Seasonal affective disorder (SAD) is a form of depression that starts in the fall and ends in the spring. This article reviews existing theories about the relationship between circadian rhythms and the disorder. Recent research indicates that as with pharmacologic antidepressants, at least 2-4 weeks are needed to demonstrate the effectiveness of bright-light therapy compared to placebo. The response to such treatment is strongest with precisely timed light exposure: treatment is optimal during the morning hours when the circadian system is susceptible to phase advance. Such clinical improvement is correlated with the magnitude of the phase shift induced. These observations suggest a model of circadian function in SAD and provide important guidelines for its treatment.