The Yawn Explained: It Cools Your Brain

Dec. 15, 2008 -- If your head is overheated, there's a good chance you'll yawn soon, according to a new study that found the primary purpose of yawning is to control brain temperature.

The finding solves several mysteries about yawning, such as why it's most commonly done just before and after sleeping, why certain diseases lead to excessive yawning, and why breathing through the nose and cooling off the forehead often stop yawning.

The key yawn instigator appears to be brain temperature.

"Brains are like computers," Andrew Gallup, a researcher in the Department of Biology at Binghamton University who led the study, told Discovery News. "They operate most efficiently when cool, and physical adaptations have evolved to allow maximum cooling of the brain."

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He and colleagues Michael Miller and Anne Clark analyzed yawning in parakeets as representative vertebrates because the birds have relatively large brains, live wild in Australia, which is subject to frequent temperature swings, and, most importantly, do not engage in contagious yawning, as humans and some other animals do.

Contagious yawning is thought to be an evolved mechanism for keeping groups alert so they "remain vigilant against danger," Gallup said.

For the study, the scientists exposed parakeets to three different conditions: increasing temperature, high temperature and a moderate control temperature. While the frequency of yawns did not increase during the latter two conditions, it more than doubled when the researchers increased the bird's ambient temperature.

A paper on the findings has been accepted for publication in the journal Animal Behavior.

"Based on the brain cooling hypothesis, we suggest that there should be a thermal window in which yawning should occur," Gallup said. "For instance, yawning should not occur when ambient temperatures exceed body temperature, as taking a deep inhalation of warm air would be counterproductive. In addition, yawning when it is extremely cold may be maladaptive, as this may send unusually cold air to the brain, which may produce a thermal shock."

The parakeets yawned as predicted.