A yawn pulls in cool air and stretches the jaw, which helps cool a warm brain. And yawns are catching.
🥱 A yawn is a big, slow breath. Your mouth opens wide.
🧠 Your brain gets warm. A yawn brings in cool air. That cools your brain.
🐶 Dogs yawn. Cats yawn. Fish yawn too!
👀 See someone yawn. Now you feel a yawn coming. Yawns are catching.
What is a yawn?
A yawn is a long, slow breath in. Your mouth opens wide and your jaw stretches. Then you let the air out.
Why does my body do it?
Your brain works hard all day, and hard work makes it warm, like a phone. A yawn pulls cool air into your mouth and nose. The stretch also moves warm blood away from your head. So a yawn is like a little fan for your brain!
Why do I yawn when I see someone else yawn?
Yawns are contagious. Contagious means they spread from one person to another, like a giggle. When you see a friend yawn, your brain copies it. Even reading this can make you yawn. Did it?
Do animals yawn?
Yes! Dogs, cats, monkeys, birds, and even fish yawn. Babies yawn before they are born. A dog can catch a yawn from you.
Not About Oxygen
For a long time, people said we yawn because our bodies need more oxygen. It sounds right. It is wrong. In 1987 a scientist named Robert Provine had people breathe extra oxygen, and they yawned exactly as often as before. Breathing air with extra carbon dioxide did not change their yawning either. So whatever a yawn is for, it is not for oxygen.
A Fan for Your Brain
The best answer scientists have today is that a yawn cools your brain. Your brain makes heat when it works, the same way a computer does, and it works best when it is not too warm. A yawn pulls in a big gulp of cooler air, stretches your jaw, and sends a rush of blood through your head. All of that carries heat away.
Here is the clue that convinced many scientists: animals with bigger brains yawn longer. A mouse yawns for about a second. A person yawns for about six. An elephant, with a huge brain, yawns longer still. Bigger brain, longer yawn, more cooling.
Yawns Are Catching
Seeing a yawn, hearing a yawn, or even reading the word yawn can make you yawn. Scientists call this contagious yawning (say: kun-TAY-jus). Most children start catching yawns around age four or five, about the time they get good at understanding how other people feel. You catch yawns more easily from family and close friends than from strangers.
When Do We Yawn?
People yawn when they wake up, when they are sleepy, and when they are bored. But they also yawn before big moments. Athletes yawn before a race. Skydivers yawn before they jump. A yawn seems to help the brain switch gears, from resting to ready.
Try This!
Count how many times you yawned while reading this page. Now sit with your family and yawn on purpose, a big slow one, and count how many people catch it within a minute. Then try it on the dog.
The Explanation Everyone Knows Is Wrong
Ask anyone why we yawn and you will hear the same answer: the body is low on oxygen, so it takes a big breath. It is tidy, it is memorable, and it failed the only real test anyone gave it. In 1987 Robert Provine and his colleagues had volunteers breathe air enriched with carbon dioxide, which should have made oxygen-starved bodies yawn more, then pure oxygen, which should have made them yawn less, and then had them exercise until they were breathing hard. Yawning did not budge. Breathing changed, yawning did not, and the two turned out to be run by different parts of the brain.
Thermoregulation: The Brain-Cooling Hypothesis
The leading explanation today is thermoregulation. The brain produces heat whenever it works and performs best inside a narrow temperature range, so it needs a way to shed heat quickly when it warms up, and a yawn does three cooling things at once. It draws a large volume of relatively cool air through the mouth and nose. It stretches the jaw, which increases blood flow in the face and skull. And the deep inhale flexes the sinuses so that cooler blood is pushed toward the brain while warmer blood is carried away.
The evidence is not just an idea: when researchers pressed a cold pack to volunteers' foreheads, contagious yawning dropped sharply while a warm pack did nothing, and people yawn most when the air is a little cooler than their body and least in extreme heat or cold, which is exactly what you would expect if the point is to dump heat into cooler air. And across the animal kingdom, yawn length tracks brain size.
Why Yawns Spread
Contagious yawning is a separate puzzle. Nearly every vertebrate yawns, but only a few species catch yawns from each other: humans, chimpanzees, bonobos, some monkeys, wolves, and dogs, which catch them from us. In children the ability appears around age four or five, the same age at which children get good at reading other people's feelings, and it is weaker in people who find social cues hard to read. Consequently, most researchers treat contagious yawning as a side effect of a brain built for empathy: watching a yawn activates the same circuits that produce one.
That fits another finding: in a year-long study of people yawning in ordinary life, a yawn spread fastest and most often between family members, next between friends, then acquaintances, and least of all between strangers, so the closer the bond, the stronger the contagion.
State Changes
People yawn when they wake and when they are sleepy, which is why yawning is linked with boredom, but they also yawn before exams, before races, and, in one classic report, in the door of an aircraft just before a parachute jump. Nobody is bored in that doorway. The common thread is a transition: the brain shifting from one level of alertness to another, and a quick cooling may be how it prepares.
Think About It
If yawning were really about oxygen, what would you expect to happen when a person breathes pure oxygen, what actually happened, and, if contagious yawning depends on understanding other minds, why might a dog catch its owner's yawn while a fish never catches anything at all?
A Behaviour Older Than Jaws Deserve
Yawning is a stereotyped action pattern: a slow, deep inhalation with the jaw gaping, a brief peak, and a faster exhalation, lasting about six seconds in humans and remarkably consistent from one yawn to the next. It appears in fish, reptiles, birds and mammals, it is present in the human fetus from the first trimester, and once it starts it cannot be stopped halfway, all signatures of a very old brainstem program rather than a learned habit. Provine, who spent much of a career on it, called yawning the behaviour that everyone does and no one studies.
The Oxygen Hypothesis, Tested and Retired
The respiratory explanation was straightforward to test. Provine, Tate and Geldmacher (1987) manipulated inspired gas in volunteers, raising carbon dioxide to 3 to 5 percent, supplying 100 percent oxygen, and adding exercise; none of the conditions changed yawn frequency, although breathing rate responded as physiology predicts. Yawning and breathing are controlled separately. Whatever the reflex is for, blood gases are not the trigger.
Thermoregulation
Andrew and Gordon Gallup proposed in 2007 that a yawn is a heat-exchange manoeuvre for the brain, and the prediction set that followed has held up well. Deep inhalation of ambient air cools venous blood returning from the nasal and oral cavities; the gaping stretch raises facial and cerebral blood flow; and flexion of the maxillary sinus walls appears to ventilate the sinuses, which sit beside the brain's arterial supply. Cooling the forehead or breathing through the nose, both of which lower brain temperature by other routes, reduced contagious yawning in their experiments, while warming had no effect.
The comparative data are the second pillar. Gallup, Church and Pelegrino (2016) showed that mean yawn duration across 24 mammal species predicted brain mass and cortical neuron number better than body mass did; Massen and colleagues (2021) extended this to 1,291 yawns from 101 species of mammals and birds and found the same scaling in both lineages, with birds yawning shorter for their brain size, consistent with their higher baseline body temperatures. Larger, hotter brains need longer cooling bouts. Small ones do not.
Contagion and the Social Brain
Contagious yawning is phylogenetically narrower than spontaneous yawning and develops later in the individual. Helt and colleagues (2010) found it emerges in children around four years of age, when theory of mind matures, and is markedly rarer in children with autism spectrum conditions, while Norscia and Palagi (2011) recorded natural yawns over a year and showed that the likelihood and speed of contagion followed social closeness: kin, then friends, then acquaintances, then strangers. Dogs catch yawns from humans (Joly-Mascheroni, Senju and Shepherd, 2008), and chimpanzees catch them from familiar group members more than from outsiders. The parsimonious reading is that contagious yawning rides on the same perception-action circuitry that supports empathy, though whether it serves any function of its own, such as synchronising the alertness of a group, remains open.
What Is Still Unknown
The brain-cooling account is the best-supported single explanation, but "best supported" is not "settled". Direct measurements of brain temperature before and after yawns exist for rats and show the predicted drop, and human intracranial data are scarce. The social function of contagion, if any, is unproven. And the deep evolutionary origin, in animals whose brains are small and whose yawns still happen, suggests the reflex may have started as something else, with cooling layered on later. It is a good field for a curious person: the question is ancient, the experiments are cheap, and the answer is not in the textbook yet.
Why Your Kid Asked
Because they just did it, probably at bedtime, and because the answer they have heard ("you need more oxygen") is one of the most durable pieces of folk physiology there is, and also wrong, which makes this a rare chance to show a child how science retires a nice-sounding idea: someone tested it.
The Short Version
Yawning is an involuntary, stereotyped reflex present in most vertebrates and in the human fetus. The oxygen explanation was tested directly by Provine, Tate and Geldmacher in 1987 and failed: neither elevated carbon dioxide, pure oxygen nor exercise changed yawn frequency, though each changed breathing. The best-supported current account is thermoregulation. A yawn draws in cool air, increases cranial blood flow and appears to ventilate the sinuses, and a series of predictions from that idea have held: forehead cooling suppresses yawning, yawning peaks in a mild "thermal window" of ambient temperature, and yawn duration scales with brain size and cortical neuron count across more than a hundred species. Contagious yawning is a separate phenomenon, present in only a handful of social species, emerging in children around four alongside theory of mind, and strongest between people who are close.
The Numbers
A human yawn lasts about six seconds, a mouse's about a second and a half, an elephant's eight or more, and when Massen and colleagues timed 1,291 yawns across 101 mammal and bird species they found duration tracking cortical neuron number in both lineages. Helt and colleagues put the onset of contagious yawning near four years, with a sharp reduction in children on the autism spectrum. In Norscia and Palagi's year of naturalistic observation, the probability of catching a yawn ran kin, then friends, then acquaintances, then strangers, and the delay before the caught yawn shortened along the same gradient. Joly-Mascheroni, Senju and Shepherd's dogs: 21 of 29 yawned in response to a human yawn, none to a control mouth movement.
Two Common Misconceptions Worth Correcting
First, the oxygen story, above; if a children's book in your house prints it, a pencil note in the margin is fair. Second, "yawning means bored or rude." Yawning tracks transitions in arousal, which is why athletes, musicians and paratroopers yawn before performing; a child yawning in class may be sleepy, may be cold, or may be shifting into focus. It is information about state, not about manners, and a warm classroom makes it worse, not better.
When to Pay Attention
Occasional yawning is universal and a burst of it in a warm room or at a transition is normal. Persistent excessive yawning, dozens of times an hour with no obvious cause, is worth a conversation with a doctor: it can accompany sleep deprivation, some medications (notably certain antidepressants), migraine, and less commonly neurological conditions that affect thermoregulation. That is the exception. The rule is that a yawning child is a child whose brain is doing exactly what it should.
Something to Do Together
The contagion experiment is free and works at dinner: one person yawns, slowly and visibly, everyone else counts who catches it and how long it takes, and then you repeat the whole thing with a stranger's yawn from a video and compare the two. Children old enough to keep a tally can test the thermal window too: note yawns during ten minutes outside on a cool evening versus ten minutes in a warm bath, and see which wins. Let them predict first. That is the whole scientific method in one bedtime.
Sources
- Provine, R.R., Tate, B.C. and Geldmacher, L.L. "Yawning: no effect of 3-5% CO2, 100% O2, and exercise." Behavioral and Neural Biology 48 (1987): 382–393.
- Provine, R.R. "Yawning." American Scientist 93, no. 6 (2005): 532–539.
- Gallup, A.C. and Gallup, G.G. "Yawning as a brain cooling mechanism: nasal breathing and forehead cooling diminish the incidence of contagious yawning." Evolutionary Psychology 5 (2007): 92–101.
- Gallup, A.C. and Eldakar, O.T. "The thermoregulatory theory of yawning: what we know from over 5 years of research." Frontiers in Neuroscience 6 (2013): 188.
- Gallup, A.C., Church, A.M. and Pelegrino, A.J. "Yawn duration predicts brain weight and cortical neuron number in mammals." Biology Letters 12 (2016): 20160545.
- Massen, J.J.M. et al. "Brain size and neuron numbers drive differences in yawn duration across mammals and birds." Communications Biology 4 (2021): 503.
- Helt, M.S., Eigsti, I.-M., Snyder, P.J. and Fein, D.A. "Contagious yawning in autistic and typical development." Child Development 81 (2010): 1620–1631.
- Norscia, I. and Palagi, E. "Yawn contagion and empathy in Homo sapiens." PLoS ONE 6 (2011): e28472.
- Joly-Mascheroni, R.M., Senju, A. and Shepherd, A.J. "Dogs catch human yawns." Biology Letters 4 (2008): 446–448.