Seasonal affective disorder (SAD) is a type of depression that seems to be related to the amount of daylight to which people are exposed. For most people it tends to be worse in the fall or winter, making it an extreme form of the "winter blahs." Some people, however, experience symptoms in the late spring or early summer.
Every year, as the days get shorter and the weather gets colder, it is estimated that about 2 or 3 out or every 100 people are affected by SAD. About 15 out of every 100 people have less severe symptoms of SAD called the "winter blues."
It is more common in women than in men and it usually begins when people are in their 20s. Older people are at lower risk. Children can also experience SAD, although it is far less common.