According to research published February in the Journal of Neuroscience, the answer links back to how daylight interacts with opioid receptors in the brain.
Lihua Sun is a postdoctoral researcher at the University of Turku in Finland and the study’s lead author. "Opioid receptors are known to be linked with brain emotional functions." explains Sun. Sunlight is also known to influence emotions and social behaviour. This is why Sun and his colleagues hypothesised sunlight and opioid receptor signalling were potentially coupled.
Opioid receptors are proteins on nerve cells, and the human body has three kinds of opioid receptors: mu, delta, and kappa. All of these receptors influence mood.
The study team examined how the length of daylight hours influenced those receptors in humans and rats. Ultimately, they found that the number of receptors they observed was dependent on the time of year the brain was imaged. This was especially true in the parts of the brain that control emotions and social behaviour.
What exactly is going on, however, is a bit of a mystery.
“While we found a causal effect of day-length on brain opioid signalling, we have not addressed the exact mechanism,” Sun explains. “Previous knowledge of the effect of sunlight on the brain opioid system is very limited, and further research is required to understand this.”
Sun hypothesises these opioid receptor changes and their association with daylight hours could explain some of the science behind seasonal affective disorder (SAD).
In the future, he’d like to examine whether modulating the brain opioid system with drugs could mitigate SAD and “seasonality in brain cognitive disorders.”
Past research in mice suggests long day-length increases serotonin and norepinephrine — neurotransmitters that affect mood — and reduces anxiety and depression. It’s possible that a to-be-created treatment that interacts with opioid receptors could help brains in darker parts of the year.
Does all of this mean DST should be killed — or extended? It depends, Sun says. Because DST does increase the amount of time people are exposed to light, and his research suggests day-length can positively influence brain functions, there’s some argument to be made for extending daylight hours.
“However, for locations where day-length can be extremely long, like in Finland, my argument is different since day-length that’s too long may have an adverse effect by inducing physiological stress,” Sun says.
In Finland — DST begins March 28 there — summer days can mean 18 to 19 hours of light, which causes its own mental health problems.
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