Cooler Heads

Yawning may be a reflex for cooling our brains off, a new study suggests.

Yawning, or acting out an outrageous Native American stereotype? Sometimes it's hard to tell.

People are less likely to yawn when their own body temperature is lower than that of the surrounding environment, the research shows – in fact, a person’s tendency to yawn actually varies with theseasons, becoming more frequent in winter and less frequent in summer.

Scientists have debated the cause(s) of yawning for centuries. Some have explained it as a sort of reflexive muscle stretch, because it often occurs along with other stretching behavior. Others have suggested that yawns may help increase alertness, and that contagious yawning might help signal members of a social group to become more alert.

The theory that yawning helps regulate brain temperature dates back at least to 2007 – but as the journal Frontiers in Evolutionary Neuroscience reports, a team led by Princeton’s Andrew Gallup was the first to measure seasonal variations in this reflex. Back in 2010, Gallup confirmed that when rats yawn, their brain temperature cools slightly, so he was eager to see if the same held true for humans.

To find out, Gallup and his team documented the yawning frequency of 80 people in the winter, and another 80 in the summer:

The proportion of pedestrians who yawned in response to seeing pictures of people yawning differed significantly between [winter and summer]. Across conditions yawning occurred at lower ambient temperatures, and the tendency to yawn during each season was associated with the length of time spent outside prior to being tested.

In other words, the longer people spent outside in cold weather, the more likely they were to yawn in response to a picture of a person yawning – and by contrast, the more time they spent outside in warm weather, the less likely they were to respond with a yawn:

Nearly 40 percent of participants yawned within the first five minutes outside, but the percentage of summertime yawners dropped to less than 10 percent thereafter. An inverse effect was observed in the winter, but the proportion of people who yawned increased only slightly for those who spent more than five minutes outdoors.

Though this study didn’t record other factors that might have played a part in people’s tendency to yawn – such as how much sleep they’d gotten, how bored they were, or how close they were to bedtime – it does provide a clear correlation between seasonal variation and yawniness. Whether that variation depends on temperature alone, or on other factors as well, remains to be seen.

It’s also worth noting that yawning may have more than one cause – for example, it might’ve originally evolved to cool off animals’ brains, but its tendency to increase alertness might’ve helped it get exapted for use as an in-group “stay alert” signal.

Anyway, the next time someone catches you yawning in a meeting, just tell ‘em, “Can’t help it – my brain’s hot.”

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