Sunlight plays a critical role in human sleep

Sleep might seem disconnected from the natural world these days. Our lives are ruled by alarms, smartphones, and overhead lights – not sunrise or sunset. But our bodies tell a different story.

A new study finds that our internal clocks and sleep schedule still march to the beat of sunlight, even in today’s high-tech world.

Scientists from the University of Michigan (U-M) found that our circadian rhythms continue to respond to seasonal changes in daylight.

Even when our daily routines are shaped by artificial lighting and unpredictable schedules, the cycles of dawn and dusk still leave their mark.

How sunlight shapes our sleep

The research has major implications for mental and physical health. It offers a new way to look at seasonal affective disorder, a form of depression triggered by the changing seasons.

“Humans really are seasonal, even though we might not want to admit that in our modern context,” said Ruby Kim, U-M postdoctoral assistant professor of mathematics. “Day length, the amount of sunlight we get, it really influences our physiology.”

“The study shows that our biologically hardwired seasonal timing affects how we adjust to changes in our daily schedules.”

Previous studies have shown a link between mood and how well our sleep matches our internal clock. This new work builds on that.

“This work shows a lot of promise for future findings,” Kim said. “This may have deeper implications for mental health issues, like mood and anxiety, but also metabolic and cardiovascular conditions as well.”

Inherited sleep struggles

The study also uncovered something else: a genetic factor. Our response to changing day length might be partially inherited. That could help explain why some people feel fine after a time change, while others struggle for weeks.

Daniel Forger, who led previous sleep studies, is an U-M professor of math and director of the Michigan Center for Applied and Interdisciplinary Mathematics.

“For some people they might be able to adapt better, but for other people it could be a whole lot worse,” he said.

Figuring out how this genetic piece fits in will take more time. But it opens the door to understanding why some people are more vulnerable to sleep disruptions than others. The research also suggests that our bodies may not operate on a single sleep-wake clock.

“A lot of people tend to think of their circadian rhythms as a single clock,” Forger said. “What we’re showing is that there’s not really one clock, but there are two. One is trying to track dawn and the other is trying to track dusk, and they’re talking to each other.”

Sleep disruptions meet seasonal clocks

To observe these rhythms in action, the researchers turned to a large dataset: thousands of medical residents participating in the Intern Health Study.

The interns wore health trackers like Fitbits and submitted data over the course of their demanding first year of training.

Medical interns often work long, irregular hours that conflict with natural sleep cycles. Despite these disruptions, the study found that their circadian rhythms still showed a clear seasonal pattern. That makes a strong case for how deeply embedded this response is in humans.

This kind of seasonal response has long been observed in animals – from fruit flies to mice. Until now, though, strong real-world evidence for this in humans had been limited.

“I think it actually makes a lot of sense. Brain physiology has been at work for millions of years trying to track dusk and dawn,” Forger said. “Then industrialization comes along in the blink of evolution’s eye and, right now, we’re still racing to catch up.”

DNA linked to rest

Interns in the study also provided saliva samples for DNA analysis. The team identified a small group of participants who had a variation in a gene already known to affect circadian rhythms in animals.

This variation made it harder for their sleep cycles to stay in sync with the seasonal shifts in sunlight.

That means shift work could be especially harmful to certain individuals – raising new questions about how genetics and work schedules affect our health. The researchers plan to investigate this further.

For now, the study shows that even with busy lives and bright lights, we still carry a deep connection to the sun. Our internal clocks haven’t forgotten what millions of years of evolution taught them – and neither should we.

The full study was published in the journal npj Digital Medicine.

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