During the fall and winter months, as days get shorter and sunlight scarce, some people “get the blues,” and soon recover. But those suffering from seasonal affective disorder or “SAD” go though a much more dramatic experience, which is often difficult to overcome without help.

People with SAD tend to sleep a lot, overeat, and consequently gain weight during fall or winter. They can feel extremely tired and are unable to maintain a regular schedule. Some feel depressed and irritable, and even loose interest in social interactions.

SAD is thought to be caused by a disturbance in the sleep-wake-cycle due to a decrease in sunlight during the winter. Less sunlight leads to low levels of the hormone melatonin, which causes the excess sleep and tiredness characteristic of SAD. When the spring returns, the symptoms tend to go away.

“SAD is a real disorder that needs diagnosis and often treatment,” said Dr. Douglas Jacobs, Executive Director of “Screening for Mental Health” and a Professor of Psychiatry at Harvard Medical School.

“Although the illness is usually not associated with severe morbidity and hospitalization, some people can get suicidal. If that happens, they should consult a health care professional immediately,” Jacobs explained. “Most of the time SAD is very mild and self-limiting, but in some situations, an evaluation is needed.

Certain disorders such as hypothyroidism (abnormally low thyroid activity) or mononucleosis, a viral infection commonly known as mono or “kissing disease” can mimic the symptoms of SAD, and can be mistaken for SAD. “That’s why it’s important to get a medical evaluation,” Jacobs said.

The treatment for SAD depends upon the severity of the symptoms, and can be as simple as getting more sunlight or spending more time closer to a window during the dark months. But if the symptoms become more severe, light therapy (exposure to bright artificial light) might be required. If they become even more severe, psychotherapy and antidepressant medication might be appropriate.

Between 10 and 20 percent of the U.S. population suffer from symptoms associated with SAD, of which 75 percent are women.