Athletes do it before competition. Concert violinists do it before going onstage. Paratroopers even do it before throwing themselves out of a plane for the first time. Of all the involuntary physical reflexes humans experience before stressful events, yawning seems not only improbable, but also kind of ridiculous—like sneezing a lot before a knife fight.
Most of us (rightly) associate these 3 to 6-second oscitations with sleepiness and boredom---not feats of daring and skill. According to Robert Provine, a University of Maryland, Baltimore County, neuroscientist and author of Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond, we really do yawn most when we're tired. "Right after waking and before bedtime," he says, "which is consistent with yawning's role in facilitating state changes: sleep to wakefulness, wakefulness to sleep, arousal to de-arousal, or vice versa."
But yawning does more than just engender physiological state changes. Human fetuses begin yawning in the womb after about 20 weeks; dogs frequently yawn when asked to do things they find difficult (bath time, fella!); and there's a good chance you've yawned just from reading this article (seeing, reading about, and hearing yawns also causes them). In short, yawning remains one of the least understood common behaviors among vertebrates---and this is especially true of stress and anxiety yawning.
"Nervous people will definitely yawn more," says Provine, "but there haven't been a lot of formal studies investigating why." While that hasn't stopped agencies like the TSA from including "exaggerated yawning" in its 92-point checklist of suspicious passenger behaviors (perhaps they should focus more on weapons?), it has meant that the emotional significance of yawning has remained a something of a scientific mystery.
Here's what is known: Reptiles, birds, mammals, and fish all tend to yawn a lot before—and in some cases during—conflict or other stress-inducing activities. In one study, male Siamese fighting fish were observed yawning multiple times during different aggressive encounters with one another. Similarly, numerous studies have shown that macaques will yawn in response to various male threats, bouts of sexual jealousy, and anxiety. In a recent study published in Neuroscience Letters, Japanese researchers used classic fear conditioning to successfully induce yawning in rats. Still, while scientists have repeatedly demonstrated the link between stress and yawning, they don't know much beyond the fact that the hypothalamus, a part of the brain involved in functions like feeding, metabolic balance, blood pressure, heart rate, and sexual behavior, seems to be involved.
Some psychologists, including Provine, suggest that anxious yawning could be an example of what's known as a displacement activity—i.e., behavior that results from an uncomfortable or stressful situation and that seems out of context. Examples in humans include scratching one's head, stroking a non-existent beard, or repeatedly tugging on an earlobe. Animals do it too. Ever see a cat go after a bird, miss, and then immediately start grooming itself? That's not some awkward attempt to play it cool. It's a displacement behavior.
Insofar as they signal stress and involve actions meant to have a calming effect, displacement behaviors also fit the whole state-changing model of yawning. But that still leaves a number of fundamental questions unanswered: Does yawning itself cause the activity changes, or do those activity changes cause the yawning? Also, what physiological purpose does yawning actually serve in these scenarios? If it's stress reduction, how exactly does that work?
One possible clue relates to yawning's perceived role in thermoregulation. According to Andrew Gallup, an assistant professor of psychology at the State University of New York at Oneonta, yawning may help keep the brain at its optimal 98.6 degree temperature. Gallup believes this is important because our brains hate being hot. A so-called hot head can result in everything from slower reaction times to poor memory performance, he says.
In a 2010 study on the impact of yawning on brain temperature, Gallup implanted probes in the brains of rats and found that even a rise in temperature of 0.18 degrees Fahrenheit triggered yawning. He also found that the skull temperature fell immediately after the rats finished their yawns—sometimes by as much as 0.7 degrees.
Gallup believes that our human brains react similarly. His theory is that when we yawn, our gaping jaws increase circulation to our skulls, effectively forcing warm blood out of the brain. The deep inhalation of our yawns also brings a flood of air into our nasal and oral cavities, which cools the cranial arteries through heat dissipation, he says.
So far, there seems to be some support for this idea. A number of recent studies have shown people do in fact yawn more in the summer than in the winter, and that cold weather and doing things like jumping in a cold pool can greatly diminish yawning frequency.
As it so happens, stress and anxiety also cause our brains to get hotter, Gallup says. And Simon Thompson, a neuroscientist and clinical psychologist at Bournemouth University in the UK, agrees with him that the yawn may be our brain's way of countering these unwelcome temperature rises.
Thompson's own research has shown that yawns often seem to be triggered by a rise in blood cortisol levels, and that they in turn serve to elevate these levels even further. Of course, our bodies also produce the hormone cortisol when we're stressed, and this spike, Thompson believes, stimulates both the production of adrenaline to make us more alert, and tells the tempearture-control portion of our brain, the hypothalamus, to cool down the brain.
It's far from a definitive scientific explanation of stress yawning, but it does seem to indicate that cool heads really do prevail.
