It usually doesn't do much good to tell someone feeling down to lighten up, but that may be just the prescription for those whose blahs are restricted to the depths of winter.

Researchers increasingly believe that light itself is the best treatment, perhaps even the cure, for the annual cycle of depression and mania that has been called since the early 1980s "seasonal affective disorder," or SAD.Researchers with the National Institutes of Mental Health and other psychiatric experts believe as many as one in 20 Americans is severely hampered by depression and fatigue that stem directly from reduced exposure to sunlight in winter.

Another 15 percent or more may be mildly affected, and some researchers say that half of those who live in northern climes find their mood at least somewhat altered in tune with the seasonal cycle. The same people experience moods varying from ebullience to sheer mania when springtime signals the return of the sun.

Far smaller proportions of people apparently suffer from "summer SAD," a similar depressive syndrome that appears to result from excess heat and humidity.

Light's interaction with mood is all the more interesting because recent research has revealed how closely tied it is to the body's daily cycle, particularly sleep. And psychiatrists have known for years that sleep and mood are closely linked.

So an idea that just a decade ago sounded radical - that exposure to bright light could ease some forms of depression and fatigue - is now well accepted.

Some researchers go so far as to say that light could be useful in non-seasonal depression as well. Perhaps anyone who is depressed, mildly or clinically, might feel at least a temporary easing of symptoms after a one-hour stroll around the neighborhood.

The seasonally affected can recognize themselves, experts say, by several tell-tale symptoms in addition to depression itself. Those with SAD are perpetually bone-tired, even though they're probably sleeping much more than usual. And they tend to binge on sweets and other carbohydrates, putting on pounds they then shed without effort come spring and summer.

The answer? Lighten up - naturally if possible, artificially if not. Severe cases benefit from light-bathing in front of banks of bulbs whose glow simulates the sun's spectrum of visible light. One researcher even invented a computerized bank of lights that imitates a Caribbean sunset at bedtime and a sunrise when it's time to rise.

The mildly affected don't need to go nearly so far.

Studies have found that many people spend only a few minutes outdoors each day during the winter. But even a cloudy or rainy day sheds far more powerful light on the eyes than the brightest home or office.

The best idea, according to researchers, is to spend at least an hour or two every day outside, no matter what the weather. Walking part way to work or taking a long stroll at lunchtime should do the trick.

It might also help to move your sofa or desk by a window. Every little bit of sunlight can help ease the burden of SAD.