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By Stephen Beech
Humans are still "seasonal" creatures despite the advances of 24-hour technology, according to new research.
Our body clocks are still "wild at heart" - tracking the seasonal changes in daylight, say American scientists.
They say their findings, published in the journal npj Digital Medicine, could enable new ways to probe and understand seasonal affective disorder (SAD).
Study author Dr Ruby Kim, Assistant Professor of mathematics at the University of Michigan, said: "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.
"The study shows that our biologically hardwired seasonal timing affects how we adjust to changes in our daily schedules."
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The research team says their findings could also open new areas of inquiry for other health issues connected to the alignment of our sleep schedules and circadian clocks.
For example, researchers, including study senior author Professor Daniel Forger, have previously shown that our moods are strongly affected by how well our sleep schedules align with our circadian rhythms.
Dr. Kim said, "This work shows a lot of promise for future findings.
"This may have deeper implications for mental health issues, like mood and anxiety, but also metabolic and cardiovascular conditions as well."
The research also showed there is a genetic component to seasonality in humans, which could help explain the huge differences in how strongly different people are affected by changes in day length.
Forger, also from the University of Michigan, said: "For some people, they might be able to adapt better, but for other people, it could be a whole lot worse."
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He says that exploring this genetic component will help researchers and doctors understand where different people fall on that spectrum, but getting to that point will take time and effort.
Forger said the new study is an early, but important, step that reframes how we conceive human circadian rhythms.
He said, "A lot of people tend to think of their circadian rhythms as a single clock.
"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."
The Michigan researchers revealed that people's circadian rhythms were tuned into the seasonality of sunlight by studying sleep data from thousands of people using wearable health devices, such as Fitbits.
Participants were all medical residents completing a one-year internship.
Interns are shift workers whose schedules change frequently, meaning their sleep schedules do as well.
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Those schedules are often at odds with the natural cycles of day and night.
The researchers say the fact that circadian rhythms in the participants showed a seasonal dependence is a "compelling" argument for just how "hardwired" the feature is in humans.
Forger said there's a lot of evidence from studies of fruit flies and rodents that animals possess seasonal circadian clocks, and other researchers have thought humans' circadian clocks may behave similarly.
Now, the Michigan team has provided some of the strongest support for the idea yet in observing how that seasonality plays out in a large, real-world study.
Forger said: "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.
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"Then industrialisation comes along in the blink of evolution's eye and, right now, we're still racing to catch up."
The study participants also provided a saliva sample for DNA testing, which enabled the researchers to include a genetic component in their study.
Genetic studies led by other researchers have identified a specific gene that plays an important role in how other animals' circadian clocks track seasonal changes.
Humans share the gene, which allowed the Michigan team to identify a small percentage of interns with slight variations in the genetic makeup of that gene.
Among that group of people, shift work was more disruptive to the alignment of their circadian clocks and sleep schedules over seasons.
The research team says that raises questions, especially about health implications and the influence of shift work on different peopl,e that they plan to explore in the future.
