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Depression Health Center

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Understanding Seasonal Affective Disorder (SAD) -- the Basics

What Is Seasonal Affective Disorder?

Seasonal affective disorder (SAD) is an extreme form of the "winter blues," depression that occurs at the same time each year, bringing lethargy and curtailing normal functioning. It was only recently recognized as a specific disorder, but since 1982, much has been learned about it and how to treat it. People suffering from SAD undergo extreme changes in mood that fluctuate with the seasons, as if they were split between a "summer person" and a "winter person."

Although a different kind of SAD can occur in the summer, the most common form ("winter depression") begins gradually in late August or early September and continues until March or early April, when the symptoms begin to dissipate. Sufferers have been known to increase their sleep by as many as four hours a night and gain more than 20 pounds as they attempt to "hibernate" the winter away. Research suggests that SAD may affect 11 million people in the U.S. each year and that an additional 25 million suffer a milder form that is indeed called the winter blues. Four times as many women suffer from SAD as men, and it tends to run in families.

As might be expected, geographical location plays the largest role in susceptibility to SAD; the nearer one lives to one of the poles, the greater the incidence. People in Canada or the northern U.S. are eight times more likely to fall victim to SAD than those living in sunny, more temperate areas like Florida or Mexico.

SAD usually begins in a person's early 20s and the risk for developing it decreases with age.

What Causes Seasonal Affective Disorder?

Researchers are still far from agreement about the precise cause of SAD and suggest it may have more than one cause. Currently, the most likely explanation involves abnormalities of brain pathways that utilize neurotransmitters (chemicals) such as serotonin. During the short days of winter, serotonin may work less efficiently in key parts of the brain that regulate mood. In addition, SAD is triggered by inadequate outdoor light and exacerbated by stress. Heredity may also play a role.

Some researchers believe that a lack of sunlight disrupts circadian rhythms, which regulate your body's internal clock.

For children, the fall onset of SAD comes at the time that school starts, and it is difficult to sort out SAD from other possible reasons for mood changes. SAD should be considered as a cause for mood changes in children.

WebMD Medical Reference

Reviewed by Joseph Goldberg, MD on February 27, 2015

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