Oct. 8 (HealthDay News) -- If your mood, energy level, and motivation decline in November but bounce back to normal in April, you may have seasonal affective disorder (SAD), one expert says.

"SAD is thought to be related to a chemical imbalance in the brain brought on by lack of light due to winter's shorter days and typically overcast skies," says Dr. Angelos Halaris, chief of the psychiatry and behavioral sciences department at Loyola University Health System.

As many as 10 percent to 20 percent of Americans may experience a mild form of SAD, according to the American Psychiatric Association. Certain people may have a genetic vulnerability to developing the condition, which affects more women than men and tends to start appearing in the teen years.

"This condition, characterized by depression, exhaustion and lack of interest in people and regular activities, interferes with a person's outlook on life and ability to function properly," Halaris said.

You can take steps to reduce the risk of developing SAD.

"If at all possible, get outside during the winter, even if it is overcast. Expose your eyes to natural light for one hour each day. At home, open the drapes and blinds to let in natural light," Halaris recommended.

If you do develop SAD, it can be "effectively treated with light therapy, antidepressant medication and/or psychotherapy. The latest treatment is a headband containing mounted lights that delivers light to your retina whether you are inside or outdoors," Halaris said.

SOURCE: Loyola University Health System, news release, Sept. 20, 2007