Many people find that their sleep and moods are linked to the seasons. Those living in temperate zones may feel like hibernating in winter and staying out all night in summer, though even those in the tropics can be affected by changing seasons. That’s because we are seasonal animals and adjust our behaviour according to cues from the environment.
Now, it turns out that our ancient adaptation to the seasons also affects our ability to adjust to modern lifestyle factors such as shiftwork – and probably jet lag, too.
This is the conclusion of a recent paper studying about 3,000 US medical interns wearing health trackers on their wrists for a year. The study also found significant differences between participants, which it linked to variations in a specific gene called SLC20A2.
On average, the medical interns’ daily step count and the time they spent awake were both higher in summer than in winter. Yet some participants showed little to no difference in their step counts between summer and winter, while some even showed opposite patterns to the main group.
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The authors used heart-rate data collected via the health trackers to calculate each person’s internal time, in other words what time it “feels like” to their body. This is determined by our circadian rhythm, the “body clock” which also affects everything from body temperature to hormone levels. The authors then compared this to participants’ activity patterns to look at to what extent their bodies were disrupted by night shifts.
Participants who showed the greatest seasonal difference in step count also showed the most disruption from winter night shifts to their sleep-wake cycle – when and how long they sleep. They were not disrupted in the same way after summer night shifts.
The researchers then looked at how these findings related to the SLC20A2 gene, since previous work had shown that the gene is involved in seasonality in mice. This gene is responsible for encoding a protein embedded in our cell membranes that allows the movement of ions (electrically charged atoms or molecules) in and out of cells. The protein is very active in neurons in the brain, where this movement of ions is important in generating the electrical signals which form the basis of all brain functions.
The researchers found thousands of differences in the sequence of the SLC20A2 gene in the participants they studied. They focused on five differences called single nucleotide polymorphisms (SNPs) and how different combinations of those SNPs (or genotypes) influenced participants’ behaviour in summer and winter. Using mathemetical modelling, they were able to show that having a particular genotype influenced participants’ circadian rhythms, physical activity and adaptability to shiftwork in winter.
Circadian rhythms and the seasons
The most reliable feature of seasons, at least in temperate countries, is the change in the proportion of light in a day (the photoperiod). Seasonal changes in plants and animals such as when they mate and migrate are thought to be a way of responding to changes in the availability of food to increase their chances of surviving and reproducing. Even humans, particularly males, demonstrate seasonality in reproductive hormones, with higher levels of testosterone in spring and summer. This is despite the fact that we do not tend to reproduce seasonally.
Light exposure via our eyes synchronises our circadian rhythms to the environment every day. A model proposed by biologists Colin Pittendrigh and Serge Daan almost 50 years ago suggests that humans’ and many other animals’ circadian rhythms are governed by two internal clocks which are coupled to each other: one that responds to dawn and one that responds to dusk. The idea is that these separately control the transitions into daytime (active phase) and into nighttime (resting phase). Biologists still use the model as a framework to explain how living things adjust to the changing length of days across the seasons.
Light signals are transmitted from the eyes to a collection of neurons in the brain called the suprachiasmatic nuclei (SCN) which communicate that information to the rest of the brain and body. The cells in the SCN are arranged in clusters that co-operate differently in response to different day lengths. Research has shown that in mice and rats, SCNs signal in synchrony in shorter days (winter), and out of phase with one another in longer days (summer).
The intensity of how synchronised these cells are leads to differences in how they transmit information about light. This contributes to individual differences in our body’s response to changes in day length, as well as to other things like shiftwork and jet lag. Also, we also all experience different amounts of natural sunlight and indoor electrical light. The amount of light you’ve been exposed to recently can affect how you adapt to the changing seasons. This is another reason not to expect yourself to adapt to these changes in the same way as other people
Night-shiftwork is also associated with poor health such as weight gain and low quality sleep. Understanding the biological basis of people’s adaptation to shiftwork will help us to mitigate this by developing personalised strategies to shift-workers’ health. And it could help people understand whether they need more rest when jet-lagged or as the seasons change.
Laura Roden receives funding from the Wellcome Trust.
