Epidemiology

Lifetime estimates for depressive and bipolar disorders with a seasonal pattern average between 0.4% and 2.9% in US, Canadian, and UK community studies.[1][2][3][4] Some estimates are as high as 9.7%.[5] This is likely due to differences in the sampling and diagnostic criteria used. Rates of seasonal affective disorder (SAD) may be slightly higher among people living in more northern latitudes.[3][6]​ Although the latitude-SAD association has been demonstrated in North American samples, this finding has not been reliably replicated in European cohorts. This suggests the influence of other factors, such as genetic variability, cultural differences, and climate.​[7]

Fall or winter onset of major depressive episodes is much more common than other seasonal-mood fluctuations.[6][8]​​​​ Approximately 25% of people with SAD have bipolar I or II disorder, with SAD being more likely in bipolar II disorder.[9] SAD appears to show an earlier onset among people with bipolar II disorder compared with those with major depression or bipolar I disorder.[10]​ The incidence of SAD may be higher in some populations with anxiety, ADHD, and premenstrual dysphoric disorders.[11][12][13]​​ Alcohol use may also increase as a means of coping with SAD symptoms in some populations.[14]

The average age of onset is between 20 and 30 years, with declining rates in older populations.[15] SAD is approximately 3 to 5 times more likely among women, which is a greater sex difference than that observed in nonseasonal depression.[16] The prevalence in children and adolescents ranges from 3.3% to 4.2%, with the incidence increasing among girls during puberty.[17][18]​​ Other studies have noted that parental ratings of depression are more severe among 16- to 18-year-olds than among 6- to 15-year-olds when assessed during the fall and winter months.[19]

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