Biological processes such as sleep, heart rate, and metabolism are regulated by the circadian clock found in nearly every cell in the human body. However, modern lifestyles challenge this natural timing mechanism in ways for which we are not well-suited. Factors like industrialization, shift work, artificial lighting, and smartphone usage significantly impact our sleep and circadian rhythms. A recent study from the University of Michigan reveals that our circadian rhythms continue to align with seasonal changes in sunlight. This result was published in the journal NPJ Digital Medicine.
Kim et al. We believe that substantial individual differences in shift work adaptation, which are vital for shift workers’ health, can be explained in part by the biological mechanisms of seasonal timing. Image credit: Sasin Tipchai.
“We may not want to admit it in today’s world, but humans are inherently seasonal,” stated Dr. Ruby Kim, the study’s lead author.
“The duration of daylight and the sunlight we receive significantly influence our physiology.”
“Our research demonstrates that the timing of biologically significant seasons plays a role in how individuals adapt to changes in their daily routines.”
“These findings could lead to new avenues for investigating and understanding seasonal affective disorders, a form of depression linked to seasonal variations.”
“It could also point to new areas of exploration regarding a range of health issues related to sleep schedules and alignment with circadian rhythms.”
“This work holds great promise for future discoveries, potentially impacting metabolic and cardiovascular health as well as mental health conditions such as mood disorders and anxiety.”
The study also indicated that humans possess a seasonal genetic component, which might explain the significant differences in how individuals are impacted by variations in daylight.
“Some individuals may adapt better, while others might fare much worse,” remarked Professor Daniel Foger, a senior author of the study.
Investigating this genetic component could help researchers and healthcare providers identify where an individual falls on the adaptation spectrum, although achieving this will require more time and effort.
For now, this study serves as an important first step in reshaping our understanding of human circadian rhythms.
“Many people tend to perceive their circadian rhythm as a singular entity,” explained Professor Foger.
“Our findings indicate that it’s not one clock, but rather two.”
“One clock tracks dawn, and the other tracks dusk. They communicate with each other.”
Researchers adjusted their studies of circadian rhythms according to seasonal sunlight by analyzing sleep data collected from thousands of participants using wearable health technology like Fitbits.
All participants were medical interns involved in a one-year internship as part of a healthcare study funded by the National Institutes of Health.
Interns are shift workers whose schedules frequently change, which also changes their sleep patterns.
Moreover, these schedules often run counter to the natural day-night cycle.
The observation that the circadian rhythm of this group demonstrated seasonal dependence is a strong indicator of how deeply ingrained this feature is in humans, which is unsurprising.
“It makes a lot of practical sense. Our brain physiology has been attuned to track dusk and dawn for millions of years,” stated Professor Foger.
“Then industrialization came along in an evolutionary blink, and we’re still trying to catch up.”
Participants in the healthcare study also provided saliva samples for DNA analysis, enabling researchers to include genetic factors in their evaluations.
Previous genetic studies have identified specific genes involved in how circadian clocks in various animals respond to seasonal changes.
Since humans share this gene, the authors could pinpoint a smaller group of interns with slight variations in their genetic makeup.
For this group, shift work was more disruptive due to the misalignment between seasonal circadian rhythms and their sleep schedules.
This leads to many questions, particularly regarding the health implications and how shift work affects different individuals.
However, these are questions researchers will seek to investigate further in the future.
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R. Kim et al. 2025. Seasonal timing and individual differences in shift work adaptation. npj digits. Pharmaceuticals 8, 300; doi:10.1038/s41746-025-01678-z
Source: www.sci.news
