Many of us may dread the approach of winter (does anyone really like scraping ice off their windshield?), but about 4 to 6 percent of people experience a more severe winter depression.
Now, new research using brain scans helps pinpoint why seasonal affective disorder — known colloquially as "SAD" — only affects some people.
"People with SAD have an unhelpful way of controlling the "happy" brain signalling compound serotonin during winter months," the BBC explained.
Using PET scans, University of Copenhagen researchers compared levels of the serotonin transporter (SERT) protein in the brains of 11 people with SAD and 23 people without it. They also compared summer data with winter data. The results, presented at a European College of Neuropsychopharmacology conference, showed seasonal differences in only the participants with SAD, whose SERT levels changed by 5 percent.
While the study was small, "we believe that we have found the dial the brain turns when it has to adjust serotonin to the changing seasons," lead researcher Dr. Brenda McMahon told the BBC. "The serotonin transporter (SERT) carries serotonin back into the nerve cells where it is not active. So the higher the SERT activity, the lower the activity of serotonin."
Since sunlight helps dial down SERT activity, as light fades in autumn, SERT levels increase.
While plenty of past research has shown a connection between SAD and seratonin, this research helps pinpoint the role of seratonin even further.
The findings also help explain why one of the common treatments for the disorder, light therapy, often works. It also makes sense that more people in northern and cloudy climates are affected. Motherboard notes:
"It's worth emphasizing that SAD isn't a thing you just have to deal with. It's common and often viewed as a bit jokey, but depression, seasonal or not, is often devastating. It might rain three inches in the next three days, but, with something as simple as sunlight-mimicking lights, it doesn't have to feel that way on the inside."
Humans aren't alone in their sensitivity to sunlight: some veterinarians suspect that even animals may experience SAD, according to C&G Newspapers.