Definition of Seasonal affective disorder:

Seasonal affective disorder (SAD) is a form of depression that occurs in relation to the seasons, most commonly beginning in winter.

Causes, incidence, and risk factors:

Seasonal affective disorder (SAD) is marked by symptoms of depression profound enough to seriously affect work and relationships. The disorder may have its onset in adolescence or early adulthood and, like other forms of depression, occurs more frequently in women than in men. Most people with the "winter blahs" or "cabin fever" do not have SAD.

The cause of SAD is not known, but is thought to be related to numerous factors including body temperature, hormone regulation, and ambient light. A rare form occurs in the summer.