Seasonal affective disorder (SAD) is a mood disorder characterized by recurrent depressive episodes that occur and resolve with changes of season. Although recurrent spring-summer depressions have been documented and may be classified as SAD, the most common form of the disorder involves onset of depression in the late fall or early winter with remission in the spring or summer. Thus, winter pattern SAD is the focus of the majority of studies that have been conducted. The only widely-used instrument to detect SAD is the Seasonal Pattern Assessment Questionnaire (SPAQ), a questionnaire that looks back in time and assesses the magnitude of seasonal change in sleep, social activity, mood, weight, appetite and energy. The SPAQ is a very simple, brief and useful screening questionnaire, but a careful clinical evaluation is still necessary to confirm the diagnosis. Using this questionnaire alone, the incidence of SAD is about 1-10% in North America and 1% in Asia. A more thorough evaluation places the incidence at 2-3% in Canada and 1% in the United States. For comparison, this is about the same incidence as manic-depressive disorder.

It has been a common belief that there is an increase in prevalence of SAD with increasing latitude (further north, with longer winters). However, more recent studies have shown that the latitude effect is not as robust as previously thought.