While gray, polluted skies have a serious and well-documented negative health impact for people who have asthma or heart disease, it has little to do with feeling gloomy.

You can blame that on the shorter days of winter, according to experts who treat in an illness called Seasonal Affective Disorder.

"I haven't seen anything convincing" to indicate air quality affects emotions or mental health, said Dr. Fred Reimherr, director of the Mood Disorders Clinic at the University of Utah. But studies have absolutely shown that people with SAD are seriously affected by the shorter days that are winter.

"(Air quality is) certainly playing havoc with asthma," said Dr. David Tomb, professor of psychiatry at the U. School of Medicine. "But air quality is less a factor than short days for people who are SAD-inclined."

Women are as many as four times more likely to have the condition, which is a "chronobiological" or body clock problem that affects some people who live north of 40 degrees latitude. It's also very treatable. SAD sufferers respond well to two things: traditional anti-depressant medications and light therapy using bright, full-spectrum light.

Activity level is important, too, Reimherr said. He encourages patients to exercise in the morning, preferably in front of the bright light. That helps shut down the secretion of melatonin, which contributes to the illness.

He also counsels them to get up at a consistent time, "maybe earlier than usual."

SAD is more common the farther north you go for Americans because the days are shorter. It's almost unheard of in warm, sunny places like Arizona.

Treatment of patients with SAD may include time in front of a bright light box every day for as much as six hours. People get used to doing everything in front of that light, Reimherr said.

Tomb has students who study in front of the light. What matters, he said, is that "it's shining right smack in front of them." A trip to the tanning salon doesn't help. "The benefit is solely the light coming in through your pupil," he said.

Like any other serious depression, SAD is a dangerous disorder, Tomb said. But it's also an "atypical" depression. While people with major depression tend to lose their appetites and thus weight and complain of inability to sleep, people with SAD frequently overeat, gain weight and sleep too much. And some SAD patients feel so good during the longer, brighter days of summer that they end up "halfway between normal and manic," Tomb said. That's also a risk when they use anti-depressants, so patients must be carefully monitored.