Definition of Seasonal affective disorder:

Seasonal affective disorder (SAD) is episodes of depression that occur at a certain time of the year, usually during winter.

Causes, incidence, and risk factors:

The disorder may begin during the teen years or in early adulthood. Like other forms of depression, it occurs more often in women than in men.

People who live in places with long winter nights are at greater risk for SAD. A less common form of the disorder involves depression during the summer months.

Other factors that may make SAD more likely include:

• Amount of light
• Body temperature
• Genes
• Hormones