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Ann Arbor, MI — Despite our reliance on smartphones, artificial lighting, and irregular schedules, new research reveals that the cycles of sunrise and sunset continue to play a critical role in regulating human sleep. A team of scientists from the University of Michigan has found that our internal body clocks, or circadian rhythms, remain deeply tied to seasonal changes in daylight, with significant implications for both mental and physical health.
Sunlight’s Enduring Influence
The study, published in npj Digital Medicine, tracked thousands of medical residents during their demanding first year of training as part of the Intern Health Study. These residents wore health trackers and provided DNA samples, allowing researchers to observe their sleep patterns and genetic makeup over time.
Even as the interns worked long, irregular hours—often in direct conflict with natural sleep cycles—their circadian rhythms still displayed a clear seasonal pattern. This suggests that, despite modern disruptions, the human body remains hardwired to respond to the natural cycles of dawn and dusk.
“Humans really are seasonal, even though we might not want to admit that in our modern context,” said Ruby Kim, a postdoctoral assistant professor of mathematics at U-M. “Day length, the amount of sunlight we get, it really influences our physiology.”
Implications for Health and Wellbeing
The findings shed new light on conditions like Seasonal Affective Disorder (SAD), a form of depression triggered by changing seasons. The study also suggests that mismatches between our internal clocks and daily schedules could contribute to mood disorders, metabolic issues, and cardiovascular conditions.
Interestingly, the research uncovered a genetic component: some individuals possess a gene variation that makes it harder for their sleep cycles to adjust to seasonal changes in sunlight. This may explain why some people adapt easily to time changes or shift work, while others struggle for weeks.
“For some people, they might be able to adapt better, but for other people it could be a whole lot worse,” said Daniel Forger, U-M professor of mathematics and director of the Michigan Center for Applied and Interdisciplinary Mathematics.
Two Clocks, Not One
The study also challenges the common belief that humans operate with a single sleep-wake clock. Instead, researchers found evidence of two separate clocks—one tracking dawn and the other tracking dusk—that interact to regulate our sleep.
This dual-clock system, deeply rooted in our evolutionary history, highlights how industrialization and modern lifestyles have only recently disrupted patterns that have guided human physiology for millions of years.
Looking Forward
The research team plans to further investigate how genetics and work schedules interact to influence health. For now, the study serves as a reminder of our enduring connection to the sun.
“Our internal clocks haven’t forgotten what millions of years of evolution taught them—and neither should we,” said Forger.
Disclaimer:
This article is based on findings reported by Earth.com and the University of Michigan. The information provided is for educational and informational purposes only and should not be taken as medical advice. For concerns about sleep or mental health, please consult a qualified healthcare professional.
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