According to the National Institute of Mental Health, seasonal affective disorder (SAD), otherwise known as "seasonal depression," is real. The symptoms may start in the late fall or early winter and go away during the spring and summer.
They state that millions of adults may suffer from SAD and it occurs much more often in women than in men. It is also more common in those living farther north, where there are shorter daylight hours in the winter.
Their research shows that people with SAD may have reduced activity of serotonin, which helps regulate mood. It also suggests that sunlight controls the levels of molecules that help maintain normal serotonin levels. However, in people with SAD, this regulation does not function properly, resulting in decreased serotonin levels in the winter.
https://www.nimh.nih.gov/health/publications/seasonal-affective-disorder