Eight-time Olympic medalist speedskater Apolo Ohno has been spotted yawning before major competitions. Army soldiers yawn more than normal right before they're going to jump from an airplane for the first time. You may have even spotted the girl one spot over during MetCon sneaking a yawn during the water break.
But it’s not that Apolo Ohno was tired before his Olympic race, or that the girl next to you, dripping in sweat, was agape because she was bored.
“It is very well documented that humans and animals yawn in anticipation of or leading up to important events. Even world class athletes, before the most monumental race or competition of their life, have been observed yawning,” says Andrew Gallup, Ph.D., an evolutionary psychologist at SUNY Polytechnic Institute who studies yawning.
A study in Adaptive Human Behavior and Physiology shows that stress elicits yawning. When you’re stressed (or anxious or excited) the sympathetic nervous system is activated, which increases core temperature. Anytime the core temperature rises, so does the temperature of the brain, Gallup says. The prevailing theory in the scientific community, which Gallup’s research has lead, is that yawning helps to essentially act as an air conditioner for your brain.
It works like this: The motor action of yawning has two basic components. First, the deep gaping of the jaw, which is essentially an extended stretch of the jaw muscles. Stretching increases blood flow to that localized area, which in this case, is your skull. The second component is the inhale. When you take a deep breath, you’re pulling in cool air from outside, which hits your blood vessels, cools that blood, and then cools your brain as it travels up your arteries.
So, when you’re nervous or excited about an impending race or exercise, your body activates a yawn to bring your temperature back down to homeostasis, Gallup explains. Mid-workout, your core temperature fires up as a response to movement, which induces a yawn to cool your brain down.
There’s another theory with some credibility, but it’s really a sister theory to the brain cooling hypothesis, Gallup says. Scientists know that yawns facilitate changes in “behavioral state”—they make you more alert, more aroused, more mentally efficient—and there’s some evidence to suggest that yawning is triggered to enhance blood flow specific to, say, the cerebral cortex, which would facilitate these state changes.
The one idea that doesn’t hold up: Science dispelled the theory that people yawn in order to bring oxygen into the blood 30 years ago, Gallup adds.
You’ll probably see more people yawning at the weight rack than the treadmills. “During steady cardio, your respiratory system needs to keep a consistent rate, so you’re actually less likely to yawn because it would disrupt that higher need,” Gallup explains. (The exception: HIIT cardio since you have rest in between hard efforts, your body isn't maintaining a consistently high breathing rate in the same way as steady state movement.)
And you might be more agape on leg day than getting after a total-body circuit. “If you’re working a specific muscle group, your body is diverting blood to that area, so yawning may be a way to reclaim some of the blood flow to your brain,” he adds.
Some people may be genetically predisposed to yawn more frequently, as research shows is the case in rats. Certain medications can up your chances of yawning, too. Some pharmaceutical drugs (like SSRI antidepressants) come with the side effect of increasing your brain and body temperature, thereby causing you to yawn more frequently, Gallup says.
Another thing that will prompt fit, healthy folks to yawn more during exercise: Walking into a hot workout environment, since your core temperature shoots up. But there’s a limit—when the ambient temperature is above or below humans' physiological comfort zone of 62 to 78 degrees, your core temperature rises or drops. But once the air exceeds your body temperature of 98.6, you stop yawning. “Inhaling the outside air is no longer going to help cool you off, so it’s maladaptive to yawn,” Gallup says. That means you’ll yawn more than normal if you go to a typical hot yoga class with rooms around 92 degrees, but not if you go to a Bikram class, where the temp is set at 105 degrees. (Being overweight or uncondititoned also causes you to heat up faster once you start moving.)
Same goes for the other direction—yawning frequency diminishes when outside temperature hits 30 or 20 degrees. Studies even show you yawn less during winter.