Some researchers link seasonal depression to the natural hormone melatonin, which causes drowsiness. When light strikes the human retina, a process in the body decreases the secretion of melatonin. Light modifies the amount of melatonin in the human nervous system and boosts serotonin in the brain. So light therapy has an antidepressant effect.

Experts now believe that light therapy may be an effective treatment for people who have eating disorders, insomnia, and major depression (unrelated to SAD). Researchers have found that when depressed patients without SAD take antidepressants and use light therapy, there is an added benefit of increased energy and improved mood.