Though scientists have put forth various theories for yawning—from fatigue to lack of oxygen—none have held up to scrutiny.
"We can put a man on the moon, but we do not understand what the function of yawning is," said study co-author Gary Hack, of the University of Maryland School of Dentistry in Baltimore.
Now, Hack and co-author Andrew Gallup, of Princeton University, propose that yawning causes the walls of the maxillary sinus to expand and contract like a bellows, pumping air onto the brain, which lowers its temperature. Located in our cheekbones, the maxillary are the largest of four pairs of sinus cavities in the human head.
Like a computer, the human brain is "exquisitely" sensitive to temperature and must stay cool to work efficiently, said Hack, whose previously collected data was combined with Gallup's in the new study, recently published in the journal Medical Hypotheses.
Sinus Solution?
In addition to potentially solving the mystery of yawning, the study may also reveal why we have sinuses, whose existence has also stumped scientists.
It's a "unified theory tying yawning, sinus ventilation, and brain cooling into a neat little package," Hack said.
Ryan Soose—an ear, nose, and throat doctor as well as director of the University of Pittsburgh Medical Center's Division of Sleep Surgery—added, "The hypothesis that these two relatively unknown things may be directly related, to me, is very intriguing."
(See "Is Your Brain Sleeping While You're Awake?")
Cadaverous Clues
In 2002 study co-author Hack and his team were dissecting a cadaver when they discovered that the back wall of the maxillary sinus was much thinner—and therefore more flexible—than described in many medical textbooks.
The researchers postulated that, when the jaw moves, the sinus wall flexes, ventilating the sinuses. (Explore the human body.)
"I'd always kept that in the back of my mind, because yawning was an exaggerated jaw movement that would have an impact on this previously undescribed pump in humans," Hack said.
Later, he came across the postdoctoral research of Princeton's Gallup, who in 2007 had become the first to suggest the brain-cooling theory for yawning.
Since 2007, Gallup had tested the idea in both animals—many of which also yawn—and people. (Read "Beyond the Brain" in National Geographic magazine.)
For instance, Gallup and his team had implanted probes into rats' brains and recorded brain-temperature changes before, during, and after the rats yawned.
The team discovered that brain temperature spikes in the run-up to a yawn, then starts to decline, and finally falls rapidly to pre-yawn temperature.
This suggests yawns are triggered by an increase in brain temperature and "actually promote brain cooling," Gallup said.
(See "Rat Made Supersmart—Similar Boost Unsafe in Humans?")
Gallup had also studied two women who suffer from chronic bouts of excessive yawning. He had asked one of the patients—who could predict her "yawning attacks"—to take her own temperature before and after the episode, he said.
The results showed her body temperature rose before the yawn and fell afterward—"directly mirroring results of the rat-brain temperature study," Gallup said.
However, "we do have to be cautious that there are only two subjects in [that] study."
Indeed, co-author Hack expects the brain-cooling yawning theory to be "very controversial—we're delving into an area that's not well understood."
Yawning Theory May Influence Medicine
Overall, understanding yawning could be a useful tool for diagnosing certain medical conditions, such as epilepsy and migraines, which are both preceded by excessive yawning, the scientists say.
The University of Pittsburgh's Soose added that the discovery might someday aid doctors in treating patients with insomnia, the most common U.S. sleep disorder. Insomniacs often have trouble regulating their body temperatures, which must drop in order for sleep to occur. (Take a secrets-of-sleep quiz.)
"You could envision some mechanism by which you could rinse or cool the sinuses to treat insomnia," Soose said. "It opens the doors to help treat insomnia in a different way."
Courtesy: NGC
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