Snakes and fish do it. Cats and dogs do it. Even human babies do it inside the womb. And maybe after seeing the picture above, you’re doing it now: yawning.
Yawning appears to be ubiquitous within the animal kingdom. But despite being such a widespread feature, scientists still can’t explain why yawning happens, or why for social mammals, like humans and their closest relatives, it’s contagious.
As yawning experts themselves will admit, the behavior isn’t exactly the hottest research topic in the field. Nevertheless, they are getting closer to the answer to these questions. An oft-used explanation for why we yawn goes like this: when we open wide, we suck in oxygen-rich air. The oxygen enters our bloodstream and helps to wake us up when we’re falling asleep at our desks.
Sounds believable, right? Unfortunately, this explanation is actually a myth, says Steven Platek, a psychology professor at Georgia Gwinnett College. So far, there’s no evidence that yawning affects levels of oxygen in the bloodstream, blood pressure or heart rate.
The real function of yawning, according to one hypothesis, could lie in the human body’s most complex system: the brain.
Yawning—a stretching of the jaw, gaping of the mouth and long deep inhalation, followed by a shallow exhalation—may serve as a thermoregulatory mechanism, says Andrew Gallup, a psychology professor at SUNY College at Oneonta. In other words, it’s kind of like a radiator. In a 2007 study, Gallup found that holding hot or cold packs to the forehead influenced how often people yawned when they saw videos of others doing it. When participants held a warm pack to their forehead, they yawned 41 percent of the time. When they held a cold pack, the incidence of yawning dropped to 9 percent.
The human brain takes up 40 percent of the body’s metabolic energy, which means it tends to heat up more than other organ systems. When we yawn, that big gulp of air travels through to our upper nasal and oral cavities. The mucus membranes there are covered with tons of blood vessels that project almost directly up to the forebrain. When we stretch our jaws, we increase the rate of blood flow to the skull, Gallup says. And as we inhale at the same time, the air changes the temperature of that blood flow, bringing cooler blood to the brains.
In studies of mice, an increase in brain temperature was found to precede yawning. Once the tiny rodents opened wide and inhaled, the temperature decreased. “That’s pretty much the nail in the coffin as far as the function of yawning being a brain cooling mechanism, as opposed to a mechanism for increasing oxygen in the blood,” says Platek.
Yawning as a thermoregulatory system mechanism could explain why we seem to yawn most often when it’s almost bedtime or right as we wake up. “Before we fall asleep, our brain and body temperatures are at their highest point during the course of our circadian rhythm,” Gallup says. As we fall asleep, these temperatures steadily decline, aided in part by yawning. But, he added, “Once we wake up, our brain and body temperatures are rising more rapidly than at any other point during the day.” Cue more yawns as we stumble toward the coffee machine. On average, we yawn about eight times a day, Gallup says.
Scientists haven’t yet pinpointed the reason we often feel refreshed after a hearty morning yawn. Platek suspects it’s because our brains function more efficiently once they’re cooled down, making us more alert as result.
A biological need to keep our brains cool may have trickled into early humans and other primates’ social networks. “If I see a yawn, that might automatically cue an instinctual behavior that if so-and-so’s brain is heating up, that means I’m in close enough vicinity, I may need to regulate my neural processes,” Platek says. This subconscious copycat behavior could improve individuals’ alertness, improving their chances of survival as a group.
Mimicry is likely at the heart of why yawning is contagious. This is because yawning may be a product of a quality inherent in social animals: empathy. In humans, it’s the ability to understand and feel another individual’s emotions. The way we do that is by stirring a given emotion in ourselves, says Matthew Campbell, a researcher at the Yerkes National Primate Research Center at Emory University. When we see someone smile or frown, we imitate them to feel happiness or sadness. We catch yawns for the same reasons—we see a yawn, so we yawn. “It isn’t a deliberate attempt to empathize with you,” Campbell says. “It’s just a byproduct of how our bodies and brains work.”
Platek says that yawning is contagious in about 60 to 70 percent of people—that is, if people see photos or footage of or read about yawning, the majority will spontaneously do the same. He has found that this phenomenon occurs most often in individuals who score high on measures of empathic understanding. Using functional magnetic resonance imaging (fMRI) scans, he found that areas of the brain activated during contagious yawning, the posterior cingulate and precuneus, are involved in processing the our own and others’ emotions. “My capacity to put myself in your shoes and understand your situation is a predictor for my susceptibility to contagiously yawn,” he says.
Contagious yawning has been observed in humans’ closest relatives, chimpanzees and bonobos, animals that are also characterized by their social natures. This begs a corollary question: is their capacity to contagiously yawn further evidence of the ability of chimps and bonobos to feel empathy?
Along with being contagious, yawning is highly suggestible, meaning that for English speakers, the word “yawn” is a representation of the action, a symbol that we’ve learned to create meaning. When we hear, read or think about the word or the action itself, that symbol becomes “activated” in the brain. “If you get enough stimulation to trip the switch, so to speak, you yawn,” Campbell says. “It doesn’t happen every time, but it builds up and at some point, you get enough activation in the brain and you yawn.”