Why Seasons Matter When It Comes To Having A Stable Circadian Rhythm

ANN ARBOR, Mich. — Millions of Americans work night shifts, but some people seem to handle the schedule disruption far better than others. Now, scientists from the University of Michigan think the same biological system that helps us adapt to changing seasons might determine how well we cope with irregular work hours.

A new study of more than 3,000 medical interns reveals that people who show stronger seasonal patterns in their daily activity are actually more likely to experience circadian rhythm disruption when working night shifts. In other words, if your step count varies dramatically between summer and winter, you might struggle more with shift work than someone whose activity stays relatively constant year-round.

The research, published in npj Digital Medicine, suggests that our evolutionary programming for seasonal changes might be working against us in our 24/7 modern world.

“Humans really are seasonal, even though we might not want to admit that in our modern context,” says study author Ruby Kim from the University of Michigan, in a statement. “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.”

Ancient Biology Meets Modern Schedules

Scientists have long known that humans, like many animals, have internal clocks that respond to seasonal changes in daylight. But this is the first large-scale study to connect seasonal sensitivity with how people handle irregular work schedules.

The research team analyzed data from medical interns who wore Fitbit devices throughout their grueling first year of residency. These interns regularly switched between day and night shifts, making them an ideal group to study circadian disruption.

Interns whose activity levels varied most dramatically between seasons were the ones who had the hardest time with night shifts. When these seasonally-sensitive people worked overnight, their heart rate rhythms became severely misaligned with their sleep patterns, a condition linked to increased risks of depression, cardiovascular disease, and other health problems.

The study found that individuals with larger summer-winter differences in daily steps and more activity during summer were more likely to experience significant heart rate-sleep misalignment—but only after winter night shift work, not summer shifts.

Two Clocks Instead of One

“A lot of people tend to think of their circadian rhythms as a single clock,” says study author Daniel Forger from the University of Michigan. “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.”

Humans and many other animals have what scientists call a “dual oscillator” system, meaning we have two internal clocks that evolved to help organisms anticipate both dawn and dusk and time their activities around seasonal changes in daylight.

People with strong seasonal responses appear to have more reactive versions of these internal clocks. While this sensitivity might have been helpful for our ancestors, who needed to adjust their behavior as days grew longer or shorter throughout the year, it can work against people in modern shift work scenarios.

The researchers found that when people with highly responsive internal clocks work night shifts, their bodies try to adapt quickly. But since most shift workers only work one or two consecutive night shifts before returning to day schedules, this quick adaptation can actually make things worse when they switch back.

The Genetics Behind This

The study also looked at genetic factors that might influence seasonal sensitivity. The researchers focused on variations in a gene called SLC20A2, which previous studies in mice had linked to seasonal behavior.

Among the thousands of interns who provided genetic samples, those with certain variations of this gene showed different patterns of seasonal activity and circadian disruption. Some genetic groups had larger seasonal swings in activity levels and experienced more circadian misalignment during night shifts, particularly in winter months.

While the genetic effects were small, as is typical for most behavioral traits, they help scientists understand the biological reasons behind why people respond differently to shift work.

The study’s findings could affect the estimated 15 million Americans who work non-traditional schedules. Currently, there’s no reliable way to predict who will struggle with shift work, leading to high turnover rates and increased health risks in many industries.

Understanding that seasonal sensitivity might predict shift work tolerance could eventually help develop better ways to identify who might struggle with irregular schedules. People with strong seasonal patterns might benefit from different lighting strategies or work schedule designs, though more research is needed to develop specific interventions.

“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,” says Forger. “Then industrialization comes along in the blink of evolution’s eye and, right now, we’re still racing to catch up.”

While this study focused on medical interns, a group known for extreme sleep deprivation and stress, the findings likely apply to other shift workers as well. However, more research is needed to understand different types of shift work, age groups, and geographic locations.

The study examined heart rate patterns and sleep timing rather than job performance or safety outcomes. While circadian misalignment is associated with various health problems, the research doesn’t directly measure whether seasonally-sensitive people perform worse during night shifts or have more accidents.

Some people’s internal clocks are simply wired differently. Shift work has been linked to many health problems, but certain people may be predisposed to have more issues than others with irregular work schedules.