Certain brain chemicals called neurotransmitters are associated with depression — particularly serotonin (ser-o-TOE-nin), norepinephrine (nor-ep-ih-NEF-rin) and dopamine (DOE-puh-meen). Most antidepressants relieve depression by affecting these neurotransmitters. Each type (class) of antidepressant affects these neurotransmitters in slightly different ways.
Seasonal affective disorder is characterized by the onset of depression during the winter months, when there is less natural sunlight. This depression generally lifts during spring and summer. Winter depression, typically accompanied by social withdrawal, increased sleep, and weight gain, predictably returns every year in seasonal affective disorder.