Don't read this if you're on your way to school, work or a date. Within a few minutes, you'll feel as if pressure is building behind your ears, your eyebrows will rise, your eyes will bulge slightly and your jaws will be wedged wide open. You will hold this position for about six seconds as you take a breath through your mouth and exhale a shorter one.

Don't panic. You're just under the spell of a contagious yawn.

If you see someone yawning, read about yawning, think about yawning or even just hear a yawn, chances are that, within a few minutes, you will have the urge to yawn.

Why? And why, for that matter, do we yawn in the first place? Does it serve a purpose? Strange as it may seem, modern science does not have the answers.

Folklore suggests that we yawn when we are bored or sleepy, when we see someone else yawn or when we need more oxygen. But none of those conditions applies to human fetuses, who yawn as soon as 11 weeks after conception, perhaps as part of the development of the musculature of the jaw and face or formation of the respiratory system.

Alternatively, some scientists say human yawning is a behavioral leftover from our evolutionary past -- all animals with a backbone yawn, including fish! -- or from our fetal development stage.

"Yawning may be a vestige," says Robert Provine, a neuropsychologist at the University of Maryland Baltimore County in Catonsville. "However, I believe that it also has certain functions postnatally." He set out to identify these functions more than a decade ago by testing propositions made in the folklore.

Provine first tested the notion that people yawn more often when bored or sleepy. He either bored people by making them watch videos of colored test patterns or he let them enjoy a music video. The subjects yawned more when watching the test patterns. To test the effects of sleepiness, he had undergraduates record for one week the times of day they yawned or stretched. The students yawned more in the evening when they felt tired and sleepy.

However, Provine also found that the students stretched and yawned after awakening in the morning when they no longer were tired or sleepy. It seems that when you stretch, you yawn, Provine says, but not the other way around.

Some other evidence points to a connection between brain circuits that trigger yawning and stretching. For example, people who are paralyzed on one side of their body and unable to flex those muscles voluntarily stretch the paralyzed side uncontrollably as they yawn.

Besides mornings, people yawn at other times when they are neither sleepy nor bored. Paratroopers yawn before a jump, concert violinists yawn before going on stage and Olympic athletes yawn before a big competition. These examples seem to be associated with anxiety, Provine says. He concludes that yawning when you're anxious, sleepy or bored must help you through changes of mental state.

Ronald Baenninger, a psychologist at Temple University, explains all of the examples of human yawning in a similar way through the "arousal hypothesis." His wife, Mary Anne, a cognitive psychologist at The College of New Jersey in Ewing, originally proposed that yawning is a way of increasing or maintaining arousal when you don't want to go to sleep. You yawn at bedtime, while driving late on a lonely highway or when you are bored.

"Very few people yawn once they are lying in bed waiting to go to sleep," Ronald Baenninger says, "because then it's okay to sleep."

Further support for the hypothesis comes from the Baenningers' findings that yawning is followed by increased wrist motion, skin conductance and heart rate -- all signs of arousal. The notion also fits with observations in animals, including dogs who yawn at the threshold of aggression or seals who yawn just before a deep dive.

The second notion that Provine tested was whether you yawn when you see other people yawning. He found that 55 percent of people yawn within five minutes of seeing a videotape of someone else yawning. The rest, at least, report the urge to yawn.

Provine also found that the sight of the gaping mouth alone did not trigger the yawns. "It was the overall configuration of the yawning face," he says.

The third notion that Provine tested was whether you yawn because of a high concentration of carbon dioxide or a low concentration of oxygen in your blood. He increased the concentration of carbon dioxide in his subjects' blood by having them inhale gas mixtures containing 3 percent and 5 percent carbon dioxide, hundreds of times more than is found in ordinary air.

"We had people huffing and puffing due to the buildup of carbon dioxide in their blood, but they didn't yawn any more than they did before," Provine says. Alternatively, he gave his subjects 100 percent oxygen to increase the concentration in their blood. This did not inhibit yawning.

"Every way we could think of, we found no evidence whatsoever for one of the most prominent, enduring pieces of folklore about human yawning," he says.

This is not to say that yawning doesn't send more blood to the brain. "Yawning is a kind of facial stretch that has important hydraulic consequences for the body and the brain," Provine says. "It stirs things up."

That probably helps you through transitions such as moving from sleep to wakefulness or wakefulness to sleep, when you're going from being alert to being bored or when you're becoming nervous or stressed, he says.

Other than spontaneous yawning, hormones such as testosterone, neurotransmitters including dopamine and various drugs stimulate yawning in laboratory animals.

Several other chemicals inhibit yawning.

"Everything we know about yawning is just derived from studies in which yawning can be induced and observed," says Antonio Argiolas, a neuropharmacologist at the University of Cagliari in Italy. "We think that these circuits we discovered with drugs, peptides and so on are also active during spontaneous yawning, but we don't know for sure."

Argiolas and his colleagues have found certain areas of the brain very sensitive to triggering of yawns. The main area is the paraventricular nucleus of the hypothalamus, deep brain structure that plays an important role in involuntary body activities and stimulates release of hormones.

It also interconnects with other brain regions that control muscles involved in opening the mouth, breathing and swallowing, among other actions.

The nervous pathways and mechanisms involved are still mysterious and produce puzzling results. For example, people with Parkinson's disease hardly yawn at all, whereas those with epilepsy yawn more frequently. Several other diseases and brain injuries affect yawning in similar ways.

"Many people say that yawning is not important," Argiolas says. "I think that is not true. Even if the importance may not be so clear, it is important to study how our brain works and to find out mechanisms that are still unknown to us."

Haleh V. Samiei is a freelance science writer and producer of "To Eat or Not to Eat," a Web site on nutrition news (www.nutribytes.com).

Topic suggested by Laurie A. Spiegel of Harpers Ferry, W.Va.