"No one knows why we yawn," says Andrew C. Gallup, a psychology professor at the State University of New York at Albany.
Now Dr. Gallup and fellow researchers have a new explanation: yawning, they said, is a way for the body to cool the brain.
The volunteers were asked to step into a room by themselves and watch a video showing people behaving neutrally, laughing or yawning.
Some volunteers were asked to breathe only through their noses as they watched. Later, volunteers were asked to press warm or cold packs on their foreheads.
"The two conditions thought to promote brain cooling (nasal breathing and forehead cooling) practically eliminated contagious yawning," the researchers wrote.
The study may also help explain why yawning spreads from person to person. A cooler brain, Dr. Gallup said, is a clearer brain.
So yawning actually appears to be a way to stay more alert. And contagious yawning, he said, may have evolved to help groups remain vigilant against danger.
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