Why do we yawn?

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Is there a purpose to a yawn? I know it means you’re sleepy, but is the body trying to accomplish something by the act of yawning?

People yawn when they’re tired, but also when they wake from a night’s sleep. We yawn when we’re bored, but also when we’re anxious, or hungry, or about to start a new activity. Yawning is contagious — we often start yawning the minute someone near us starts.

“There are so many triggers. People who sky-dive say they tend to yawn before jumping. Police officers say they yawn before they enter a difficult situation,” said Adrian Guggisberg, a professor of clinical neuroscience at the University of Geneva.

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Reading about yawning makes people yawn. You are probably yawning right now.

But the physiological purpose of a yawn remains a mystery. “The real answer so far is we don’t really know why we yawn,” Dr. Guggisberg said. “No physiological effect of yawning has been observed so far, and that’s why we speculate. It’s possible yawning doesn’t really have a physiological effect.”

Until about 30 years ago, scientists explained yawning as a way for the body to take in a large amount of air in order to increase oxygen levels in the blood in response to oxygen deprivation. But the oxygenation hypothesis was discarded after being disproved by a series of experiments published in 1987.

One current theory is that yawning is a brain cooling mechanism “that functions to promote arousal and alertness,” according to Andrew Gallup, an assistant professor of psychology at the State University of New York Polytechnic Institute in Utica, who has published studies on the topic.

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Yawning consists of a deep inhalation of air accompanied by a powerful stretching of the jaw, followed by a shorter expiration of air and rapid closing of the jaw.

“Collectively, these patterns of behavior increase blood flow to the skull, which can have a number of effects, one of which is cerebral cooling,” Dr. Gallup said. “When our body temperature is warmer, we feel more tired and sleepy, and it could be that evening yawns are triggered to try to antagonize sleep onset, so we yawn at night in an attempt to maintain some state of arousal or alertness.”

Sleep triggers a steep reduction in brain and body temperature, he said, so it is also possible “we yawn to just further facilitate the change from waking to sleeping.”

One thing for sure: yawning is catching. One person’s yawn can trigger yawning among an entire group. People who are more empathic are believed to be more easily influenced to yawn by others’ yawns; brain imaging studies have shown that when humans watch other people yawn, brain areas known to be involved in social function are activated. Even dogs yawn in response to seeing their owners or even strangers yawn, and contagious yawning has been noted in other animals as well.

The spread of yawning could potentially serve to “promote coordinated arousal among members of the group, synchronizing their mental state, potentially protecting it by alerting it to external threats more rapidly than it would be otherwise, “ Dr. Gallup said.

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