It's that time of year when darkness descends like a heavy blanket beginning in mid-afternoon in much of the country. For some people, it also brings a desire to stay in bed and wait for spring.

Seasonal affective disorder, or SAD, affects an estimated 6% of Americans, causing depression, lethargy, irritability and a desire to avoid social situations. It can also create an urge to overeat, particularly carbohydrates. As many as 15% of people in the U.S. may have a milder version that includes only some of these symptoms. The incidence rises along with the distance from the equator: Roughly 8% of Canadians, 10% of Britons and as many as 20% of Scandinavians suffer from SAD this time of year.

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