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Seasonal Affective Disorder (SAD)

SAD (Seasonal Affective Disorder) is a type of winter depression that affects an estimated half a million people every winter between September and April, in particular during December, January and February. SAD was first noted before 1845, but was not officially named until the early 1980’s.

What Causes SAD?

As seasons change, there is a shift in our “biological internal clocks” in the brain that regulates our circadian rhythm. This biological clock responds to changes in season and especially to the different patterns of light. This can cause our biological clocks to be out of “step” with our daily schedules. Other research shows that neurotransmitters, chemical messengers in the brain that help regulate sleep, mood, and appetite, are also disturbed in SAD.

Symptoms include:

  • Low mood, reduced interest in normally pleasurable activities
  • Decreased concentration
  • Oversleeping (often an increase of 4 hours or more each day)
  • Decreased energy and fatigue
  • Intense craving for carbohydrates and sweets
  • Weight gain
  • Withdrawal from social contacts
  • Depression

Most people suffering from SAD show signs of a weakened immune system during the winter and are more vulnerable to infections and other illnesses. SAD symptoms disappear in the spring, either suddenly with a short period of hyperactivity, or gradually, depending on the intensity of sunlight in the spring and early summer.

Who is at Risk?

Across the world the incidence increases with distance from the equator. It occurs throughout the northern and southern hemispheres and is extremely rare in those living within 30 degrees of the Equator, where daylight hours are long, constant and extremely bright. SAD may begin at any age but the main age of onset is between 18 and 30 years. More women than men are diagnosed as having SAD. Children and adolescents are also vulnerable.


As the cause is lack of bright light, the treatment is to be in bright light every day by using a light box or a similar bright light therapy device. The preferred level of light is about as bright as a spring morning on a clear day and for most people sitting in front of a light box, allowing the light to reach the eyes, for between 15 and 45 minutes daily will be sufficient to alleviate the symptoms.

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