Seasonal affective disorder (SAD) is a condition in which a person feels depressed at a certain time each year. Most people experience SAD during the fall and winter months when days are shorter and there is less light.

The cause of SAD is not clearly known. It may be related to changes in the amount and intensity of sunlight in the different seasons. Seasonal affective disorder is sometimes called "the winter blues."

Treatment of SAD includes light therapy and, if necessary, medicines.