Illustrated photo by William Stitt
Seasonal Affective Disorder (SAD) is a type of depression that comes and goes with the seasons, typically starting in the late fall and early winter when the days grow short and you get less and less sunlight. SAD normally goes away during the spring and summer.
A study funded by National Institute of Mental Health has found that most SAD symptoms coming from daily body rhythms that have gone out-of-sync with the sun. Rhythms that have lost their bearings due to winter's late dawn and early dusk accounted for 65 percent of SAD symptoms; re-aligning them explained 35 percent of melatonin's antidepressant effect in patients with delayed rhythms, the most common form of SAD.