Humans Are Intrinsically Seasonal Creatures Driven by Circadian Rhythms

Despite our modern lifestyle, with artificial lighting and controlled indoor environments, recent research from the University of Michigan reveals that humans still retain a natural, biological connection to seasonal changes through our circadian rhythms. These internal biological clocks continue to respond to variations in daylight throughout the year, influencing our physiological processes.

According to study lead Ruby Kim, a postdoctoral assistant professor of mathematics, humans are fundamentally seasonal beings. "Day length and sunlight exposure heavily impact how our bodies function, even if we might not consciously realize it," Kim explains. The research indicates that our evolutionary programming ties us to the Earth's natural light cycle, guiding not only sleep patterns but also mood and health.

This insight bears significant implications for understanding seasonal affective disorder (SAD), a form of depression linked to reduced sunlight during winter months. Further, the findings could shed light on how seasonal shifts influence other health conditions, such as metabolic and cardiovascular diseases. Studies have also shown that the synchronization of sleep with circadian rhythms has a direct impact on emotional well-being, mood, and even cognitive performance.

The study uncovered a genetic component to this seasonal rhythmality, identifying a gene shared with animals known to possess seasonal circadian clocks. Variations in this gene might explain why some individuals adapt more easily to changing daylight lengths, while others experience greater disruption, especially in shift-work scenarios.

Data from wearable devices, like Fitbits, collected from medical interns—who often work irregular and disruptive shifts—demonstrated that our internal clocks are tuned to seasonal light patterns. The research suggests that there are actually two interconnected circadian clocks within us: one tracking dawn and the other dusk, with both communicating to regulate seasonal physiology.

"Our findings emphasize that circadian rhythms are not single, rigid clocks but complex, seasonally influenced systems," states Daniel Forger, a senior author of the study. This understanding opens new pathways for personalized medicine approaches, considering genetic makeup to better manage issues related to circadian misalignment.

Understanding human seasonality at a deeper level may help optimize treatment strategies for mood disorders, improve shift-work adaptation, and enhance overall health and well-being by aligning our lifestyles more closely with our intrinsic biological rhythms.