Sometime between the last scent of the fall harvest and the first dusting of snow, many people fall into a deep depression that may linger for as long as six months. Much more than a vague seasonal slump, this psychic chill is accompanied by weight gain, lethargy, lack of motivation, low sex drive, a craving for high-carbohydrate foods and in the most severe cases, suicidal thoughts.

No longer does the sun light up mornings and hold on to day well past the dinner hour. Now darkness greets the early riser and drapes the landscape when he's on his way home from work. For many athletes, both casual and professional ones, it's a time of dull workouts, poor performances and a general lack of zip. Most of us shrug it off as "winter blahs" and look to indoor diversions to get through the dark season.

But now, following a number of recent studies, research psychiatrists have given a name to, and are offering remedies for, what ails people during the winter. They call it SAD—Seasonal Affective Disorder—and claim the affliction appears to be caused by a chemical disruption of the body clock triggered by light deprivation. These therapists are ridding SAD patients of their winter depression by exposing them to very bright indoor lighting or directing them outdoors every day, no matter how weak the winter glow. The success rate, they say, is a remarkable 80% to 90%.

SAD was so named by Dr. Norman E. Rosenthal of the National Institute of Mental Health (NIMH) in Bethesda, Md. Like many discoveries, this one was part accident, part common sense. In the early 1980s, Dr. Rosenthal was working with a group of people who became severely depressed in the winter but snapped out of it when spring came around. Rosenthal noticed that some of the people brightened when they visited a southern climate. But those who went south experienced relapses soon after they returned north.

One of Rosenthal's colleagues, Dr. Alfred Lewy, monitored one such depressed man and found a wide swing in his body's production of the hormone melatonin, which is synthesized by the brain's pineal gland. When he was exposed to strong light, his melatonin levels dropped and his depression disappeared. But when he was deprived of supplemental light, melatonin production soared and SAD was back.

Dr. Lewy placed the patient under broad-spectrum light for a few extra hours every day, in essence duplicating a spring day. The therapy worked. Since then, indoor-light therapy has been tried on hundreds of winter-depressed patients. For the vast majority of patients, three to 10 days after treatment begins the depression leaves, energy picks up, and sleeping patterns return to normal.

Last year Dr. Rosenthal treated several children, including two competitive swimmers whose SAD symptoms—tantrums, crying spells and fatigue—would appear in December and continue until early spring. With two to four hours a day under extra light, the kids perked up. Both the swimmers noted faster times, "an uncharacteristic winter improvement in performance," as Dr. Rosenthal and his colleagues wrote in the American Journal of Psychiatry.

So, do abnormal levels of melatonin cause winter depression, and if so, is the cure for dark-season doldrums a well-lit workplace? The medical profession is certainly not one to rush recklessly to judgment. Dr. Lewy, who now works with up to 40 seasonally-depressed patients every year at the Oregon Health Sciences University in Portland, says the body's internal clock, which slows in the winter, may have more to do with it than melatonin. "The body clock follows the dawn, and when the dawn is later in winter, the clock slips in some people," Lewy says.

Why such a disruption of the body's circadian rhythms would bring on severe depression is still a mystery. In his treatment, Dr. Lewy makes his patients wake up several hours early and sit under bright lights, essentially beating the dawn to the punch. The special lights are five times brighter than normal indoor illumination.

How, then, can one explain why SAD patients feel better during a trip to San Diego or St. Augustine? "Going south [at this time of year] lengthens your day," Lewy says. He has found that some patients need as little as 30 extra minutes of strong light daily to ward off the winter blues. For everyone who has always yearned to spend the last days of winter watching baseball's boys of summer in spring training, Dr. Lewy has just offered medical justification.

Although there was a time around the turn of the century when sunlight was believed to be a cure for everything from menstrual cramps to baldness, Dr. Daniel F. Kripke, who has been encouraged by his experiments with nighttime phototherapy on severely depressed patients at the Veterans Administration Hospital in San Diego, says legitimate use of light therapy is "just now in its infancy. But we think it's going to prove to be a very useful tool."

Like other experts in the field, Dr. Kripke still does not know why light triggers a chemical reaction in the body. One theory—laughed at by some, embraced by others—is that humans tend to go into a sort of prehibernating phase during the dark months. This would explain the SAD symptoms of weight gain, the need for 10-14 hours of sleep, lethargy and a craving for carbohydrates.

"It's been looked at as a preparatory state for winter, not quite hibernation but something close," says Dr. David Avery, a psychiatrist at Harborview Medical Center in Seattle. However, recent research casts some doubt on the hibernation theory.

All that the researchers seem willing to agree on is that light, entering the body through the optic nerves, somehow cures, or at least lessens, winter depression. Many humans simply can't live in the dark without getting cabin fever—which is a less scientific, but essentially accurate, description of SAD.

If you work indoors in typical light, don't get out much for lunch or other long breaks, and live in a city north of the 40th parallel—a line approximately between Philadelphia and Reno—where there is less than 10 hours of sunlight a day in December, you are probably light-starved in winter, although that doesn't necessarily mean you will become depressed.

The sunlight measured at a window on a typical clear summer day is 2,500 lux; on a cloudy day, it's about 1,000 lux. But an average home or office is lit with a mere 500 lux. In Fairbanks, Alaska, where Dr. Carla Hellekson has found widespread cases of SAD beginning as early as September, a winter day may bring less than four hours of natural light, some of it registering as low as 60 lux.

For those who can't afford a sojourn in Palm Springs or seasonal sessions with a shrink, the experts say one can chase the winter blues a long way by getting outdoors every day. Early morning is an especially good time. But doctors warn that seasonally depressed people should not expect to be able to treat themselves with normal lighting.

What does help, according to Dr. Hellekson and others, is to sit near a south-facing skylight in the home or office, so that one can "take the sun," as the Victorians used to advise. Another suggestion is to go skiing—cross-country or downhill—since a bright winter day on snow provides almost as much light as a summer outing.

There's even hope for Sunday afternoon football-or basketball-watching couch potatoes: A mere 30 minutes a day of strong light may be enough to counter lethargy. Just by spending half-time outside, you may have something to cheer about.

Timothy P. Egan lives in Seattle, where it is cloudy an average of 228 days per year.

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