Seasonal affective disorder, or SAD, is a form of mood disorder or depression directly affected by the seasons. Sufferers of SAD find that their emotional state is quite normal during the summer months, but as the days grow shorter, colder and greyer during the autumn and winter, their moods seem to match the seasons. The seriousness of SAD can range from the mild ennui of dysthymia to a serious attack of clinical depression, which can require hospitalization.
It is believed that SAD is related to a lack of serotonin and that exposure to wide-spectrum artificial light may improve the condition by stimulating seratonin production. Light therapy appears to be effective in treating SAD, but the exact mechanism of the effect is still unknown.
One recent trial seemed to indicate that shining a bright light behind the sufferers' knees would be beneficial, but when the trial was duplicated on a larger scale, the results were negative.