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Patrick Guilfoile: Why people yawn

Yawning is a common yet odd phenomenon. Without warning, we open our mouths wide, and take in a large gulp of air.

Yet, in spite of how frequently we yawn, there hasn’t been a compelling understanding about this behavior until recently.

A common explanation for yawning is that it allows more oxygen to get to the brain.

Yet previous research showed this was not the case.

Scientists measured the frequency of yawning in subjects either breathing 100 percent oxygen or a mixture of 95 percent air, 5 percent carbon dioxide, with regular air used as a control (containing about 0.04 percent carbon dioxide).

If getting more oxygen to the brain was the purpose of yawning, breathing pure oxygen should decrease yawning, and breathing 5 percent carbon dioxide should increase yawning.

However, neither treatment had an effect on the rate of yawning.

Another hypothesis is that yawning might help cool the brain.

Initial evidence, from both mice and people, showed a transient increase in brain temperature just before yawning and a decrease immediately afterward, supporting this idea.  

A further extension of this work was to determine whether there was a different rate of yawning at different temperatures.

Specifically, the notion was that there would be a temperature window where yawning would be more common.

Too low a temperature, and yawning might result in over-cooling; too high a temperature and no cooling would occur.

Researchers did an initial study to test this idea in Tucson, Arizona.

The study was based on the premise that yawning is contagious.

Researchers showed pedestrians pictures of people yawning and determined how often they yawned as a result.

One group was tested during the summer in Arizona (average temperature 98 degrees), the other group during the winter in Arizona (average temperature, 72 degrees).

As predicted in the model, the rate of yawning was highest during the “winter” when the temperature was about 70, as this was cool enough to have an effect on brain temperature.

However, there were still other possible explanations, such as length of day, which could lead to the differences in yawning during the summer and winter.

To further test this model, scientists in Vienna, Austria studied another group of pedestrians during the summer (average temperature, 67 degrees) and winter (average temperature, 35 degrees).

In this case, the expectation was that yawning would be greater during the summer, since the outside temperature would be cool enough, but not too cool, to have an effect.

Yawning should be less prevalent during the winter in Vienna, since the outside temperature might cause too much cooling.

In fact, that is what researchers discovered.

This work also indicated that day length did not appear to be a factor; the days were short in Tucson in the winter, when yawning was more common, and the days were long in Vienna in the summer, when yawning was more common.

The factor they had in common was a temperature near 70 degrees.

Consequently, this new research indicates that yawning may be nothing more than a strategy to air-condition our brains.

More information is available in the article by Jorg J.M. Massen and others, “A thermal window for yawning in humans: Yawning as a brain cooling mechanism.” Physiology & Behavior 130 (2014) 145–148

PATRICK GUILFOILE has a doctorate in bacteriology and is the associate vice president at Bemidji State University.