Episodes of depression triggered by change in season especially at the onset of winters. It is characterized by recurrent depression usually beginning with the season and ending with it. The usual symptoms are decreased activity, social withdrawal, loss of interest in sex, increased craving for carbohydrates, increased appetite, oversleeping and weight gain.

It is believed to be caused by increased melatonin secretion(hormone secreted by pineal gland of brain). Other than medications it is uniquely treated by exposure to light (light therapy) which appears to suppress melatonin production and its depressive effects