Seasonal Affective Disorder - Causes

Scientists are not sure what causes SAD, but a hormone known as melatonin may be involved. A hormone is a chemical that occurs naturally in the body and controls certain body functions. Melatonin (pronounced mell-uh-TOE-nin) is thought to act as an internal body clock, "reminding" people when to go to bed at night and when to get up in the morning. If the hormone does not function properly, a person's body rhythms may be disturbed, which could lead to depression.

The amount of melatonin produced by the body is partly a function of the amount of daylight the body is exposed to. The more daylight, the less melatonin the body makes. The less daylight, the more melatonin. Scientists know that our bodies produce more melatonin at night and during the winter, when days are short.

One function of melatonin is to make the body feel sleepy. Some researchers think SAD is caused by an excess of melatonin during the winter months. People with too much melatonin may feel tired and depressed.