox.¹⁵ According to some, the radical pair mechanism is sensitive even in the absence of a static magnetic field, in addition to the fact that it can detect extremely weak magnetic fields.¹⁶ Initially, the electron pair is in the singlet state. Due to the hyperfine interactions with environmental nuclear spins, these pairs undergo de-coherence, leading to singlet-triplet evolutions (please refer to Figure 3 in Beason, 2005¹⁷ for an illustration of singlet-triplet evolutions). The inclination of the molecule to the Earth’s magnetic field is the key for this evolution and leads to chemical end products whose concentration is the cue for a chemical signal that correlates with the Earth’s magnetic field orientation.²⁶⁹ For the migration of birds, ferromagnetism, radical pair method, and “gravity vector” theory are also suggested as possible mechanisms,^{270,271,272,273} i.e., there is a compass and map response in navigation that imparts information about the gravity, polarity, inclination, declination, and magnetic field intensity.

Migratory nature of other species including humans

In addition to birds, the marine opisthobranch mollusk Tritonia diomedea is also found to orient itself based on the geomagnetic field, and it has a correlation with the circa-lunar rhythm.²⁷⁴ Moreover, a range of animals (vertebrates and invertebrates) demonstrate sensitivity toward magnetic fields.^275,276 The prevalence of this sensory modality is seen in birds, sea-turtles,²⁷⁷ honeybees,^278,279,280 fishes,²⁸¹ sharks,²⁸² arthropods, and mammals like bats,²⁸³ as well as in bacteria^284,285,286 and protozoa.²⁸⁷ It has been shown that external manipulations in the magnetic field can be sensed by the pineal gland and eyes of rodents.^288,289 Lately, Baconnier et al. studied the microcrystals (calcite) found in the pineal gland of humans^290,291 that exhibit the piezoelectric effect,²⁹² which in turn is thought to potentially allow the detection of electromagnetic stimuli.²⁹³

The fact that humans, rats, and mice encode a similar number of genomes reveals common ancestry between humans and rodents.²⁹⁴ Given this common ancestry and evolution of humans from these mammals (and those mammals from birds and other species lower down the evolutionary hierarchy), similar effects of geomagnetism on human biology and brain function cannot be ruled out. Robin Baker, a biologist, started an investigation of human magnetoreception in the late 1970s with a series of experiments that involved chair, bus, and walkabouts.^295,296,297 However, his experiments failed to be replicated by other scientists, thereby refuting his hypothesis of the homing ability of humans.^{298,299,300,301,302} The observation of successful canoe navigation in difficult situations without instruments led to controlled experiments, which found that it manifests from innate unconscious ability of humans in magnetoreception.³⁰³ Despite such attempts, definitive conclusions cannot be drawn about magnetoreception in humans.

Effects of magnetic field on the human brain

Given some empirical results supporting magnetoperception in humans as discussed in the previous section, it is appropriate to look at the literature that investigates the effects of magnetism on the human brain in other contexts, i.e., even when the magnetism is not based on the earth’s magnetic field.³⁰⁴ We first consider the impact of anthropogenic electromagnetic waves/radiation, and later look into simulated static/dynamic (electromagnetic in nature too) magnetic fields on brain function. In an electromagnetic wave, both electric and magnetic fields fluctuate with time and sustain each other. Some studies have shown that electromagnetic waves in the environment have the potential to trigger seizures in epileptic patients.^305,306

Technological advancements have caused increasing amounts of electromagnetic radiation in our environment, and there is a wide literature on its effects on brain function,^307,308 which can also be possibly linked to magnetoreception in humans. For example, brain function is modulated by electromagnetic activity due to GSM (Global System for Mobile Communication) signal.³⁰⁹ This aspect has been studied extensively, and systematic reviews of the corresponding literature have been published.^{12,307,308,310} Therefore, we will not delve into the details of this topic.

The brain is composed of about 100 billion neurons that communicate through electrical impulses. Faraday’s law states that¹⁴ applying a time-varying magnetic field to an electrical circuit induces current flow. Transcranial magnetic stimulation (TMS) utilizes this idea to induce currents in neurons using an external time-varying magnetic field,³⁰⁴ and it has been used to treat mental illnesses.^{311,312,313,314} On the other hand, recent studies indicate that static magnetic fields can also stimulate the brain. Applying transcranial static magnetic stimulation (tSMS) for around 10 min reduces motor excitability.³¹⁵ As per the laws of magnetism, randomly oriented dipoles in a ferromagnetic material can be aligned in the presence of a bar magnet. At the microscopic level, the mechanism of action of tSMS is thought to be that the ions in the neurons can be influenced by static magnetic fields because oriented ions change the membrane mechanical properties, thus altering neuronal excitability and modulating neuronal firing.³¹⁶ Additionally, stimulation of the visual cortex using tSMS was found to alter cortical activity, as indicated by a significant increase in alpha power.³¹⁷ This change in cortical activity was accompanied by a corresponding improvement in response time during a difficult visual search task, suggesting that tSMS affects the neural processes involved in visual perception. The exact mechanisms through which a static magnetic field affects cortical activity remain an active area of research. The magnets used in the tSMS literature produced about five orders of magnitude stronger magnetic fields compared to the Earth’s magnetic field. This begs the question whether the Earth’s weak magnetic field is capable of modulating human brain function and whether seasonal variations in geomagnetism can produce corresponding seasonal variations in brain function. Do humans, like some animals and birds, possess magnetoreception that causes changes in brain function?

Systematic review: Geomagnetism and MRI

Search strategy: the following search query: “(((“geomagnetism”) OR (“geomagnetic”) OR (“earth’s magnetic field”)) AND ((“fMRI”) OR (“functional MRI”) or (“functional Magnetic Resonance Imaging”) OR (“functional Magnetic Resonance”) OR ((“magnetic resonance imaging”) AND (“Brain”)))) NOT (Review)” returned 16 (2: Pubmed, 5: WOS, and 9: Scopus) results. After removing duplicates, 12 articles were reviewed for relevancy. Reading the title and abstract, followed by the full text of the papers, no articles were found to be relevant. The PRISMA flowchart is presented in Figure 10.

Figure 10.

PRISMA flowchart concerned with the effects of geomagnetism on the brain measured using MRI

Exclusion criteria: theory-based studies or those using MRI as an example term were not included. Book chapters, reviews, and studies not related to the topic were also excluded.

Evidence: even though the search strategy based on PRISMA did not return any relevant articles, a manual search identified two articles involving exposure of pigeons and zebrafish to high magnetic fields, followed by an assessment of their behavioral change. Homing pigeons demonstrate the ability to sense the Earth’s magnetic field for navigation.³¹⁸ One study used these homing pigeons to test whether MR imaging imposes any effect on their magnetic sensing abilities. Two groups of the species were put in a 3T MRI scanner with either a constant (no sequence) or a spatially varying (gradient echo and echo planar imaging sequences) magnetic field. The third group did not experience the MR field. On release, a day later, the pigeons that experienced varying magnetic fields in the MRI scanner lost prompt orientation and showed variability in the vanishing bearing that stems from interference with the bird’s ability to sense the magnetic field. The study indicated that the magnetic field of the MR unit temporarily disrupted the orientation of these birds,³¹⁹ which poses the question of whether the magnetoreception of other animals could also be hindered. Likewise, it was observed that when zebrafish (the model organism) larvae were exposed to high magnetic fields, tiny calcium carbonate crystals found in the inner ear, also called otoliths, fused to cause atypical swimming behavior.³²⁰ These data suggest that magnetoreception ability of humans, if any, is worth researching, considering our common ancestry with birds and commonality in genes with zebrafish.

Systematic review: Geomagnetism and EEG

Search strategy: since the geomagnetic effect is disrupted inside an MRI, literature with other modalities such as EEG plays an important role in identifying any plausible effects, especially in humans. The following query was used to identify papers on the effect of geomagnetism on brain responses measured by EEG: “(((“geomagnetism”) OR (“geomagnetic”) OR (“earth’s magnetic field”)) AND ((“EEG”) OR (“electroencephalogram”) OR (“electroencephalography”))) NOT (Review)”. The comprehensive search returned 76 articles (12-PubMed, 24-WOS, and 40-Scopus), of which 47 were unique. For additional references, a manual search was performed, and 12 studies were added. The search results were examined by reading the paper title, abstract, and in some cases the full article. 28 (16 + 12 = 28) studies were found to be relevant, which are presented in Figure 11 and listed in Table 6.

Figure 11.

PRISMA flowchart concerned with the effects of geomagnetism on the brain measured using EEG

Table 6.

Studies investigating the effects of geomagnetism on the brain measured using EEG

Study	Year of publication
Clinical and electroencephalographic observation on epilepsy during solar eclipse 1980115	1981
Convulsive threshold in humans and rats and magnetic field changes: Observations during total solar eclipse116	1981
Dynamics of interhemispheric asymmetry as altered by a geomagnetic field321	1984
Effect of magnetic micropulsations on the biological systems: A bioenvironmental study322	1985
Preliminary report on the effect of elf magnetic pulsations on human subjects323	1985
Dependence of a sleeping parameter from the NS or EW sleeping direction324	1987
Influence of the earth’s magnetic field on resting and activated EEG mapping in normal subjects325	1993
Qualitative and quantitative assessment of exposure to geomagnetic field variations on the functional status of the human brain326	1995
Dependence of human EEG spatial synchronization on the geomagnetic activity on the day of experiment327	1998
Human EEG responses to controlled alterations of the Earth’s magnetic field328	2002
Electrophysiological and neurochemical analysis of the biological effects of disturbances of Earth’s magnetic field329	2002
Effects of geomagnetic activity variations on the physiological and psychological state of functionally healthy humans: Some results of Azerbaijani studies330	2007
Evidence of a nonlinear human magnetic sense331	2007
Effects of geomagnetic activity and atmospheric power variations on quantitative measures of brain activity: Replication of the Azerbaijani studies332	2010
Experimental simulation of the effects of sudden increases in geomagnetic activity upon quantitative measures of human brain activity: Validation of correlational studies333	2012
Geomagnetic storm decreases the coherence of electric oscillations in the human brain during work on computer334	2013
Influence of geomagnetic activity on recurrence quantification indicators of human electroencephalogram335	2013
Amplitude-frequency and spatiotemporal restructuring of bioelectric activity of human brain at strong disturbances of the geomagnetic field336	2013
Greater electroencephalographic coherence between left and right temporal lobe structures during increased geomagnetic activity337	2014
Effects of the geomagnetic field on the recurrent characteristics of the electroencephalogram338	2014
LORETA predicts electromagnetic sensitivity and “hearing voices” in a predictable, increasingly prevalent subpopulation: Possible QEEG-based differential diagnosis339	2015
Estimation of the effects of geomagnetic and solar activity on the human brain bioelectrical processes with structural function347	2016
Assessment of the effects of geomagnetic and solar activity on bioelectrical processes in the human brain using a structural function344	2018
Humans may sense Earth’s magnetic field342	2019
The effect of environmental factors on the cognitive functions of cadets at a military institute346	2019
Transduction of the geomagnetic field as evidenced from alpha-band activity in the human brain341	2019
Functional state of the brain of elderly women at rest and in mental stress under varying geomagnetic conditions345	2020
Sleep of poor and good nappers under the afternoon exposure to very weak electromagnetic fields340	2022

Exclusion criteria: we excluded studies falling within the following categories: not using EEG, review, geological fault (static geological magnetic anomalies), general ambient electromagnetic noise (such as GSM signals), Schumann resonance (a specific type of electromagnetic radiation), and telepathy in the context of geomagnetic fields.

Evidence: several studies have tried detecting human brain responses to controlled changes in the Earth’s magnetic field. EEG-evoked potentials in response to a 200 μT change in the ambient magnetic field experienced by human subjects have been reported.³³¹ Quantitative EEG (qEEG) has shown variability in theta activity, particularly in the right parietal area, in response to ambient magnetic field changes as small as 20 nT³³³—and even subtler fluctuations, down to 4 nT, have been shown to influence sleep-related brain activity.³⁴⁰ Recently, it was shown that EEG activity varies in response to different geomagnetic field stimuli^341,342; specifically, alpha-ERD³⁴³ was correlated with magnetic field rotations. These artificial manipulations in the magnetic field experienced by humans are designed not to override the ambient magnetic field, but they simulate the magnetic field changes that one would experience while moving in natural conditions. The researchers concluded that this change in alpha-ERD was due to ferromagnetism rather than radical-pair postulation (cryptochrome hypothesis as mentioned in section named “The hypotheses behind the migratory nature”). Alpha-ERD is typically observed when there is external awareness due to a stimulus. Since the transition from alpha waves to alpha-ERD implies the brain is being put to conscious work, the observation of alpha-ERD during magnetic field change might signify the possibility of the brain being put to work in response to the change in the magnetic field. However, these results are correlational in nature, and it remains unclear whether changes in the magnetic field cause alpha-ERD, which can happen only if humans have the ability of magnetoreception.

Claims were made by Ruhenstroth-Bauer et al. in the 1980s on how direction influences one’s sleep or brain response. They observed a decrease in REM latency in the E-W direction in comparison to N-S,³²⁴ which led to their follow-up study where participants’ EEGs were recorded while performing a sensorimotor task. Similar results followed, with the E-W group exhibiting lower alpha power than the N-S group.³²⁵ Even subjects exposed to magnetic fields of varying frequencies and amplitudes inside controlled environments show inhibition in brain activity facing the North, while facing East promotes a relaxed yet alert state.^322,323 These direction-specific effects are hypothesized to be caused by the N-S alignment of the geomagnetic field. On the other hand, studies that failed to find a significant association between experimentally induced alterations in the Earth’s magnetic field and brain function measured through EEG used atypical field intensities (twice the ambient magnetic field).³²⁸ At these higher magnetic field intensities, even birds either do not respond or show a reduced response.²⁶¹

The upper atmosphere of the Sun, also known as the corona, continuously releases charged particles into the atmosphere in the form of the solar wind. During periods of intense solar activity, such as solar flares or coronal mass ejections, the flow of these particles can intensify dramatically, causing a disturbance that penetrates into the Earth’s magnetosphere due to the interference between the two magnetic fields, leading to geomagnetic storms. The resulting disturbance in the planet’s magnetic field leads to a phenomenon known as a geomagnetic storm. EEG studies that analyzed bioelectrical activity in the brain due to geomagnetic storms, commonly termed as heliogeophysical factors, have focused on various EEG signal characteristics derived using qEEG,^332,337 cross-correlational analysis,^327,330,344 amplitude-frequency coupling,^326,336 cross-spectral analysis,³³⁴ and recurrence plots.^331,335,338 The association of these EEG characteristics with planetary indices of geomagnetic activities (GMAs) such as Kr, Ak, local daily K, local 3-h K, D, H, and Z indices have been investigated.^{327,329,334,336,338} The planetary indices are an interpretable way of comparing EEG (bioelectrical activity) parameters with GMA. Both right hemisphere^{327,330,332,345} and left hemisphere^321,338,346 EEG activity have been shown to be associated with GMA. In particular, inter-hemispheric connections increase^337,339 and intra-hemispheric connections decrease³³⁴ with increasing GMA. Kanunikov et al. showed that GMA-induced changes in EEG activity are global in nature rather than localized to certain brain regions.³³⁵ The effects were more prominent on days when solar activity (class C and M) and geomagnetic activity (K-index of 4–7) changes were noticed simultaneously.^344,347

A study was conducted in Moscow with 15 healthy volunteers’ EEG data obtained between 9/2004 and 6/2005 on random days with and without the prevalence of geomagnetic storms. A decrease in the theta rhythm was observed by a factor of two or more when the participants were working on a computer on days of geomagnetic storm or 24 h post the event. However, there was no change observed in their ECG, blood pressure, or respiratory rate.³³⁴ This suggests that the observed cortical changes were likely due to a central neurophysiological response to geomagnetic disturbances rather than a physiologic autonomic response. Similarly, geomagnetic disturbances (Kp = 4) have been shown to decrease theta EEG activity as in the previous study, both at rest and during performance of a task.³⁴⁵ This indicates that GMA likely affects both spontaneous and evoked brain function.

In addition to geomagnetic effects on brain function in healthy individuals, altered brain function in individuals with brain disorders due to GMA has also been reported. For example, a retrospective study showed a positive correlation between grand-mal epileptic seizures and geomagnetic activity.³⁴⁸ Samoylova et al. showed that children exposed to increased GMA during their prenatal development and the first year have an increased risk of multiple sclerosis (MS) in their adulthood.³⁴⁹ In addition, increased hospital admissions of psychiatric patients have been reported to correlate with positive solar radio flux and magnetic disturbances in the ionosphere,³⁵⁰ both of which can influence the Earth’s magnetic field. Interestingly, the influence of solar activity on GMA can be moderated by gravitational alignment. Some researchers have proposed that the gravitational alignment during full and new moon phases affects how solar particles interact with the Earth’s magnetosphere. For instance, during a new moon, the moon may obstruct some solar emissions from reaching Earth, potentially correlating with observed reductions in psychiatric admissions on those days.^147,351

Some of the observed effects of lunar phase and eclipses discussed before have also been cast in the framework of geomagnetism (instead of gravity) by some researchers. For example, four decades ago, a group not only showed the reduction of the convulsive threshold in humans and rats undergoing electroconvulsive treatment during a total solar eclipse but also made measurements of the geomagnetic field during the eclipse and showed a relatively large deviation from baseline during the solar eclipse.¹¹⁶ While they attribute changes in the convulsive threshold to changes in geomagnetism, the underlying mechanism is yet unclear. During a 1980s total solar eclipse, Srinivas et al. found no significant correlation between geomagnetic activity, EEG, and seizure occurrence. However, their study focused on a small group, which is insufficient for drawing definitive conclusions.¹¹⁵ While these associations remain debated, they raise the broader question of whether geomagnetic fluctuations could subtly affect neural function. Similar to the unresolved mechanisms of magnetoreception in birds, it is unclear whether classical physics is sufficient to explain the change in the membrane potential of neurons due to changes in geomagnetism just based on the application of Faraday’s law.

Discussion

The purpose of this systematic review is to identify how the variations of seasons, gravity, and geomagnetism (that we denote here as seasonal factors) affect brain function. This review is one of the first attempts in the literature that chronicles evidence for the plausibility of these seasonal factors contributing to the observed (some of which is unexplained) variability in brain function as measured by anatomical, resting-state and task-based MRI, as well as EEG. We found that the literature on this subject is sparse, and the quality of evidence is sometimes not very robust. In addition, most studies are observational in nature. Hence, it is difficult to deduce causal or mechanistic conclusions from the literature. That being said, our objective was to provide a synthesis of different pieces of evidence that, when seen together (rather than in isolation), points to the hypothesis that these seasonal factors might affect brain function and contribute to its variability, some of which we observe in fMRI-based brain activation and connectivity and cannot be explained by other known factors.

First, the literature search on the effect of seasons on brain function suggests that variation in the intensity and duration of light, along with variability in temperature, has an association with human mood and behavior.^8,39,61,64 The clinical manifestation of this effect can be observed in SAD.⁴⁶ We found reports in literature that there is a change in brain size in shrews²⁰ and hippocampal volume in humans during the gloomy winter months with shorter days.³² The reduced intensity and duration of light during the winter months are associated with lower mood, sometimes enough to be classified as depression, with a higher probability of this effect in women.

During the colder months, blood vessels constrict to conserve heat.³⁵² In individuals with compromised cardiovascular health, this could lead to changes in perfusion throughout the body.³⁵³ The brain is particularly sensitive to changes in perfusion. Studies have shown a connection between reduced blood flow and depressed mood,³⁵⁴ which can be reversed with blood flow restoration.³⁵⁵ Further, Meyer et al.⁸ demonstrated that behavioral as well as neural responses underlying cognitive processes such as working memory, attention, and executive function were dependent on exogenous factors such as light exposure (varying by season and location), temperature, and endogenous physiological regulators like circadian rhythms and hormone variability. The implications of these findings are 3-fold. First, there is usually a large unexplained variance in brain measurements such as fMRI and EEG, which is thought to be a putative factor in their lack of reproducibility. The contribution of the effects of seasonal variations is rarely considered as part of the problem. We believe that these factors need to be taken into account to reduce the unexplained variance in brain data and improve their reproducibility. Second, the findings have implications for treating mental health disorders, given the effect of seasons on their presentation and severity. Finally, the findings also have the potential for creating schedules for optimal cognitive performance (and increased productivity and efficiency) in healthy individuals.

Coming to our second (of three) topics (concerning gravity and brain function), changes in the relative positions of the Sun, Moon, and Earth not only give rise to seasons but also lead to events such as eclipses and continuous variation in lunar phases. This in turn affects the resultant gravitation field experienced on Earth. The variation in this gravitational field across the globe causes tides in nature (e.g., ocean tides) and “biological tides” such as in the ventricles of the brain filled with CSF.¹³¹ Although these variations are significantly small, the fact that the human brain responds differently to both hypo- and hypergravity motivates us to consider the possibility that even small variations in gravitational field can potentially impact brain function.^202,211,356 Due to increased space travel, there is considerable literature concerning hyper- and hypogravity, including weightlessness and their effects on brain function. However, there is a dearth of literature on the effect of slight changes in the gravitational field on brain function. Investigating this aspect would be critical to understanding the effect of seasonal changes in the gravitational field, experienced by humans on Earth, on brain function.

The idea that lunar phases and eclipses affect the human brain stems from age-old folklore. Initially, this idea was supported by early qualitative data presented in some scientific platforms that indicate an increase in hospital admissions due to escalation of symptoms or intensity of neuro-psychiatric illness during full moon and eclipses.^138,147,148 It is hypothesized that an imbalance in pressure in the fluid through the ventricles and inter-connecting channels causes the ventricles to swell, pushing the brain toward the skull (similar to what happens in hydrocephalus^357,358).¹⁴⁷ It is postulated that this speculative situation may induce alterations in brain function and occasionally exacerbate neuro-psychiatric disorders. Moreover, the heightened nocturnal luminance during full moon phases has been associated with alterations in human sleep architecture.¹¹⁷ However, in today’s world, people spend more time in artificial luminance and spend less time outdoors on a busy work day. Thereby, it is hard to speculate whether sleep disturbance is due to increased luminosity or some other complex factor. Conversely, during new moon phases, a reduction in ventricular, hemispheric, hippocampal, and lentiform volumes has been observed, suggesting an inverse relationship between gravitational field and brain structure volumes.¹¹⁴ These observations support the notion that, much like the rhythmic patterns observed in nature, human physiology and brain function may also exhibit rhythmicity in response to environmental cycles.

The empirical data discussed above do not speculate about the mechanisms underlying the observed changes in human behavior due to lunar phases and eclipses. In order to address this aspect, we synthesize literature from a different domain. Given the strides made in space exploration, much work has happened on the effect of hypogravity (which is experienced during space travel) on brain function. Under conditions of hypogravity and weightlessness, the brain experiences an increased blood flow that results in facial swelling. This occurrence is similar to situations involving prolonged physical activity or inversion therapy (where the head is positioned lower than the heart). On one hand, too much blood flow to the brain increases the intracranial pressure that leads to tissue damage, and on the other (in case of hypergravity), an inadequate supply of blood results in tissue death. Thereby, we can say that a change in terrestrial gravitational field can potentially induce changes in cerebral perfusion and CSF clearance and thereby cause specific changes in brain physiology. How exactly this alters brain function and how much change in gravity is needed to change brain function (and behavior) perceptibly (i.e., the effect size) is yet unknown. Nevertheless, it is clear that there are plausible mechanisms through which gravity (and hence different lunar phases and eclipses) can affect brain function.

The third and final topic from our systematic review is that of an impact on human brain function due to changes in the Earth’s magnetic field. The geomagnetic field experienced by humans waxes and wanes with time due to the ever-changing forces that constitute the Earth’s magnetic field.³⁵⁹ These changes are more evident during magnetic storms caused by the interaction of the Earth’s magnetic field with that of the Sun, which is seasonal in nature. A high intensity or extreme solar activity causes the Earth’s magnetosphere to compress,^360,361 which leads to a change in the global magnetic field. The potential explanation for the impact of geomagnetic activities on brain connectivity^{326,327,329,332,334,336,337,338} and cognitive function³³⁴ is based on human magnetoperception.

While there is sparse literature on the perception of magnetic fields in humans, magnetoreception in the animal kingdom, specifically birds, has been studied extensively. The fact that animals^{277,278,279,282,284,285,287,289} and a variety of bird species possess the ability to sense the changes in the weak magnetic field of the Earth and use that cue to traverse thousands of miles^{260,267,274,275,276,277,282,362} is fascinating in all sense. Given the common ancestry between humans and animals/birds,^289,294 the possibility of human magnetoreception cannot be dismissed.^295,296,297 Research on human magnetoreception dates to the late 1970s when biologist Robin Baker proposed the question and presented data to back the idea.^295,296,297 However, the idea has not made much progress due to failed replication attempts by his peers.³⁶³ It is unclear whether the failure to replicate was due to a small effect size or a genuine lack of effect. After all, absence of evidence is not evidence of absence. In fact, Baker was able to show magnetoperception in animals, which was replicated by his peers. More recent data suggest that Baker might have been onto something. For example, it was demonstrated in 1991 that humans can feebly orient northward when prompted after being spun around.³⁶⁴ In 2003, it was shown that human eyes’ sensitivity to light can be modulated by the magnetic field around the person’s head.³⁶⁵ Further, it was shown in 2011 that the protein cryptochrome, which mediates fruit flies’ magnetic perception, is also present in humans.³⁶⁶ Therefore, one needs to keep an open mind about the possibility of magnetoperception in humans.

Nevertheless, the question of whether humans are capable of sensing the Earth’s magnetic field (consciously or unconsciously), and if so, the underlying mechanisms, is yet unresolved. An experiment with homing pigeons inside a 3T MRI scanner observed an alteration in the magnetoperception of those pigeons exposed to varying magnetic fields inside the scanner.³¹⁸ This study is an important finding after that of Baker’s^295,296,297 and requires further investigation. Whether humans possess magnetic sense or not and if they do, the neural basis of the same and the intentional or unintentional adaptation toward it is definitely worth exploring. With EEG, the study conducted by Wang et al., 2019 that observed alpha-ERD due to deliberate manipulation of the magnetic field is worthy of attention.³⁴¹

Since we have discussed multiple factors that might contribute to seasonal variability of brain function, one might wonder which of these factors are more likely to dominate. Only rigorous experimentation can answer that question. However, we can draw some inferences based on the literature as well. Figure 12 presents a summary of the research categories and their percentage contribution to the total number of studies cited by us in support of the effect of seasonal factors on brain function. Although the literature regarding the effects of lunar phase and eclipses is larger, the strength of evidence is, in fact, strongest for the effect of seasons (light exposure and temperature).

Figure 12.

Summary showing the percentage contribution of each research category for each systematic search presented in this manuscript

Limitations of the study

The goal of this review was to present accrued evidence (both for and against) relating brain function with factors that vary seasonally. We emphasize the importance of cohesive, well-powered and well-controlled brain imaging studies to investigate and analyze such effects, given the lack of indisputable evidence both for and against such a hypothesis. Long-standing empirical observations of seasonal effects on various life forms, including humans, serve as a springboard for investigating the underlying mechanisms at play.^1,2

There are some limitations in the literature (and hence in this review) that one needs to keep in mind. First, available literature on this subject is sparse, sporadic, and often disconnected across different disciplines. Therefore, we have aimed to bring together evidence from different fields of study and provide a synthesis. Second, a substantial amount of evidence (both for and against the hypothesis) are not of great quality, and hence, it is impossible to come to definitive conclusions. However, we argue that the hypothesis is worth considering, given all the available evidence taken together. Therefore, future work must ensure rigorous experimentation in this area so that definitive evidence emerges. Third, most studies investigating the relationship between brain function and environmental rhythms have adopted between-subjects case-control designs. We believe that these are not ideal, but rather, within-subject longitudinal experimental designs would parse out the inter-subject variability and allow us to match the variation of biological activity over a specific timescale with corresponding variation in environmental variables over the same timescale. This would allow us to account for the variability in the data introduced due to space and time (e.g., as we have discussed, the time, location, and what time of the year the data are collected seems to affect the measurements). Fourth, converging evidence from different modalities and experimental methods is needed, so that the drawbacks of a modality or idiosyncrasies of certain experimental methods can be negated. Fifth, most studies report empirical observations, which is fine as a first step, but we need studies that test mechanistic hypotheses so that we understand the mechanism of the purported effects. Scientific understanding cannot be based on just empirical observations. To the best of our knowledge, a comprehensive investigation assessing the above hypotheses using functional neuroimaging data does not exist. We would like to point out that a negative result refuting our hypotheses would also be a significant contribution to the nascent field of neurochronobiology.

In conclusion, we evaluated the potential impact of seasonal factors and environmental rhythms on brain function by laying the groundwork through an in-depth collation of research from disparate fields and disciplines. We have considered three of these factors: (1) changing seasons and associated changes in light exposure, temperature, and weather; (2) changes in the strength of gravitational field on Earth due to the Earth’s relative distance with celestial bodies (Sun, Moon, and planets), leading to lunar phases and events such as eclipses; and finally, (3) the effect of changes in geomagnetism. We found that the evidence for the first factor is fairly well established while hypotheses for the other two factors are worth considering but require further rigorous experimentation to test them. Experiments aimed at understanding the underlying mechanisms within this young and exciting field are needed in the future.

Resource availability

Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Joshita Majumdar (jzm0148@auburn.edu).

Materials availability

This study did not generate new materials.

Data and code availability

• •This study is a systematic review. The data analyzed in this study were extracted from databases such as PubMed, Web Of Science, Scopus, and Google Scholar, which are publicly available resources.
• •MATLAB’s “unique” function was used to remove duplicate studies based on DOI, and no custom code was developed.
• •Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

The authors appreciate the support of the Department of Electrical and Computer Engineering at Auburn University.

Author contributions

Resources, G.D.; project administration, G.D.; conceptualization, G.D.; methodology, J.M. and G.D.; investigation, J.M., D.R., and G.D.; visualization, J.M.; writing—original draft, J.M.; writing—review & editing, J.M., D.R., and G.D.; supervision, G.D.

Declaration of interests

The authors have no competing interests to declare with respect to the contents of this manuscript.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the author(s) used ChatGPT in order to paraphrase certain texts. After using this tool or service, the authors reviewed and edited the content as needed and they take full responsibility for the contents of the publication.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Deposited data
Database for systematic review	Pubmed, Web of Science, and Scopus	The included studies are presented in Tables 1, 2, 3, 4, 5, and 6
Software and algorithms
MATLAB_R2022b	‘unique' function	https://www.mathworks.com/products/matlab.html

Method details

There are three broad sections in this systematic review viz. a) seasonality, b) gravity, and c) geomagnetism. We aimed to identify the effects of seasonality, gravity, and geomagnetism on brain function. In our review, we do not restrict ourselves to any particular brain function (such as perception or cognition). We have included all types of brain function. PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines³⁶⁸ were followed to search for relevant articles. The guideline aids to identify, inspect, screen, and describe appropriate studies to be included. Prior to the database search, the inclusion-exclusion criteria were established.

We included studies that were 1) in English, 2) had references to brain function or brain activity, 3) were investigations based on MRI or EEG (for their widespread utilization in brain research), and 4) had relevant mention of the effects of seasonality, eclipses, lunar phases and/or geomagnetic field on brain function. Reviews mentioning brain tumors or brain cancer were excluded. Gender differences and animal studies were not excluded. Specific inclusion and exclusion criteria for each section are mentioned within the appropriate sections. Due to the array of keywords justified for each broad division, the database search was broken into several segments to extract as many articles as possible (e.g., MRI and EEG were in two different search queries). In some instances, varied search terms were used to identify studies more explicitly (e.g., the word ‘eclipse’ was used as a verb in several articles that were excluded). Following the query results, each article was screened meticulously (by reading both the title and abstract), which resulted in the final list of articles. In addition to PubMed, Web of Science (WOS), and Scopus (until September 2024), manual searches were conducted as well, when and where required, to add to the results of the database searches as undertaken by several other researchers. Manual searches were auxiliary to the primary search using the DATABASE search engines. It implies hand-typed searches (using search engines, specifically Google Scholar) or inspecting the reference lists. For all the searches, a spreadsheet was maintained to identify and mark out the duplicates (which were removed using Microsoft Excel’s conditional formatting functionality of duplicate values and also a MATLAB Code - when and where appropriate). The inclusion criteria are exemplified by the keywords presented with each section (Seasonality, Gravity, and Geomagnetism) and the exclusion criteria are mentioned separately. Each search is mentioned and described at several points throughout the manuscript.

Published: November 20, 2025