Do the bleak winter months get you down -- more than you think they should? Maybe you have seasonal depression, also known as seasonal affective disorder or SAD. Seasonal depression is a mood disorder that happens every year at the same time. A rare form of seasonal depression, known as "summer depression," begins in late spring or early summer and ends in fall. But in general, seasonal affective disorder starts in fall or winter and ends in spring or early summer.

What causes seasonal affective disorder?

There are two seasonal patterns with SAD. One starts in the fall and continues through the winter, and the other starts in late spring or early summer. The fall-onset type, often referred to as "winter depression," is better known and easier to recognize -- and we know more about it than we know about its counterpart.

Hormones manufactured deep in the brain automatically trigger attitudinal changes at certain times of year. Experts believe that SAD is related to these hormonal changes. One theory is that reduced sunlight during fall and winter leads to reduced production of serotonin in the the brain. Serotonin is a neurotransmitter that has a soothing, calming effect. The result of there not being enough serotonin is feelings of depression along with symptoms of fatigue, carbohydrate craving, and weight gain.