WACO, Texas (Cox News Service) - The weather's growing colder. The days have gotten shorter.

Suddenly, your mood has worsened. While you might think you have a case of the winter blues, your feelings could be something more. Seasonal affective disorder is a form of depression that recurs every fall and winter but usually subsides when spring arrives. It's a mood disorder, characterized by episodes of clinical depression, mania or both.

"It really has to do with the amount of light a person gets," said Martin Rosenzweig, a psychiatrist at the University of Pennsylvania School of Medicine. "It's kind of like a hibernation. You know, bears go through a time when all they do is sleep, they eat a lot and want to be left alone. Humans go through it too, but it just affects some people worse than others."

Rosenzweig said the disorder's intensity depends upon geographical location. For North America, people will usually be affected by the disorder from about October to March.

While the illness is more common in the North, don't rule it out in warmer temperatures. In Waco, psychiatrists said they see patients suffering from the disorder.

Stephen Mark, a Waco psychiatrist, said less than 2 percent of our population is affected by the disease, but he has treated patients for it. On average, between 5 and 10 percent of Americans suffer from seasonal affective disorder, Rosenzweig said.

"It really affects people who live in places where it's cloudy and rainy most of the time," Mark said.

According to the National Institutes of Health, seasonal affective disorder, or SAD, is rare. For those susceptible, it tends to first occur in adolescence or early adulthood and stays with a person for life.

Mark said the illness affects chemicals in the brain through the eyes. The illness is a biochemical disorder, not an emotional one, he said. Symptoms include lack of energy, loss of concentration, decreased interest in work or other significant activities, increased appetite with weight gain, increased sleep, social withdrawal and irritability.

Although these symptoms are similar to those of basic depression, experts say the two shouldn't be confused.

Treating the disorder is relatively easy. Having the person sit in front of a full-spectrum light is the best method, medical experts say. Known as light therapy, patients vary their times sitting under the bright light.

"Most light bulbs are not full-spectrum," said Rosenzweig. "There are light boxes available for a couple hundred dollars that people can use. All it takes is a half hour to an hour a day for people to feel better. (The treatment) is literally like turning the lights on for them."

Rosenzweig recommends patients sit about 14 inches away from the light box but not directly stare at it. The light should be able to penetrate the retina, but patients can perform tasks while using the box.

"I've had patients read the newspaper, exercise and do other things," he said. "The results are dramatic. Just once a day in the morning, and people feel so much better."

Both Rosenzweig and Mark suggest, if people can't afford a light box, they get outdoors into the sunshine as often as possible. Rosenzweig even has recommended to some of his patients they move to an area with plenty of sunlight during winter months. For severe cases, doctors can prescribe medication to combat the depression to work in conjunction with therapy.

"This really can be a disabling illness," he said. "It can be very difficult for people to function. They can't go to work or even get out of bed."

Rosenzweig also suggests people who think they suffer from SAD should see a psychiatrist.

"People don't recognize it as a very treatable disease, but it is," he said. "It can be fierce, but it's treatable, and people feel so much better."

Copyright 2001 Cox News Service. All rights reserved.