You probably yawned while reading that headline — or you will before you finish this article. Yawning is one of the most universal things the human body does, shared with creatures as different from us as fish, parrots, and lions. And yet, for something so ordinary, it hides a genuinely surprising fact: scientists still don’t fully agree on what yawning is for. What they have uncovered, though, is far more interesting than the tired old explanation everyone grew up with.

What a yawn actually is

A yawn feels lazy, but mechanically it is anything but. A yawn is the opening of your airway and mouth to take a long, deep breath of air — a complex muscular movement in which, within seconds, your entire airway fully dilates while surrounding muscles powerfully stretch or tense, most significantly around your throat. Your jaw drops near its limit, your eyes often water, your eardrums shift. It is one of the most all-encompassing reflexes your body performs, and it is remarkably hard to stop once it starts. The medical term for it, if you ever need to impress someone, is oscitation.

It also shows up astonishingly early. Yawning is a reflex that appears early in human development — fetuses yawn in the womb — and it persists throughout the entire lifespan. Whatever it does, the body has considered it worth keeping from before we are even born.

The myth that refuses to die

Ask almost anyone why we yawn and you’ll hear the same answer: we’re low on oxygen, and a yawn gulps in a fresh supply. It is tidy, intuitive, and almost certainly wrong. Researchers have explicitly tested that hypothesis, and the results concluded that breathing and yawning are controlled by different mechanisms. In experiments, giving people extra oxygen or more carbon dioxide did not reliably change how much they yawned. If the oxygen theory were right, those changes should have mattered. They didn’t.

So if not oxygen, then what? This is where the science gets genuinely intriguing — and where the leading modern explanation points somewhere unexpected: your brain’s temperature.

The brain-cooling theory

The idea that has gathered the most evidence over the past two decades is that yawning helps regulate the temperature of your brain — a bit like a radiator kicking in. The brain-cooling hypothesis suggests that brain temperature decreases with yawning due to the intake of cool air and changes in facial blood flow. When the brain becomes warmer than its stable temperature, we might yawn to cool it down: cooler blood from the body floods into the brain while warm blood circulates out.

The supporting evidence is surprisingly direct. Studies in both animals and humans show that yawning occurs before, during and after instances of abnormal thermoregulation, like heat stress and high body temperature. In carefully controlled experiments, scientists found they could reliably manipulate yawn frequency by changing ambient temperature and the brain and body temperature of the individual. In rats, researchers showed that rises in brain temperature reliably trigger yawning — followed, neatly, by a measurable drop in brain temperature. The mechanism even has supporting details: when you yawn, you inhale a large amount of air that cools the blood vessels in your head and neck, the stretching of facial muscles helps pump blood, and the tears a big yawn produces release a little heat too.

Why you yawn when you’re tired or bored

The cooling theory dovetails neatly with the situations where yawning is most familiar: waking up, drifting toward sleep, and long stretches of boredom. Yawning primarily occurs during periods of state change, commonly following transitions between sleeping and waking. Brain temperature naturally shifts around those transitions, and a yawn may help nudge it back into the comfortable zone.

There’s a second, related role: alertness. Some research suggests that yawns are initiated alongside increases in cortical arousal, so yawns themselves may function to promote alertness. When you yawn, your heart rate briefly increases and your facial muscles tense — small jolts that may help you snap awake. In other words, the yawn that hits you in a dull meeting may not be your body giving up on staying awake; it may be your body’s attempt to keep you going. The average adult, for the record, yawns around 20 times a day.

The strangest part: catching a yawn

Now to the phenomenon that makes yawning truly weird — its contagiousness. Reading about yawning, watching someone else yawn, even just hearing one is often enough to set you off. This is a textbook example of what scientists call echophenomena — the automatic imitation of another’s actions. And it is not unique to us: contagious yawning has been documented in chimpanzees, bonobos, dogs, and even parrots, but only in social species. Solitary animals don’t seem to catch yawns at all, which is a strong hint that the behaviour serves some social purpose.

What is that purpose? Two leading ideas compete. One is simple mimicry — the same unconscious copying that makes us mirror the posture or speech of people around us, thought to be linked to the brain’s mirror neuron system. When you watch someone yawn, brain regions involved in emotional processing and social cognition light up as though you were yawning yourself. The other idea ties contagion to empathy: catching a yawn may be a faint signal of how readily you tune in to others’ internal states.

Does it really measure empathy?

This is the claim you’ve probably heard — that more empathetic people catch yawns more easily — and it deserves a careful look, because it’s both partly supported and genuinely contested. On the supporting side: studies show susceptibility to contagious yawning correlates with empathic skills in neurotypical people, and we are much more likely to catch a yawn from a close loved one than from a stranger. Contagious yawning also doesn’t fully emerge in children until around age four or five, the same window in which the social brain matures.

But the picture isn’t clean. Other studies have failed to reproduce the empathy link entirely. One careful experiment found that simply instructing people to pay attention to a yawning video erased differences between individuals — suggesting plain attention, not empathy, was doing some of the work. As one leading yawning researcher put it bluntly, the jury’s still out. The most you can responsibly say is that empathy may play a role, alongside attention and sheer tiredness, which remains one of the strongest predictors of whether a yawn spreads.

Why being unable to stop one is normal

If you’ve ever tried to suppress a yawn and felt the urge grow stronger, that’s real — and revealing. Using brain stimulation, researchers found that the “urge” to yawn is actually increased by trying to stop yourself, because contagious yawning is driven by excitability in the brain’s primary motor cortex. The harder you fight it, the more you stoke the very circuit producing it. That same line of research has a serious payoff: understanding the brain excitability behind contagious yawning may help scientists better understand — and perhaps one day treat — conditions like Tourette syndrome, where involuntary movements are also tied to motor-cortex excitability.

The honest bottom line

So why do we yawn? The most likely answer is that a yawn is a small thermostat for the brain, kicking in around the moments we shift between sleep and wakefulness and helping to keep us alert. Why is it contagious? Probably because, somewhere in our social evolution, syncing up with the people around us was useful — possibly to coordinate group rhythms or stay collectively vigilant. What’s remarkable is that a reflex this common, this old, and this universal still hasn’t fully given up its secrets. The next time a yawn ripples across a room, you’re watching one of biology’s small, unsolved mysteries play out in real time — and, almost certainly, joining in.