The Facts

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.


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 centres 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.

Symptoms and Complications

SAD usually occurs during the late fall or winter months - usually between October and April. Symptoms include:

  • lethargy and fatigue (low energy level)
  • withdrawal from friends and family
  • inability to focus or concentrate
  • sadness, anxiety, and despair
  • change in appetite with cravings for sweets and starches
  • weight gain
  • headaches
  • irritability
  • increased sleep

In children, symptoms include irritability, problems in school, and difficulty getting out of bed.

Some people, especially those who experience SAD in the summer, may have an opposite pattern of symptoms, such as loss of appetite and weight loss as well as insomnia.

Making the Diagnosis

Even if someone is not predisposed to depression, SAD can affect them. The diagnosis for SAD is made when certain criteria are met.

Your doctor will ask you questions about your symptoms and perform a physical exam. Your doctor may also order blood tests to rule out other medical conditions (e.g., low thyroid) that have similar symptoms to SAD.

Treatment and Prevention

There are many options for treating SAD. It is usually treated with light therapy, exercise, and good nutrition, as well as antidepressants in more pronounced and severe cases.

Daily (professionally supervised) phototherapy with exposure to a specific type of light (full-spectrum white light) can sometimes eliminate SAD quite quickly. Phototherapy is delivered in a phototherapy device ("light box") that can be purchased or rented on a monthly basis from a private supplier or in medical device stores. People can read, but not sleep, for the several hours a day they receive light therapy.

Symptoms should subside within a few days, but they may reappear if therapy is stopped. Phototherapy should always be administered in consultation with a physician, as there are some people who should not use it.

Exercise is very helpful, especially if it's aerobic and combined with light, for example, walking outdoors on a bright winter day or exercising on a treadmill in front of a light box.

Because many people with SAD gain weight, a healthy and low-fat diet is important to help keep weight gain to a minimum.

Counselling and therapy may also be part of a treatment plan for SAD.