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DURING MY last winter in North Dakota, I nearly snapped.
Because I grew up there, the cold, darkness and isolation brought on by this nether season were entirely familiar; but that year, 1983, was distinctive.

During one 10-day stretch, temperatures never got above zero, with wind chills as low as 100 degrees below.

My car died. My friends' cars died. I spent a lot of time alone in my dark, virtually windowless studio apartment.

By March, I hated most of the other forces at work in my world: my love life, my financial situation, my professional status. But mostly, I was sick of winter.

Physical rigors aggravated by general dissatisfactions: That was how I'd always interpreted the toll on my psyche that year.

I didn't know it then, but during this winter of discontent researchers at the National Institute of Mental Health in Bethesda, Md., were in the midst of studies establishing a relationship between seasons and moods.

They went on to define a condition they called seasonal affective disorder, or SAD. When I heard about it, I wondered: Could that have been my problem?

The influence of the seasons on health has been observed for ages. Hippocrates noted that "some are well or ill adapted to summer, others are well or ill adapted to winter."

Such notions were generally pooh-poohed by modern medicine until the mid-1980s. Now research indicates that most of us experience at least some degree of seasonal change.

In a 1987 telephone survey of 416 adults in Montgomery County, Md., 92 percent of the respondents reported varying degrees of seasonal change in mood and behavior.

Dr. Norman Rosenthal, National Institute of Mental Health director of seasonal studies, and author of "Seasons of the Mind: Why You Get the Winter Blues and What You Can Do About It" (Bantam, 1989), says that up to one-quarter of the population may suffer from SAD or a mild variation of it.

"To me, that's a lot," he says -- more than he and his colleagues had expected. "If you had told me when we started our research that a quarter of the population was affected, I would have told you it was nonsense."

The discovery -- or rediscovery -- of seasonal influences on people began with animal research.

The seasons have long been known to affect animals profoundly: Many are fertile only during certain times of the years. Light, it seems, triggers seasonal reproductive changes through the hormone melatonin.

Every night, melatonin is secreted into the bloodstream by pea-size pineal gland at the base of the brain. This secretion tapers off at dawn, marking the duration of darkness and providing animals with a seasonal time cue.

How light suppresses secretion and whether melatonin causes similar cues in people are unclear, but in 1982 Dr. Alfred Lewy of mental health institute made a crucial discovery: Melatonin secretion in humans also can be suppressed by light.

This, and the fact that information from the eye travels along nerve pathways to the hypothalamus -- part of the limbic system that regulates emotions and basic body functions -- led researchers to try using bright lights to treat a man named Herb Kern.

Kern came to the mental health institute bearing a number of notebooks in which he'd kept scrupulous track of regular seasonal fluctuations in his mood and behavior going back some 15 years.

He figured light had something to do with his emotional swings and asked researchers for help. They suggested he artificially lengthen his winter day by six hours, using a 2-by-4-foot box that emitted high-intensity fluorescent light softened by a diffusing screen.

The results were dramatic. Kern's mood improved noticeably within three days.

The classic symptoms often begin in the fall with a feeling of anxiety over the approaching winter.

During winter months, sufferers typically feel like hibernating. They become lethargic, spend more time sleeping, crave sweet and starchy foods, gain weight, withdraw from other people and lose interest in sex.

A less common summer syndrome brings opposite symptoms in many cases: insomnia and loss of appetite and weight. (Sex drive falls in summer, too.)

SAD is four times as likely to show up in women. The reason for this isn't clear, but the association of depression with premenstrual syndrome and the existence of postpartum depression suggest that endocrine changes may influence mood.

Researchers estimate that in the United States, 6 percent of us -- about 10 million people -- suffer from full-blown SAD. These are the people affected most severely and explicitly, the hibernators who find seasonal change so debilitating that it can affect jobs and relationships.

They suffer from the disorder defined in the "Diagnostic and Statistical Manual," which specifies the conditions necessary for a seasonal depression to be clinically identified. Among them:

Depression occurs within a particular 60-day period -- say, from the beginning of October to the end of November.

Full remissions also occur within a particular 60-day period -- say, mid-February to mid-April.

These changes have happened at least three times in three separate years, two of which were consecutive.

Seasonal episodes outnumber non-seasonal episodes by more than 3 to 1.

There haven't been any extenuating seasonal stresses, such as being unemployed every winter.

The scientists who conducted the Maryland survey, led by Dr. Siegfried Kasper of the University of Bonn, Germany, say that clinical SAD represents the extreme end of a spectrum of seasonality affecting a large percentage of the population.

Within that spectrum are people who don't meet the clinical criteria for SAD but notice the same symptoms to a lesser degree.

Their seasonal doldrums may make them feel less energetic, less efficient and less sociable, but they are able to work around their difficulties, consider themselves normal and don't seek treatment.

There is no laboratory test for SAD: Diagnosis is based solely on case history.

Generally, treatment consists of sitting, often in one's own home, at a specified distance from the front of a metal box containing bright fluorescent lights.

The illumination from a typical light box, which costs around $450, is about five times brighter than that found in a well-lighted room.

How much time is spent in front of the box depends on the person, the time of year and the geographic location.

"The procedure is innocuous, and you know quickly if it's working," says Dr. Michael Terman, director of the light-therapy unit at New York State Psychiatric Institute, Columbia-Presbyterian Medical Center, in New York City. "Within seven to 10 days, you know unambiguously if the symptoms have gone away." Most patients improve significantly within two to four days.

There are other ways to treat depression, and often patients undergoing light therapy have tried other methods, or may find them beneficial.

Light therapy can be applied in conjunction with psychotherapy, or it can complement the use of antidepressant medication, often allowing the patient to take lower drug dosages.

"Many people experience a deep sense of surprise that such a simple environmental manipulation can affect them to the core," he explains. "We're not used to being so deeply tied to our environment."

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