Circadian rhythm
A reduction in exposure to sunlight during winter months and the shift in times or sunrise and sunset can affect a person's Circadian rhythm/internal clock, serotonin levels and melatonin levels. Symptoms of SAD may be more pronounced in women (75% of SAD cases), in younger adults (18-30), those with depression or a family history of depression, and it is also more common in those who live more than 30 degrees north or south of the equator.