Your Body Clock Still Follows Ancient Seasonal Rhythms

Despite electric lights and indoor living, human circadian rhythms remain surprisingly wild at heart, tracking seasonal changes in daylight just like our ancestors did millions of years ago.

New research from the University of Michigan reveals that our biological clocks operate as dual systems—one tracking dawn, another following dusk—and these ancient mechanisms significantly influence how different people adapt to modern shift work schedules.

The discovery emerged from studying over 3,000 medical interns whose demanding schedules create a natural experiment in circadian disruption. These first-year residents frequently switch between day and night shifts, challenging their internal clocks in ways that mirror the struggles faced by millions of shift workers worldwide.

Two Clocks, Not One

“A lot of people tend to think of their circadian rhythms as a single clock,” explained Daniel Forger, University of Michigan professor of mathematics and the study’s senior author. “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.”

This dual-oscillator system isn’t unique to humans. Scientists have documented similar mechanisms in fruit flies, mice, and other animals where separate neural populations coordinate to anticipate both sunrise and sunset. The research provides some of the strongest evidence yet that humans retain this evolutionary heritage.

Study author Ruby Kim, a postdoctoral assistant professor of mathematics at Michigan, noted the profound implications: “Humans really are seasonal, even though we might not want to admit that in our modern context. Day length, the amount of sunlight we get, it really influences our physiology.”

Seasonal Patterns in Daily Life

The researchers analyzed step count data from wearable devices worn by medical interns across latitudes ranging from 21.3°N to 48°N throughout the continental United States. The results revealed clear seasonal variations:

• Peak summer activity: 8,454 daily steps on average
• Lowest winter activity: 7,589 daily steps on average
• Latitude correlation: Stronger seasonal differences at higher latitudes where daylight variation is more extreme
• Wake duration changes: Longer periods awake during summer (15.83 hours) versus winter (15.52 hours)

While these differences might seem modest, they represent statistically significant patterns that emerged despite the chaotic, non-natural schedules of medical residency. The fact that seasonality persisted in this population underscores how deeply embedded these rhythms are in human biology.

The Shift Work Challenge

What makes the findings particularly intriguing is how seasonal timing affects individual responses to shift work. The researchers developed a sophisticated metric called “HR-sleep misalignment”—measuring how well heart rate circadian rhythms sync with actual sleep patterns.

They discovered that people with stronger seasonal activity patterns experienced greater circadian disruption after winter night shifts. Essentially, individuals whose biology remains more attuned to natural light cycles struggle more when forced into schedules that conflict with seasonal rhythms.

This creates a fascinating paradox: those most connected to natural rhythms may be most vulnerable to modern schedule disruptions.

Genetic Influences on Seasonal Sensitivity

The study uncovered a genetic component to seasonal sensitivity by examining variations in the SLC20A2 gene, which recent animal research linked to seasonal timing in mice. Human participants with certain genetic variants showed altered patterns of activity, sleep duration, and circadian alignment throughout the year.

Mathematical modeling suggested these genetic differences affect how morning and evening oscillators communicate within the brain’s circadian control center. Some individuals appear to have more responsive internal clocks that react quickly to schedule changes—but this responsiveness becomes problematic during brief shift rotations common in medical residency.

Counterintuitively, those with the most adaptable circadian systems often experienced greater misalignment because their clocks began adjusting to night shifts just as they returned to day schedules.

Real-World Implications

The research carries significant implications for understanding and treating various health conditions linked to circadian disruption. Seasonal affective disorder, cardiovascular disease, metabolic disorders, and mood disturbances all involve misaligned biological rhythms.

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

Previous research by Forger’s team demonstrated strong connections between circadian-sleep alignment and daily mood fluctuations. The new findings suggest these relationships may vary seasonally, potentially explaining why some people experience predictable mood changes throughout the year.

Beyond Individual Differences

The study analyzed nearly 87,600 data points from participants spread across diverse geographic locations, providing unprecedented insight into how latitude affects human circadian behavior. Participants at higher latitudes—where seasonal daylight variation reaches 8.2 hours annually—showed stronger seasonal activity patterns than those closer to the equator.

These geographic patterns mirror those found in animal studies, reinforcing the evolutionary conservation of seasonal timing mechanisms across species.

The Evolutionary Perspective

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

This evolutionary mismatch helps explain why shift work remains so challenging despite decades of research into circadian biology. Our internal clocks evolved for predictable natural light cycles, not the artificial schedules demanded by modern society.

Understanding these fundamental constraints could inform better strategies for managing shift work, potentially through personalized approaches based on individual genetic profiles and seasonal sensitivity patterns.

The researchers acknowledge their findings raise more questions than they answer, particularly regarding health implications and optimal intervention strategies. But the work establishes a crucial foundation for future research into personalized circadian medicine.

As Kim noted, “For some people they might be able to adapt better, but for other people it could be a whole lot worse.” The key now lies in identifying where individuals fall on that spectrum and developing targeted approaches to support those most vulnerable to circadian disruption.