Spring fever isn't just an excuse.

Scientists have found a biochemical explanation for the conventional wisdom that spring can banish the depression that afflicts many during the dark depths of winter.

They have discovered that the brain measures the length of each day, using the eyes as sensors, and the information enables it to regulate the ebb and flow of brain hormones that influence emotion and behavior.

When days are short, the pineal gland in the brain makes more of the substance melatonin, which brings on depression. As the days grow longer -- a phenomenon that accelerates as spring approaches -- the pineal gland makes less melatonin and life again seems worth living.

Researchers also have discovered that the depression, which they call SAD for Seasonal Affective Disorder, can be cured artificially by lengthening the "day" with electric lights.

Bright lights -- much brighter than those in most homes or offices -- have been used to cure victims of the most severe form of SAD, which can leave people almost suicidally depressed. After three or four days of light treatments, scientists at the National Institute of Mental Health in Bethesda report, patients' moods lift dramatically.

The treatment must continue, however, because three or four days after a patient goes back to naturally short days, SAD sets in again.

More convenient, though more gradual, is nature's method. As the Earth's movement along its orbital path causes the North Pole to tip more toward the sun, the days grow not only warmer but longer. Today, for example, the sun will shine 17 minutes longer than it did last week at this time. By next Sunday, daylight will last 18 minutes longer than today.

The longer bright light enters the eyes, the longer nerve signals from the eye can suppress the pineal gland's manufacture of mood-depressing melatonin.

In some reptiles the pineal has long been called a third eye because it lies at the surface of the brain, just inside a thin patch of skull that lets in light. In mammals, which have evolved brain tissue that covers the pineal, light signals must come from the eye, traveling a short way down Bright lights -- much brighter than those in most homes or offices -- have been used to cure victims of the most severe form of SAD, which can leave people almost suicidally depressed. the spinal cord and then back up to the pineal, which lies near the center of the brain.

The pineal gland's function has long been a mystery, but Lawrence Tamarkin, a biologist at NIMH, and his colleagues think that it serves many mammals as a mechanism for coordinating annual reproductive cycles so that births occur during the best season for offspring survival. This action may also occur in human beings.

Tamarkin suspects that melatonin is the pineal's reproduction-regulating agent. He has found that melatonin levels fluctuate daily in every mammal species tested, going up at night and down in the day, the total varying with the day's length. The substance can be found in blood as well as brain fluids.

Its effect on reproduction varies according to the species -- in some cases it makes the testes grow or shrink -- but in general it stimulates more frequent mating during the time of year that precedes the optimal birth season by the length of the gestation period.

"We think something similar may be happening in people," said Frederick Jacobsen, a psychobiologist at NIMH. Patients severely afflicted with SAD are more likely to have been born in June and July than are people in general. "The parents of our patients may have been responding somehow to the days getting shorter the previous fall."

Jacobsen said there is a strong familial tendency in SAD.

He and Norman E. Rosenthal, head of NIMH's SAD research program, say they think that the affliction is fairly common, especially in more northern latitudes where annual day length changes are most extreme. In addition to depression, they say, the most typical symptoms are a slowdown in physical activity, oversleeping, overeating and, curiously, a craving for carbohydrates.

To test whether day length was a factor, Rosenthal exposed groups of patients to special lights for three hours every morning and three hours every evening, stretching their effective day length to that of summer.

The special lights were chosen because they include the entire color spectrum of sunlight and emitted a brightness of 2,500 lux, which is equivalent to the intensity measured at a window on a clear spring day. Ordinary room lighting, by contrast, is only around 100 to 200 lux.

The bright light worked. The melatonin role was confirmed when tiny injections of melatonin quickly brought back the patients' depression.

Other patients who were kept awake the same length of time but exposed to only 500 lux showed little or no improvement.

Researchers also suspect, but have not done studies to prove, that factors other than the seasons may alter the effective day length for some people.

Those who work at night, for example, may be exposed to such short periods of daylight that they are always somewhat depressed, though presumably less so in summer.

Blind people who have little or no nerve transmission from the eye also may suffer.

However, the researchers say the effects should vary greatly among people, just as they do for those affected by purely seasonal change.