Seasonal affective disorder, or SAD, is a type of depression that seems to be related to the amount of daylight to which people are exposed.

About one third of those affected have actual SAD and the other two thirds have some symptoms of SAD or "winter blues." It is three to four times more common in women than in men and it usually begins when people are in their twenties.

It is believed to be caused by a decrease in the person's exposure to sunlight.

It may be linked to the body's internal clock, which controls temperature and hormone production.

SAD may also be related to the levels of melatonin in the body, a hormone secreted by the pineal gland.

SAD is more common among people who live in northern climates, or among people who move from a sunny, southern climate to a more northern climate.