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:

The disorder may begin in adolescence or early adulthood. Like other forms of depression, it 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 it is thought to be related to numerous factors, including:

• Ambient light
• Body temperature
• Hormone regulation

A rare form occurs in the summer.