We all yawn, usually when we are tired, bored or just waking up, and sometimes simply because someone nearby did it first. For years, the explanation seemed simple: we yawn to take in more oxygen and get rid of carbon dioxide. It sounds logical, and it is almost certainly wrong. So why do we yawn? One of the leading explanations is that yawning helps regulate the temperature of the brain, and it may also be tied to alertness and social behaviour. The evidence is intriguing, but scientists still do not have one definitive answer.
The oxygen idea was put to the test
In 1987, psychologist Robert Provine and his colleagues tested the oxygen idea directly. They had volunteers breathe air with extra oxygen or extra carbon dioxide. If yawning were a response to low oxygen or high carbon dioxide, changing those gases should have changed how often people yawned. It did not. Their breathing rates changed, but their yawning did not, which showed that yawning and breathing are controlled by different mechanisms, as a review in Frontiers in Neuroscience explains. A yawn obviously involves a deep breath, but getting more oxygen into the blood does not appear to be the reason we do it.
The brain-cooling theory
In 2007, psychologists Andrew Gallup and Gordon Gallup proposed what became known as the thermoregulatory, or brain-cooling, theory. A yawn combines a big stretch of the jaw, a deep breath and changes in blood flow around the head and neck, and together these could help carry heat away from the brain, as ZME Science explains. The theory makes a testable prediction. If yawning cools the brain by drawing in air, it should work when the air is cooler than your body, and become much less useful when the air is as warm as you are. That idea, that yawning works within a certain temperature range, is called the thermal window.
Why we yawn less on very hot days
You might expect a cooling mechanism to kick in most on a scorching day. The research suggests the opposite. In 2011, Andrew Gallup and Omar Eldakar showed pictures of people yawning to pedestrians in Tucson, Arizona, in two seasons. In winter, when it averaged about 22°C, 45% of people yawned; in early summer, at about 37°C, close to body temperature, only 24% did, as the study reports. At 37°C, breathing in the outside air would do little to cool the brain. A 2014 study in Vienna then found the other side of the window: people yawned less in the near-freezing winter than in the mild summer, suggesting yawning works best around 20°C, according to the University of Vienna. That is exactly what scientists want from a theory: a specific prediction that could have failed, but did not.
Is brain cooling definitely the answer?
Not yet. Gallup and his colleagues have built up a substantial body of supporting evidence, and a 2012 review argued that no study had contradicted the theory’s predictions. But other researchers are more cautious, and the question is far from settled. One of the most interesting recent clues comes from Australia. In 2026, researchers at UNSW Sydney and Neuroscience Research Australia used real-time MRI to watch what happens inside the head and neck of 22 volunteers as they yawned. They found that yawning sent cerebrospinal fluid, the clear fluid that cushions the brain, out of the skull along with blood, the opposite of what happened during deep breathing, and that blood flow into the brain surged, as UNSW reports. The researchers suggested yawning might help clear waste from around the brain, as well as possibly help cool it, while stressing that its exact role is still uncertain. Brain cooling, in other words, is a serious, well-supported hypothesis, but not yet the whole story.
Why we yawn when we are tired
We often yawn when we are tired, before sleep and as we wake up. That has led researchers to wonder whether yawning is linked to shifts in alertness and brain state, as well as temperature. The fact that so many animals yawn suggests it is an ancient behaviour, not just a human response to boredom, and it may well do more than one job.
Why yawns are contagious
Then there is the strangest part. You see someone yawn and, seconds later, you are yawning too. Hearing a yawn, thinking about one or even reading about it can be enough. There has long been a debate over whether contagious yawning is linked to empathy and social bonding, but the evidence is mixed. A 2017 study at the University of Nottingham found that differences in how excitable people’s motor cortex is, the part of the brain that controls movement, explained about half the variation in how easily they caught a yawn, according to the University of Nottingham. In other words, some people’s brains are simply more prone to it. Contagious yawning is not just a human thing, either; it has been seen in dogs, among other social animals, as ZME Science notes. So if you have yawned while reading this, you are not imagining it.
So, what does a yawn actually do?
The honest answer is that we do not know for certain. The oxygen theory does not hold up. The brain-cooling theory has made clever predictions that have been confirmed, and new research on brain fluids adds another possibility. Yawning may well do several things at once: help regulate the brain, mark a change in alertness, and connect us socially. So next time you yawn, do not think of it as your body gasping for air. It may be something far more interesting. If you are curious how your brain reacts to temperature in other ways, see what is brain freeze?, and for how the body keeps its temperature in balance, see what is homeostasis? And if you have made it this far without yawning, you are probably about to.
This article is general information only and is not medical advice. If you develop a sudden and persistent increase in yawning, especially with other unusual symptoms or significant daytime sleepiness, speak to your GP, or call Healthdirect on 1800 022 222.
Sources
- Gallup and Eldakar: The thermoregulatory theory of yawning, Frontiers in Neuroscience (2012)
- Gallup and Eldakar: Contagious yawning and seasonal climate variation (2011)
- University of Vienna: Yawning to cool the brain (2014)
- UNSW Sydney: A good yawn does more than you think (2026)
- University of Nottingham: A neural basis for contagious yawning (2017)
- ZME Science: Yawning to cool the brain