Intrinsic biological processes, encompassing sleep patterns, cardiovascular rhythms, and metabolic rates, are fundamentally governed by internal circadian clocks present within nearly every cellular unit across the organism. Contemporary existence imposes unprecedented strains on this finely tuned temporal regulation system, presenting challenges for which it was not originally designed. Advances in industrialization, the prevalence of shift work, artificial indoor illumination, and the ubiquitous presence of smartphones have profoundly impacted our sleep quality and the synchronized timing of our physiological systems. Emerging scholarship from the University of Michigan indicates that our fundamental circadian mechanisms retain a primal connection to the natural world, actively tracking seasonal fluctuations in daylight. The findings have been formally presented in the esteemed journal npj Digital Medicine.
Kim et al. posit that the considerable variances in how individuals adapt to shift work — a critical factor for the health of those in such professions — may find partial explanation in biological mechanisms responsible for seasonal timekeeping. Image courtesy of Sasin Tipchai.
“It is demonstrably true that humans exhibit seasonal periodicity, irrespective of our modern inclination to disregard this fundamental aspect,” stated Dr. Ruby Kim, the lead investigator in this research endeavor.
“The duration of daylight, signifying the extent of solar exposure, exerts a tangible influence on our physiological functions.”
“This study unequivocally demonstrates that our inherent, evolutionarily ingrained seasonal timing significantly modulates our capacity to acclimate to shifts in our daily operational schedules.”
“This revelation holds the potential to unlock novel avenues for investigating and comprehending Seasonal Affective Disorder, a form of depression intrinsically linked to cyclical environmental changes.”
“Furthermore, it could pave the way for new lines of inquiry into a spectrum of other health concerns that are directly correlated with the synchronization of our sleep cycles and internal biological clocks.”
“This body of work offers substantial promise for future discoveries. It may hold profound implications not only for mental health conditions, such as mood disorders and anxiety, but also for metabolic and cardiovascular ailments.”
The investigation also brought to light a significant genetic influence on this inherent seasonality in humans, which could shed light on the pronounced disparities in how individuals are affected by variations in daylight duration.
“For certain individuals, the process of adaptation might be more fluid, whereas for others, it could present considerably greater challenges,” remarked Professor Daniel Forger, the senior author of the study.
Delving deeper into this genetic predisposition will equip researchers and clinicians with the ability to ascertain where individuals stand on this adaptive spectrum, though achieving this level of understanding necessitates further dedicated effort and time.
As it stands, this research represents a nascent yet crucial step that fundamentally redefines our conceptualization of human circadian rhythms.
“A prevalent misconception is that individuals possess a singular circadian clock,” Professor Forger commented.
“What our findings reveal is the existence of not one, but rather two distinct temporal regulators.”
“One of these is oriented towards monitoring the onset of dawn, while the other is tasked with tracking the arrival of dusk, and these two systems maintain an active dialogue.”
Through the meticulous analysis of sleep data collected from thousands of participants utilizing wearable health monitoring devices, such as Fitbits, the research team ascertained that individuals’ circadian rhythms were finely attuned to the seasonal patterns of solar illumination.
All participants were drawn from the cohort of medical residents engaged in a year-long internship, having previously consented to participate in the Intern Health Study, which received funding from the National Institutes of Health.
Interns are characterized by their engagement in shift work, resulting in frequent alterations to their professional schedules and, consequently, their sleep patterns.
Moreover, these work schedules frequently diverge from the natural diurnal cycle of light and darkness.
The observation that circadian rhythms within this specific population exhibited a discernable seasonal dependency provides compelling evidence for the deeply ingrained nature of this characteristic in humans, a finding that is not entirely unexpected.
“Indeed, this aligns logically with evolutionary principles. The intricate architecture of the brain has been engaged for millennia in the meticulous tracking of dawn and dusk,” Professor Forger asserted.
“Then, with the advent of industrialization, a development that occurred in the briefest of evolutionary timescales, we are presently engaged in a rapid effort to adapt and reintegrate.”
The Intern Health Study participants also contributed saliva samples for genetic analysis, thereby enabling the scientific team to incorporate a genetic dimension into their investigation.
Prior genomic studies have identified a particular gene that plays a pivotal role in how the circadian clocks of other animal species track seasonal variations.
Humans possess this same gene, a fact that facilitated the authors’ identification of a small proportion of interns displaying minor genetic variations within this specific gene.
For this subset of individuals, the disruptive impact of shift work on the temporal alignment of their circadian clocks and sleep schedules was amplified across the changing seasons.
This finding, in turn, generates numerous pertinent questions, particularly concerning the implications for health and the differential impact of shift work across diverse individuals.
However, these are precisely the inquiries that the research team intends to pursue in their forthcoming investigations.
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R. Kim et al. 2025. Seasonal timing and interindividual differences in shiftwork adaptation. npj Digit. Med 8, 300; doi: 10.1038/s41746-025-01678-z
