Seasonal affective disorder is a form of depression with symptoms that occur during the winter months, usually going into remission during the spring and summer. Although it was first noted before 1845, it has only received wide public attention in the last decade or so.

It has long been known that sunlight, as it peaks and ebbs over the year, affects many animals’ seasonal activities, such as hibernation or reproductive cycles. Apparently, humans are no exception.

Researchers have tied SAD to melatonin, a sleep-related hormone that the human pineal gland produces and releases in the dark. Production of the hormone seems particularly active during winter, when the days are shorter and darker.

In 1980, a researcher named Dr. Alfred Lewy discovered that the very bright light blocks the release of melatonin in people – and relieves winter depression. Patients generally respond to bright light therapy within four days of starting treatment – relapsing within four days after stopping the therapy.

Because so many people respond to bright light treatment, it’s assumed that light has an antidepressant effect, and there have been no research findings to indicate anything to the contrary. A definite link between the patient’s response and the way light affects melatonin, however, has not yet been established.

The disorder usually begins in early adulthood, and four times as many women as men are affected. For most people with SAD, January and February are the worst months.

The symptoms for SAD are rather specific to avoid misdiagnosis for other depressive disorders:

• Regularly occurring symptoms of depression (sad, anxious or "empty" moods; decreased energy and interest, etc.) during the fall/winter months of at least three different years – two of them consecutively
• At least three times as many instances of depression within a two-month time frame as during other times of the year
• No other factors that could account for regular changes of mood (become unemployed every winter, etc.)
• Excessive eating and sleeping, weight gaining

Phototherapy (light treatment or therapy) has been effective in relieving SAD. Patients are seated three feet away from a full-spectrum florescent light (about 12 times brighter than ordinary room light) and asked to glance at it about once a minute. The only side effects it seems to cause are occasional eyestrain and headaches.