The syndrome of seasonal affective disorder (SAD) is defined as a history of major depressive episodes that recur regularly at a particular time of year, typically fall or winter, and completely remit in the spring. It has been hypothesized that photoperiod-related changes in the duration of melatonin secretion may be involved in the seasonal mood cycles of SAD in human beings, suggesting that artificial bright lights (mimicking daylight) might be used to treat SAD. This article reviews studies on the use of bright light therapy and antidepressant medication for the treatment of SAD. Studies have found that bright light therapy and antidepressant medication are both effective for the treatment of SAD. Bright light therapy may also be effective for treating nonseasonal depression.