Your body clock still follows the ancient seasonal rhythm

Despite electric lights and indoor life, human circadian rhythms are still surprisingly crazy, tracking the seasonal changes in sunlight, just like our ancestors millions of years ago.

New research from the University of Michigan shows that our biological clock is the role of a dual system (one tracks dawn and the other follows dusk), and these ancient mechanisms significantly influence how different people adapt to modern transfer work schedules.

The findings came from studying more than 3,000 medical interns whose schedules demandfully created natural experiments with circadian disruptions. These first-year residents often switch between daytime and night shifts, challenging their internal clocks in a way that reflects the struggles faced by millions of shift workers around the world.

Two clocks, none

“A lot of people tend to think of their circadian rhythm as a single clock,” explains Daniel Forger, a professor of mathematics at the University of Michigan and senior author on the study. “What we’re showing is that there isn’t one clock, but there are two clocks. One is trying to track dawn, the other is trying to track dusk, and they talk to each other.”

This dual oscillator system is not unique to humans. Scientists have documented similar mechanisms in fruit flies, mice and other animals, where separate neural populations coordinate to predict sunrise and sunset. The study provides some of the strongest evidence that humans have preserved this evolutionary legacy.

“Humans are really seasonal, even if we may not want to admit it in a modern context. The length of the day, the amount of sunshine we get, we do affect our physiology,” said Ruby Kim, an assistant professor of postdoctoral mathematics in Michigan.

Seasonal patterns in daily life

The researchers analyzed step count data for wearable devices with a latitude range of 21.3°N to 48°N in mainland America. The results showed significant seasonal variation:

• Summer peak activities: Average 8,454 steps per day
• The lowest winter activities: Average 7,589 steps per day
• Latitude correlation: Higher latitudes have stronger seasonal variations, while sunlight changes are more extreme
• Wake-up duration changes: Wake longer in summer (15.83 hours) to winter (15.52 hours)

Although these differences seem moderate, they represent statistically significant patterns in spite of the confusing non-natural residence schedule. The fact that seasonality persists in this population emphasizes how deep these rhythms are in human biology.

Shift job challenges

What makes the discovery particularly interesting is how seasonal timing affects an individual’s response to transfer work. The researchers developed a complex metric called “HR-Sleep misalignment” that measures the synchronization of heart rate circadian rhythm with actual sleep patterns.

They found that people with stronger seasonal activity patterns suffered greater circadian damage after winter transitions. Essentially, biology still struggles more with people who are more difficult with natural light cycles when forced to conflict with the schedule of seasonal rhythms.

This creates a fascinating paradox: the people most relevant to the rhythm of nature may be most vulnerable to the destruction of modern timelines.

The effect of genetics on seasonal sensitivity

The study found genetic components to seasonal sensitivity by examining variation in the SLC20A2 gene, which was recently associated with seasonal time in mice. Participants with certain genetic variants showed changes in activity, sleep duration and circadian alignment patterns throughout the year.

Mathematical modeling shows that these genetic differences affect how oscillators communicate in the morning and evening within the brain’s circadian control center. Some people seem to have a faster internal clock that can quickly respond to changes in schedules, but this responsiveness becomes problematic during short shift rotations commonly found in medical residence.

Counterintuitively, those with the best circadian system often experience greater misalignment as their clocks start to adapt to night shifts, just as they return to schedules.

The meaning of the real world

This study has a significant impact on understanding and treating a variety of health conditions associated with circadian disruption. Seasonal affective disorders, cardiovascular diseases, metabolic diseases and mood disorders all involve misaligned biological rhythms.

“This work shows a lot of hope for future discoveries,” King explained. “This may have a deeper impact on mental health issues such as mood and anxiety, but also has metabolic and cardiovascular disease.”

Previous research by Forger’s team showed that the link between circadian alignment and daily mood swings is strong. New findings suggest that these relationships may differ seasonally, which may explain why some people experience predictable mood changes throughout the year.

Beyond individual differences

The study analyzed nearly 87,600 data points from participants, distributed across a variety of geographical locations, providing unprecedented insights into how latitude affects human circadian behavior. Participants at higher latitudes (seasonal daylight changes reach 8.2 hours per year) showed stronger seasonal activity patterns than participants near the equator.

These geographical patterns reflect the geographical patterns found in animal studies, thereby enhancing the evolutionary protection of the species’ seasonal timing mechanisms.

Evolutionary perspective

“Brain physiology has worked for millions of years to try to track dusk and dawn,” Foger observed. “Then industrialization emerged in the eyes of evolution, and now, we are still competing.”

This evolutionary mismatch helps explain why circadian biology has been studied for decades, but the transformation work remains so challenging. Our internal clocks evolved out of predictable natural light cycles, rather than artificial timelines required by modern society.

Understanding these basic constraints can provide better strategies for managing transformation efforts and adopt a personalized approach through individual-based genetic profiles and seasonal sensitivity patterns.

The researchers acknowledged that their findings raised more questions than they answered, especially about health effects and best intervention strategies. However, this work lays an important foundation for future research on personalized circadian rhythms.

As Kim points out, “For some, they may be able to adapt better, but for others, it may be worse.” The key now is to identify where individuals fall into that range and develop targeted approaches to support those most vulnerable to circadian disruption.