Seasonal affective disorder (SAD), aka seasonal depression, occurs in areas that experience lower levels of sunlight during certain times of the year.
In the United States, it’s most prevalent in the northern regions of the country—i.e., Alaska, the Pacific Northwest, the Midwest, and the Northeast. The further north you live1 (i.e., the further you are from the equator), the more susceptible you are.
According to a 2015 article from Depression Research and Treatment, SAD is four times more common in women than men and cases typically begin between the ages of 18 and 30.
While seasonal depression is a clinical diagnosis, a subsyndromal type of SAD with milder symptoms called S-SAD, or “the winter blues,” is more common. For example, 15% of the Canadian population and 20% of the U.K. population experiences the winter blues, while only 2%-6% and 2% experience SAD in Canada and the U.K., respectively.