Every winter up to 50 percent of the populations of northern American cities have noticeable shifts in energy and mood.

Most victims experience winter depression, caused by the annual decrease in light, states a report in the magazine American Health. But a smaller number suffer in summer; their mood problems are thought to be provoked by humidity and heat.Extreme cases of these seasonal changes in behavior are called seasonal affective disorder (SAD) - a pattern of depression, and sometimes mania, that comes at one time of the year and evaporates in another. SAD may seriously affect as many as 10 percent of the people who live in northern latitudes, according to scientists.

Just as the annual decrease in light can trigger the winter blues, increasing light is the key to lifting the season's bleak mood. And a lamp can be as effective as natural sunlight. Innovative devices such as "phototherapy" panels - banks of very bright lights - can deliver rays that banish SAD.

Thomas Wehr, chief of the clinical psychology branch at the National Institute of Mental Health (NIMH) believes the use of a simple environmental agent - light - to change behavior has profound philosophical implications for the future of the psychiatric profession as well. Many patients who end up in SAD clinics have already had a great deal of traditional talk therapy that did nothing to relieve their depression symptoms. Their response to phototherapy - up to 70 percent - presages the dawn of a new branch of drugless, talkless psychiatric treatment.

Notes Wehr, "If you exposed yourself to light and three or four days later you felt much better, your whole attitude was different, your ability to function was dramatically improved and you had become more creative, what's to talk about?"

A recent collaborative survey conducted in four U.S. latitudes, each with its different length of day and angles of winter sun, showed how geography helps to shape mood. The survey showed that fewer than 2 percent of Florida residents experienced seasonal mood problems, while the figure jumped to 6 percent in New York and 10 percent in New Hampshire.

Michael Terman, director of the light therapy unit of the New York State Psychiatric Institute in New York City, conducted the New York area segment of the latitude study. He found that while 6 percent of his subjects have true SAD, 18 percent fit into the subsyndromal category and another 26 percent experience a slowdown of body and mind.

"When you add the three groups together," he says, "half the New York area - more than 4 million people - suffer from some sort of seasonal shift in what is their normal behavior."

Terman says when he wants to be sure he sleeps well, he turns on his "tropical dawn machine" - a computerized device he invented to re-create a Caribbean-style dawn and sunset. The electrical sun starts to sink as he gets into bed at 11 p.m., and he's fast asleep by 11:25. Sunrise happens promptly at 7 a.m., when he wakes feeling alert and refreshed.

"I spent a winter with the machine and I was always asleep and awake at the right times, despite my chaotic habits," says Terman.