Summary: Despite widespread use of electric lighting and indoor living, human sleep and circadian rhythms remain strongly influenced by seasonal changes in daylight. New research from the University of Michigan shows that our internal clocks track shifts in day length across seasons, affecting sleep timing, mood, and health.
By combining wearable sleep data from medical residents with genetic analysis, the study found that human circadian systems respond to both dawn and dusk and that genetic differences make some people more sensitive to seasonal changes. This dual-clock model deepens our understanding of physiological seasonality and suggests new directions for researching conditions such as seasonal affective disorder and other health issues tied to circadian misalignment.
Key Facts:
- Dual Circadian Clocks: Humans appear to have two interacting internal oscillators—one that tracks dawn and another that tracks dusk.
- Genetic Sensitivity: Genetic variation can make some individuals more vulnerable to sleep disruption and circadian misalignment when daylight changes.
- Health Implications: Season-driven misalignment between sleep schedules and circadian timing can influence mood, metabolic health, and cardiovascular risk.
Source: University of Michigan
It is easy to assume that artificial light and constant indoor schedules have erased seasonal effects on human sleep. However, the University of Michigan study reveals that our circadian biology still responds to the changing length of day across the year.
“Humans really are seasonal, even in our modern environment,” said Ruby Kim, a U‑M postdoctoral assistant professor of mathematics and lead author on the study. The research demonstrates that natural variations in day length continue to shape physiological timing, influencing how people adjust to changes in daily schedules.
Understanding seasonal circadian timing could improve how we approach seasonal affective disorder and other conditions linked to circadian disruption. Earlier work from this research group has shown that mood and mental well-being are closely tied to how well sleep schedules align with internal circadian timing.
“This line of research has important implications not only for mental health—mood and anxiety—but also for metabolic and cardiovascular conditions,” Kim added. The study further found a genetic contribution to seasonal sensitivity, which may explain why some people cope with seasonal changes more easily than others.
“Some individuals adapt better to changing schedules, while others experience more serious disruption,” said Daniel Forger, U‑M professor of mathematics and director of the Michigan Center for Applied and Interdisciplinary Mathematics and senior author of the study. Identifying genetic markers of sensitivity could eventually help clinicians predict who is most at risk and tailor interventions accordingly, though more research is needed.
Kim, Forger and their colleagues analyzed sleep patterns from thousands of medical interns enrolled in the Intern Health Study. Interns work rotating shifts throughout their yearlong internships, making them an informative group for studying how irregular schedules interact with natural day–night cycles. Each participant wore a consumer health device (such as a Fitbit) that recorded sleep timing and also provided a saliva sample for DNA analysis.
The researchers found clear seasonal variation in circadian alignment: shifts in day length from summer to winter correlated with greater misalignment after shift work. Mathematical modeling suggested that seasonal timing influences the rate at which circadian systems adapt to new schedules, with different effects on morning and evening oscillators. In other words, one internal clock may be more responsive to dawn cues and the other to dusk cues, and the balance between them affects how quickly people adjust.
The study also explored genetic polymorphisms previously associated with seasonality in animals. Human variants of a conserved gene known to affect seasonal timing were present in a minority of interns; those with these variants experienced greater disruption from shift work when seasonal daylight varied. While this finding does not yet enable clinical prediction, it supports the idea that biological mechanisms evolved for seasonal timing contribute to interindividual differences in shift work adaptation.
About this genetics and circadian rhythm research news
Author: Matt Davenport
Source: University of Michigan
Contact: Matt Davenport – University of Michigan
Image credit: Neuroscience News
Original Research: Open access. “Seasonal timing and interindividual differences in shiftwork adaptation” by Ruby Kim et al., published in npj Digital Medicine.
Abstract
Seasonal timing and interindividual differences in shiftwork adaptation
Millions of shift workers in the United States face higher risks for depression, cancer, and metabolic disease, yet individuals vary widely in how they respond to shift work. This study identifies a conserved biological system of morning and evening oscillators—originally evolved for seasonal timing—that may help explain these interindividual differences.
Analyzing seasonal patterns in medical interns working rotating shifts, the researchers found that summer–winter variation correlates with increased circadian misalignment following schedule changes. Mathematical modeling indicates that seasonal timing affects the speed of adaptation to new schedules and predicts different responses of morning versus evening oscillators.
Genetic analyses examined polymorphisms linked to seasonality in other animals and found that human variants influence how quickly circadian rhythms respond to changes in schedule. The findings support the hypothesis that biological mechanisms for seasonal timing contribute to the large differences observed among individuals in their adaptation to shift work—differences that have important implications for worker health.