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Seasonal depression, also called seasonal affective disorder (SAD), is a mental health condition that people tend to experience at specific times of the year.2 Researchers don’t know exactly what causes SAD, but it is believed that having less exposure to daylight (which is typical during the long, dark winter months for much of the United States) throws off levels of serotonin, a mood-regulating chemical in the brain.
Source: Verywell Health