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.
The cause of this 'phase delay' and why it has such a pronounced effect on mood and behaviour, is likely to be multifactorial, involving abnormalities at various levels along the retino-hypothalamic tract, its links with the pineal gland, and the metabolism of melatonin and serotonin. Familial studies suggest a higher incidence of SAD among first-degree relatives, 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 mean prevalence of SAD is two times higher. Over all prevalence studies, the correlation between prevalence and latitude was not significant. A significant positive correlation was found between prevalence and latitude there was a trend in the same direction were estimated at 9.6%