EVERYONE KNOWS the symptoms: restless energy, shortened attention span, daydreaming.

"It's as if some ancient, primeval barometer starts to rise in people," one local weatherman says. "And it affects everthing, not just people: chickens, ducks, rabbits, old mules . . . even the snakes seem friendly."

You won't find spring fever listed in any medical dictionary. Its symptoms often are explained as just pleasant memories reawakened by shirtsleeve temperatures, budding trees and fresh fragrances. But authorities will tell you there are physical foundations for these effects - springtime changes in health, body chemistry and even attitudes that suggest a complex relationship between the human body and its surrounding.

"Spring is a time of tremendous contrasts in the weather," says Dr. Helmut Landsberg, emeritus professor of meteorology at the University of Maryland and author of "Weather and Health," an introduction to the science of biometeorology. "The body's adaptation to these changes is what causes what we call spring fever."

Spring is a difficult transition period for mechanisms that regulate normal body temperature. Ordinarily the body takes several days to adjust its rate of metabolism, food and fluid intake and such cooling devices as perspiration. Such adjustment lags are experienced by those who leave Washington for a vacation in the subtropics.

The abrupt changes of spring weather - marked by the greatest air temperature extremes of the year and by stormy collisions between northern cold fronts and southern highs - also require rapid and continual adjustments in the body that can cause considerable physiological stress.

Normally we take these changes more or less in stride by changing the weight of our clothing and the content of our diet, though adaptation usually causes more discomfort for older people, whose bodies adjust more slowly. Failure to make these adjustments can lead to serious consequences. More cases of heat stroke, for example, are reported in late spring among older people still wearing heavy winter clothes. Rapid weather changes also bring on weather pains in bones and joints of rheumatism sufferers.

There is wide agreement, of course, that most people seem to feel better in spring. The national Center for Health Statistics reports that illnesses requiring visits to a doctor or restricted activity - especially respiratory infections like colds and flu - reach an annual low in spring. Days missed from work also decline from wintertime peaks, according to the Bureau of Labor Statistics.

But there is reason to believe that weather changes, especially the abrupt ones of spring, may have a more subtle influence on such mental states as attention span, reaction time, even mood. In one Large-scale German study, reaction times tended to be fastest during periods of stable high barometric pressure - the hallmark of fair weather - and slower during rapid pressure changes. Reaction times were found to be especially slow at times of peak weather front activity, periods also linked in many people to mental depression and even increased suicide rates.

Some scientists speculate that these changes in the nervous system may be connected to electrical surges in the atmosphere during heavy frontal activity. One California scientist has written that the concentration of ions in the atmosphere seems to affect the brain's balance of certain neurohormones - the same chemicals recently studied for their association with the brain's natural pain-killers and with cases of severe depression.

Another intriguing possibility is that the delicate balance of these neurohormones may fluctuate in annual rhythms, called circannual rhythms, controlled not by day-to-day weather changes but by a little-understood biological clock. These rhythms have been extensively studied in animals and plants.

Human circannual rhythms are known to exist, for example, in blood chemistry (white blood cells are most abundant in spring), blood pressure (reduced in springtime) and birth rate (lowest in spring, highest in fall).

Perhaps these suggestive research trails will one day help us understand more precisely what triggers spring fever symptoms and other human reactions as well. But for now these phenomena remain puzzling and sometimes, in the words of one endocrinologist, they seem "almost like voodoo."

We can still explain so little, says Dr. Landsberg, that "you're likely to get more information about spring fever from a poet than from a scientist."