Seasonal Affective Disorder (SAD) is a form of depression that usually starts in late fall as the days get shorter. It subsides as spring approaches and there are more hours of daylight.

Women are most likely to experience SAD, while children and adolescents are least likely to be hit by it, says information from the Medical College of Wisconsin. A person has to experience SAD for at least two consecutive years to be properly diagnosed with the disorder.

In most cases, SAD symptoms are not as severe as major depression and it is not linked to a traumatic event, as with major depression.

Typical SAD symptoms include depressed mood, anxiety, irritability and general loss of interest or motivation. However, compared to people with major depression, those with SAD eat more and may binge on foods with lots of carbohydrates. People with SAD seldom have thoughts of suicide.

It's believed that SAD may be caused by changes in a person's natural body rhythms caused by reduced exposure to sunlight.

Increased exposure to light is standard treatment for SAD. Normal room lighting isn't sufficient. SAD treatment requires a lightbox with a minimum lux value of 2,000 to 2,500. Some lightboxes have a lux value as high as 10,000.

A lightbox needs to be used for 30 minutes to two hours per session and should be located within 3 feet of the person using it. The user doesn't have to stare at the light, but the light must be able to enter the eyes to affect the brain and the person's mood. Many people read while using a lightbox.

A lightbox may not be sufficient to relieve SAD symptoms. Other treatments may include regular exercise and prescription antidepressants.

More information

The National Mental Health Association has more about SAD.