Seasonal affective disorder, or SAD, also known as winter depression is an affective, or mood disorder. Most SAD sufferers experience normal mental health throughout most of the year, but experience depressive symptoms in the winter. SAD is rare, if existent at all in the tropics, but is measurably present at latitudes of 30°N (or S) and higher.

Connections between human mood, as well as energy levels, and the seasons are well-documented, even in healthy individuals. Particularly in high latitudes (50°N or S) it is common for people to experience lower energy levels during the winter. Colds and flu also peak during this time, and most people get less outdoor exercise than in the summer.

Norman Rosenthal, MD, has estimated that the prevalence of SAD in the adult US population is between 1.4 percent (in Florida) and 9.7 percent (in New Hampshire).

Seasonal mood variations are believed to be related mostly to daylight, not temperature. For this reason, SAD is prevalent even in mid-latitude places with mild winters, such as Seattle. Prolonged periods of overcast weather can also exacerbate SAD. Normal "winter blues" can usually be dampened or extinguished by exercise and increased outdoor activity, particularly on sunny days, resulting in increased solar exposure. SAD, however, is a more serious disorder, sometimes triggering dysthymia or clinical depression. It may require hospitalization.

Various etiologies have been suggested.[1] One possibility is that SAD is related to a lack of serotonin and that exposure to full-spectrum artificial light may improve the condition by stimulating serotonin production, although this has been disputed.[2],[3] Another theory is that melatonin produced in the pineal gland is the primary cause,[4],[5] since there are direct connections between the retina and the pineal gland. Some studies show that melatonin levels do not appear to differ between those with and without SAD. However, mice incapable of synthesizing melatonin appear to express "depression-like" behaviors[6],[7] and melatonin receptor ligands produce antidepressant-like effect in mice[8]. Light therapy appears to be effective in treating SAD, but the exact mechanism of the effect is still unknown.

Treatment

Full-spectrum bulbs, "light boxes" and "sunlight lamps" can be purchased as specialty lighting products for those suffering from SAD. These light boxes are many times brighter than regular indoor lighting and should be administered with guidance from a psychiatrist. Natural sunlight is considered to be the best source of light.