Yawning may no longer be a wide-open question.
A new theory proposed by Gary Hack, DDS of the University of Maryland School of Dentistry and Andrew Gallup, PhD of Princeton University suggests that yawning cools the brain, and ýthe sinuses may play a key role.
In the current edition of the journal Medical Hypotheses, Gallup and Hack tackle the question of why humans yawn, and explore the possible function of the human sinuses.
Yawning occurs not because you are tired, bored, or even need oxygen, they argue. Instead, they say, yawning helps to regulate the brain's temperature. "The brain is exquisitely sensitive to temperature changes and therefore must be protected from overheating," the authors write. "Brains, like computers, operate best when they are cool."
The scientists propose that the walls of the human maxillary sinus (pictured in green above) flex during yawning like a bellows, which in turn facilitates brain cooling. The theory helps explain the function of the human sinuses, which is still debated among scientists. In fact, Hack says everything concerning the human sinuses is debated. "Very little is understood about them, and little is agreed upon even by those who investigate them. Some scientists believe that they have no function at all," he says.
Essentially the authors present a hypothesis, not previously proposed, that the human sinuses play a role in brain cooling that is driven by yawning.
Beyond the physiological curiosity, the brain cooling theory of yawning also has practical medical implications. Bouts of excessive yawning often precede the onset of seizures in epileptic patients, and predict the onset of pain in people with migraine headaches, explains Gallup. Hack and Gallup predict that excessive yawning might be able to 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 nervous system damage and sleep deprivation," says Gallup, a postdoctoral research associate at Princeton.
Previously Gallup conducted experiments to test the yawning theory. For example, he implanted thermocoupled probes in the frontal cortex of rats to measure brain temperature before, during, and after yawning. He found that yawning was preceded by rapid increases in brain temperature and followed by corresponding decreases in brain temperature. Gallup also published a case 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 and or postponements of her yawning symptoms.
In 2002, Hack presented findings on the potential role of the maxillary sinus in facilitating brain cooling during functional movements of the jaw. He postulated that human jaw musculature may flex the thin walls of that sinus, ventilating the sinus system and aiding in brain cooling.
Hack, an asociate professor in the Department of Endodontics, Prosthodontics, and Operative Dentistry, and Gallup began collaborating last year. They thought the sinus wall flexing may ventilate the human sinuses similar to that process which is known to occur in birds. "Therefore the proposed ventilation process may assist in controlling brain temperature and insuring the maintenance of integrated functions of the brain," says Hack.