It has long been observed that the incidence of depression varies with the seasons. For example, depression and suicide seem to be more frequent in the population during the spring months in men but more common in the autumn and winter in women. In the 1980s a small group of patients considered to be suffering from manic-depressive illnesses were noted to follow a cyclical pattern with depression in the winter and mania in the summer. It was also noted that latitude had an effect on these illnesses in that moving north made depression worse and moving south had a beneficial effect (at least for those living in the northern hemisphere). The term seasonal affective disorder (SAD), has been coined to describe this small group of sufferers.

Melatonin (Circadin) helps as a treatment of seasonal affective disorder.