Why Do We Yawn?

You yawn and you probably assume you know why. You needed more oxygen. Or maybe your body was getting rid of carbon dioxide. You’ve heard this explanation your whole life.

It’s wrong.

In the 1980s, psychologist Robert Provine at the University of Maryland ran a straightforward test: he had subjects breathe air with elevated CO2 concentrations, elevated O2 concentrations, and normal air. If yawning were about gas exchange — either taking in oxygen or expelling CO2 — the altered gas mixtures should change yawning frequency.

They didn’t. The oxygen and carbon dioxide levels of the air made no difference to how often people yawned.

The most common explanation for one of the most common things humans do was simply incorrect.

What We Know For Sure

Before getting to the best current theory, here’s what’s established:

Yawning is ancient and widespread. Fish yawn. Reptiles yawn. Birds yawn. All mammals yawn. Human fetuses begin yawning in utero at around 11–12 weeks of gestation — before they have any respiratory function that could explain it.

Yawning is not voluntary. You can initiate a yawn deliberately, but spontaneous yawning happens without your input. It has involuntary physiological characteristics — coordinated jaw movement, facial muscle activation, a brief forced inhalation — that are controlled by brainstem mechanisms, not cortical decision-making.

Yawning peaks at transitions. The most reliable predictor of yawning frequency is not tiredness per se, but transitions between states. Yawning peaks in the 30–60 minutes before sleep and the 30–60 minutes after waking. It also peaks at other state transitions: before high-stakes performance (athletes report yawning before competition; musicians yawn before performing), during boredom, and when shifting from high focus to low-focus states.

Yawning is suppressed by social engagement. People yawn significantly less when engaged in tasks that require attention and social awareness. This is one reason you rarely yawn during an exciting conversation but frequently yawn during a dull lecture.

The Brain Cooling Hypothesis

The most supported current theory, developed by Andrew Gallup at Princeton, is that yawning functions as a brain-cooling mechanism.

The brain is metabolically demanding and temperature-sensitive. Small changes in brain temperature can affect alertness, processing speed, and — at extremes — cognitive function. The brain has its own temperature regulation systems, but additional cooling mechanisms may be beneficial during temperature transitions.

Here’s where yawning fits: a yawn involves opening the jaw extremely wide (stretching the associated facial muscles and temporomandibular joint), followed by a forced inhalation through the wide-open mouth. This inhalation draws ambient air across the upper palate and sinuses, near the carotid blood supply to the brain. If the ambient air is cooler than the brain’s current temperature, this may briefly cool the blood flowing upward.

Additionally, the stretching of the jaw muscles and facial tissues may increase blood flow to the face, facilitating heat dissipation.

The evidence:

Yawning is temperature-sensitive. In studies where ambient temperature was experimentally manipulated, participants yawned more when air temperature was close to (but not above) body temperature, and less when air was cool. This is what you’d expect if yawning is trying to cool the brain — it’s useful when ambient air is cool enough to be thermally helpful.

Brain temperature fluctuates at wake/sleep transitions. Core brain temperature rises before sleep onset and falls during sleep. This matches the timing when yawning is most frequent — the transitions where temperature is changing.

Cooling the forehead reduces yawning. In studies where participants held cool packs against their foreheads, yawning frequency decreased. Holding warm packs, or breathing through their noses only (reducing the open-mouth inhalation component), yawning increased.

The State Transition Component

Even with the brain-cooling hypothesis, the timing of yawns — particularly the pre-competition yawning that athletes describe, or the yawning during boring lectures — suggests something additional.

A second (possibly complementary) hypothesis treats yawning as a state-shifting mechanism — something the body does to change its level of arousal.

Pandiculation — the combined stretch-and-yawn that most people do upon waking — is well-documented across mammals as an arousal signal. It’s not just humans: dogs, cats, and many other animals pandiculate when transitioning from rest to activity. The combined muscular and respiratory action may serve to increase blood flow, activate proprioceptive feedback (the sensory signals from muscles that help you know where your body is), and shift the nervous system from a low-activity state toward a higher one.

On this account, yawning before athletic competition isn’t about being tired — it’s about priming the arousal system for what’s about to happen.

Both mechanisms — brain cooling and state modulation — may operate simultaneously. They’re not mutually exclusive.

The Ear Thing

One often-overlooked component of yawning: it frequently changes the pressure in your ears.

The involuntary jaw movement during a yawn forces the eustachian tubes (the small canals connecting the middle ear to the throat) to open briefly. This is the same mechanism as chewing gum on an airplane — the eustachian tube opening allows pressure equalization between the middle ear and the external environment.

This may explain why people often yawn when ascending or descending in elevation — the yawn reflex may be partly triggered by changes in external pressure, which the body then compensates for through the jaw-opening mechanism.

This component doesn’t explain yawning in general, but it suggests that the yawning mechanism serves multiple functions simultaneously — another example of the body repurposing an action that’s already there for one reason to serve additional purposes.

The 5-Second Rule

A yawn lasts, on average, about 5–6 seconds. This is remarkably consistent across individuals and species that have been studied. Not 2 seconds, not 15 seconds — consistently 5–6.

No one has fully explained why the mechanism terminates at this specific duration. The best available interpretation: the brainstem circuitry controlling yawning runs a program with a fixed completion time, and the physical mechanism (the jaw stretch, the forced inhalation, the facial muscle activation) takes approximately that long to execute.

What’s interesting is that this consistency suggests yawning isn’t just a loose, variable behavior that happens when you happen to open your mouth wide. It’s a coordinated program, executing predictably, in a body that decided to build it and keep it across hundreds of millions of years of vertebrate evolution.

For something with no agreed explanation, it has an extremely specific and reliable form.

That’s the puzzle: the machine is clearly well-designed. We’re still figuring out exactly what it was designed to do.