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It’s been known for a long time that changing seasons affect our circadian rhythm and sleep patterns. However, most people also intuitively think of summer moods compared to winter moods as being markedly different. Now new research from Finland has shown for the first time how your brain literally changes its neurobiology in response to variations in daylight hours throughout the year.
A group of more than 20 Finnish neuroscientists led by Lihua Sun at the Turku PET Centre and the University of Turku sought to uncover why the seasons affect our mindsets and sociability so significantly. In Scandinavian countries Seasonal Affective Disorder, aptly known as the abbreviation SAD, is a major problem for many people through long Winter months of very short days, which typically provide 16 hours less sunlight than summer days.
For example, 8% of Swedes develop some form of depressive disorder due to the Winter blues. This is in stark contrast to summer days when it is known that negative emotions are much easier to subdue. However, changes in circadian rhythm don’t account for such a prevalence of reduced wellbeing, for instance jet lag is not associated with such effects even for frequent flyers such as airplane staff. This group hypothesized that there may be neurobiological changes occurring from our relationship with the sun, then took to the lab to investigate the theory.
The researchers focused on changes in opioid receptors in the brain, which have a well-established role in regulating our moods and emotions. Specifically, they looked to see if the number of these receptors changed in response to number daylight hours throughout a year.
They took regular measurements using Positron Emission Tomography (PET) brain scans combined with a radioactive tracer that binds to the brain's opioid receptors. 204 volunteers participated in the study.
Interestingly, they studied both humans and rats, using rats as a kind of litmus test to isolate daylight effects from other potential human influences on mental state, such as variations in cultural or societal trends.
In both humans and rats, days of the year with less sunlight were associated with a significant reduction in the quantity of opioid receptors. However, during the Finnish midsummer when there is almost no night time, the quantity also dropped off. The sweet spot for the brain was found to be across days with around 13-17 hours of sunlight, when the receptors were at their most abundant.
A key finding was that these opioid related changes in were most prevalent in brain regions that deal with how we actually feel and process emotional states. Lihua Sun summarized the findings,
"In the study, we observed that the number of opioid receptors was dependent on the time of the year the brain was imaged. The changes were most prominent in the brain regions that control emotions and sociability. The changes in the opioid receptors caused by the variation in the amount of daylight could be an important factor in seasonal affective disorder."
It’s an eye-opening insight that sunshine doesn’t only change how we look on the outside, but actually has a transformative effect on our brain from a physiological perspective - literally shaping the boundaries of what we can actually feel. It’s unknown why humans and likely most mammals have evolved to be neurologically sensitive to sunlight exposure, but these findings do show that we are intimately connected to our nearest star.
The research sheds fresh light on alternative treatments for SAD that could focus on the neurobiology of the brain, rather than behavioral or environmental factors. It also supports the notion of vacationing to sunnier climates during the winter months, to help rejuvenate both the body and mind. The lack of access to winter holidays due to COVID-19 lockdowns may have exacerbated the struggles that many people have had with anxiety and depression, or at least underline the wellness importance of making it through to 2021 spring and summer.
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