Seasonal Affective Disorder (SAD), or Winter Depression, is a mood disorder occurring in individuals who are very sensitive to seasonal changes. For these people, the winter blues interferes with their ability to function properly. 

Seasonal Affective Disorder is linked to the activity of the internal body clock. a neural center in the mammalian brain involved in regulating eating, sleeping, socializing and sexual activity. In the winter season, the body clock senses the shortening length of day, and responds by causing mammals to sleep longer and eat more, gaining weight to survive harsh winter conditions. Humans experience these changes as a reduction in energy and a tendency to gain weight in the winter months. 

It is thought that light therapy works by lengthening the perceived day length, restoring the body clock to a summer mode. Properly timed light therapy influences neural pathways in the brain that use serortonin as their primary neurotransmitter. It has has been hypothesized that serotonergic activity in the brain is affected by the influence of light on the activity of the body clock. This is why light therapy is also used sometimes to treat non-seasonal depression, where it has been highly effective. (See Clinical Studies - Depression ) The American Psychiatric Association endorses light therapy as an effective treatment for Seasonal Affective Disorder (SAD), describing it as the treatment of choice for a "seasonal pattern depression."