A surprising finding published in the most recent issue of Clinical Psychological Science:
Seasonal affective disorder (SAD) is based on the theory that some depressions occur seasonally in response to reduced sunlight. SAD has attracted cultural and research attention for more than 30 years and influenced the DSM through inclusion of the seasonal variation modifier for the major depression diagnosis. This study was designed to determine if a seasonally related pattern of occurrence of major depression could be demonstrated in a population-based study. A cross-sectional U.S. survey of adults completed the Patient Health Questionnaire–8 Depression Scale. Regression models were used to determine if depression was related to measures of sunlight exposure. Depression was unrelated to latitude, season, or sunlight. Results do not support the validity of a seasonal modifier in major depression. The idea of seasonal depression may be strongly rooted in folk psychology, but it is not supported by objective data. Consideration should be given to discontinuing seasonal variation as a diagnostic modifier of major depression.
Here’s one passage from this very interesting paper:
There have been other reports that support this finding and cast doubt on sunlight exposure as a causal factor in depression. Hansen et al. (2008) reported no increase in depression in northern Norway during the twomonth-long “dark period” (p. 121). A large-scale study of residents of Tromsø, Norway, a city north of the arctic circle and also subject to the two-month polar night, found neither an increase in self-reported mental distress during the polar night nor a decrease in reported mental distress during the polar day (Johnsen, Wynn, & Bratlid, 2012).