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Big Question: Why do we yawn?

The expert: Dr. William Cullinan, dean of the College of Health Sciences and director of the Integrative Neuroscience Research Center

"With respect to the ultimate purpose of the yawn, you can search all you want but nobody really knows. Many theories have been proposed for it — cooling of the brain, a shift in arousal/attention state, but none of them are adequate. It is often taught that it is a response to low oxygen or high carbon dioxide levels, but this is not borne out by recent evidence. (Also, while there is a higher oxygen/carbon dioxide exchange with a yawn, that could be accomplished in other ways, like simply increasing respiratory rate).

"More perplexing is why yawning is contagious (but only in humans and some non-human primates). Interestingly, one study claimed that contagious yawning is absent in autistic children. A yawning center in the brain has been proposed, but the evidence is weak. This simply falls into the class of ‘we have no real idea.’

"The biggest mystery concerning yawning may well be this: Humans yawn as early as 11 weeks in the womb, where there is no gas exchange as the lungs have yet to inflate. Perhaps a yawn is a type of healthy stretch of the respiratory system.

"One thing is certain — a good yawn can be satisfying, as you have probably experienced by simply reading this!"