Each afternoon, many people find themselves slipping into a solitary struggle against nature. Sometime around 3 o'clock, they crash. Minds start to wander, eyelids droop, temperatures drop, breathing slows -- there is an almost overwhelming urge to . . . nod . . . off.
Most working adults fight the urge by sitting, slouching, furtively lying down, or sneaking out for a walk or cup of coffee.
It is odd that such a pervasive biological need is regarded as a secret shame. Industrialized societies in general equate naps with laziness, irresponsibility, immaturity and senility.
Such societies are geared to what biologists call a "monophasic" model consisting of rigidly separated periods of sleep and wakefulness. Even in the scientific fields of sleep research and sleep-disorders medicine, napping traditionally has been considered unhealthy and inappropriate. Daytime sleepiness, in fact, is a classic sign of a problem, and many doctors view it with suspicion.
But as intriguing new data keep rolling in, this picture is starting to change.
The urge to nap is increasingly being viewed as a biological imperative, a daily dip in the body's natural circadian rhythm, and may be the only basic human need that gets short-changed because of its negative image.
There is a sneaking suspicion that, like many animals, human beings may actually be "biphasic" creatures, in which activity patterns are separated by two troughs of sleep, nocturnal and mid-afternoon. Studies show naps occur in predictable time frames, rather than randomly.
And lunch, commonly blamed as provoking drowsiness, may be innocent. The napping urge hinges on time of day, rather than food intake, and its intensity may rely on sleep lost the previous night.
About 55 percent of the American population are either active or closet nappers, while 45 percent stoutly maintain they never nap. But even the most virtuous non-nappers will zonk out on cue when somebody like David Dinges gets a hold of them.
Dr. Dinges, a biological psychologist at the University of Pennsylvania, is among the few scientific experts to concentrate on the subject of napping. He is boldly bent on changing society's mind about the practice and may even save lives in the process -- sleepy people are responsible for between 60 and 90 percent of all industrial accidents, he estimates.
Napping allows us to whittle away at our sleep deficit, which, since the end of World War II, has slowly grown into a form of chronic stress to millions of people forced to manage competing needs of careers and families.
Although much napping stems from a chronic sleep debt, Dr. Dinges believes, he considers that only part of the story. The body has a pronounced tendency to sleep in the midafternoon, even if well rested. "Look at an airplane or train at 3:30 p.m. -- everybody's sacked out. This doesn't happen at other hours."
The biological long view teaches that sleep and wakefulness are interacting phases in our daily cycle of existence, and there's no proof which one is our true natural state.
It formerly was thought the human sleep-wake cycle was equally divided into 12 hours. Now, though, research indicates we pass through two sleep zones each day -- lengthy nocturnal sleep and shorter midafternoon sleep attacks -- separated by zones of increased alertness.
Nap research, Dr. Dinges stresses, is merely in its infancy. Regardless of whether one naps or not, feelings of alertness in most people generally continue to increase into the early evening hours.
But Dr. Dinges' most striking finding may be that in most people, performance is enhanced hours later after having napped, indicating that the practice may indeed capture the essence of the "chief nourisher in life's feast," as Shakespeare described sleep.
Naps are good, Dr. Dinges sums up. Science absolves the napper.