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 of every 100 people are affected by SAD. About 15 out of every 100 people have less severe symptoms of SAD called the "winter blues."
SAD 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.
The cause of SAD is unknown. It is believed to be caused by a decrease in the person's exposure to sunlight. It may be linked to the body's internal clock, which controls temperature and hormone production.
SAD may also be related to the levels of melatonin in the body, a hormone secreted by the pineal gland. The nerve centers in the brain that control daily rhythms and moods are stimulated by the amount of light entering the eyes. During the night, the pineal gland produces melatonin, which makes people drowsy. In the morning, the bright light of the sun causes the gland to stop producing melatonin. On dull winter days, not enough light is received to trigger this waking up process and for some people it may become very difficult to get up in the morning.
In addition, on dark winter days, less light comes into the eyes during the daylight hours, and this may cause some people to feel "low."
Geography seems to play a role. SAD is more common among people who live in northern climates, or among people who move from a sunny, southern climate to a more northern climate. SAD can affect anyone, even if they are not already predisposed to depression. If someone has a history of depression and lives in a more northern climate, they may be more susceptible.