Seasonal affective disorder (SAD), also known as winter depression, winter blues, summer depression, summertime sadness, or seasonal depression, is a mood disorder subset in which people who has normal mental health throughout most of the year experience depressive symptoms in the winter or summer.
Familial studies suggest a higher incidence of SAD among first-degree relatives, with genetic factors accounting for between 29-69% of variance in seasonal mood symptoms. This suggests that genetic aberrations may underlie the various abnormalities, which cause SAD symptoms. Melatonin secretion occurs later in the night, and for longer periods during the early morning compared to healthy individuals.
The prevalence estimates of SAD across 20 retrospective studies varied from 0% to 4.7%. All prospective population studies, except one, find seasonal variations in mood, depressive symptoms usually peaking in winter. SAD was more prevalent at higher northern latitudes, but the prevalence varied across ethnic groups. SAD has also been identified in children and adolescents. it has also been found in bulimia nervosa, anxiety disorders and other psychiatric illnesses.