Yawning seems like a basic bodily process, but it turns out that it's much more complex — and mysterious — than you probably realize. Unlike blinking (which keeps your eyes moisturized) or sneezing (which expels irritants from our noses), yawning doesn't have such an obvious function. "Yawns receive little attention because they seem insignificant," says Andrew Gallup, PhD, an assistant professor of psychology at SUNY Oneonta. "The association with sleep has kind of been taken at face value — that it’s something that signals that the body is fatigued or alertness is dropping." But Dr. Gallup's research suggests that the purpose of yawning is far more important than that: It helps keep our brains from overheating.
To understand how that's possible, you've got to understand what's happening when you yawn. Dr. Gallup describes it as a "powerful gaping of the jaw and inhalation of air, followed by a more rapid closing of the jaw and expiration of the air." Stretching your jaw, like stretching any part of your body, encourages circulation in that area. And that's important because changes in circulation affect both the rate of blood flow and the temperature of blood that's coming to the brain, which are two major factors in regulating our overall temperature. "We're warm-blooded, so we maintain relatively constant brain and body temperature," Dr. Gallup explains. "But if you monitor moment-to-moment changes in brain temperature, it’s constantly fluctuating to a slight degree."
Dr. Gallup's theory is that when you yawn, the incoming blood is actually slightly cooler than the brain. So, yawning may be one mechanism by which we keep our brains from overheating. In a 2007 study published in Evolutionary Psychology, Dr. Gallup's team found that people yawned less frequently when they had cold packs on their heads compared to when they wore warm packs. In two more recent studies, he surveyed people during winter and summer months in a warm city (Tucson, AZ) and a cooler one (Vienna, Austria). In both studies, the season had a significant effect on how often people yawned. This idea also helps explain why we so often associate yawning with waking up and falling asleep. It's at these transition points that our bodies go through their major changes in temperature and might need a little help keeping it in check. When we're getting ready for bed, we see a significant drop in body temperature, perhaps aided by a big yawn or two. And then when we're waking up, our bodies are getting warmer very rapidly, Dr. Gallup explains, which may trigger changes in our need for circulation.
There are other prevalent theories about why we yawn. One that's incorrect is the idea that we yawn in order to take in more oxygen. You're gulping in a lot of air, so it isn't necessarily surprising that most people think yawning has some sort of respiratory function, admits Dr. Gallup. However, that hypothesis has been thoroughly tested and debunked, he says. In studies, participants who receive less oxygen don't yawn more frequently than those in normal conditions. And exercising, which can double our breathing rate, doesn't seem to make us yawn more often, either. Another hypothesis — that actually has some evidence to support it — is that there's some sort of social function to our yawning. Although Dr. Gallup says there's no consensus about what that function might be, we do know that yawns can be contagious. You're more likely to yawn if you see, hear, or even just think about someone yawning, Dr. Gallup explains. And while many animals yawn spontaneously (for physiological reasons), far fewer yawn contagiously. Those that do tend to be particularly social species, such as humans, chimps, and even parakeets. Some research has suggested that this social connection might be mirrored in our levels of empathy. The general idea is that those who are more empathetic are more susceptible to contagious yawning. One particularly bold study found that those who score higher on measures of psychopathology are actually less vulnerable to contagious yawning. And while some research does support the empathy connection, Dr. Gallup says there are other studies in this line of thinking that have been inconclusive or contradictory. Even if yawning isn't a window into our personalities, it's clear that it's a useful behavior. The fact that we share it with so many other species (especially mammals like ourselves) suggests that it has been evolutionarily beneficial. We're just not quite sure how — yet.