Open your mouth wide, stretch the muscles of your jaw and upper body, take a slow breath in, and then exhale quickly. What have you done? You have yawned. Many animals, including humans, yawn. They do it involuntarily.

The signal that initiates a yawn comes from a particular brain region, the paraventricular nucleus (PVN) of the hypothalamus. It stimulates other brain cells in both the brain stem and the hippocampus to produce the muscle contractions we call a yawn. The PVN also makes chemical messengers that may induce yawning. Producing one called adrenocorticotropic (ACTH) increases dramatically during sleep and just before waking—which may explain why we yawn at morning rise.

But why do you yawn when you're not ? Despite what you may have heard, yawning has nothing to do with increasing the body's oxygen supply. In experiments, subjects yawn just as much in oxygen-rich air as they do in an oxygen-poor atmosphere. Yawning is, however, a response to boredom. When researchers showed students ages 17-19 music videos and color bar test patterns, those who saw the test patterns yawned nearly twice as often as those who watched videos, and their yawns lasted longer.

But boredom isn't the only yawn factor. If someone you're conversing with yawns, chances are, you will too. "Yawning is extraordinarily contagious," says Robert Provine, a pioneer of yawning research. "Seeing a person yawn triggers yawns. Reading about yawning causes yawns. Sitting in a room thinking about yawning triggers yawning," he says. Some experts think this happens because yawning evolved as a means of communication. It may help animals, including humans, coordinate their behavioral responses to changing conditions in .



That may explain why some people are more susceptible to contagious yawning than others. Psychologist Steven Platek and his team at Drexel University in Philadelphia gave 65 college students tests. The tests measured their , or how well they perceived and responded to the mental states of other people. Platek then observed through a one-way mirror (so his subjects didn't know they were being watched) how the students responded as they watched videos of people yawning. The students who scored high for empathy yawned more often in response to the videos than their less compassionate peers. This suggests that yawning is a form of social communication.

But there's more. A new theory suggests that yawning cools the brain, and the sinuses may play a key role. In the current edition of the journal Medical Hypotheses, Andrew Gallup of Princeton University and Gary Hack of the University of Maryland argue that yawning helps to regulate the brain's temperature. "The brain is exquisitely sensitive to temperature changes and therefore must be protected from overheating," they write. "Brains, like computers, operate best when they are cool."

Gallup and Hack propose that the walls of the human maxillary sinus (pictured in green here) flex during yawning like a bellows, which in turn facilitates brain cooling. The theory helps explain the function of the human sinuses, which is little understood and hotly debated.

Gallup has conducted experiments to test the yawning theory. In one, he implanted thermocoupled probes in the frontal cortex of rats to measure brain temperature before, during, and after yawning. He found that rapid increases in brain temperature precede yawning and decreases in brain temperature occur immediately after.

Gallup has also published a case study of two women with chronic and debilitating bouts of yawning 5 to 45 minutes in length, occurring as many as 15 times per day. Both women showed signs of dysfunctioning brain temperature regulation. Mirroring the results of the brain temperature study, one woman took oral temperature measurements before and after yawning episodes, which showed a significant drop in temperature. After receiving that information, the woman reported that methods of behavioral brain cooling provided relief or postponement of her yawning symptoms.

The brain-cooling theory of yawning is more than a curiosity. It may have practical medical applications. Bouts of excessive yawning often precede the onset of seizures in epileptic patients and may predict the onset of pain in people with migraine headaches. Hack and Gallup predict that excessive yawning might be used as a diagnostic tool in identifying dysfunction of temperature regulation. "Excessive yawning appears to be symptomatic of conditions that increase brain and/or core temperature, such as central damage and sleep deprivation," Gallup says.

Faith Brynie is the author of 101 Questions about Sleep and That Kept You Awake Nights...Until Now.