Why do we yawn?
A big slow breath, a wide-open jaw, and a mystery scientists still argue about.
Open wide
A yawn is one big, slow breath. Your jaw drops wide open, your chest fills up more than a normal breath, you hold it for a moment, and then you let it out with a sigh. The whole thing takes about six seconds. Try not to yawn while you watch this. Hard, isn’t it?
A yawn is a fixed pattern: a long breath in with the mouth wide open and the jaw dropped, a short hold at full lung volume, then a quicker breath out, often with a stretch of the arms and back (pandiculation). It lasts about 4 to 7 seconds. The jaw opens at the two jaw joints just in front of the ears, where the lower jaw hinges on the temporal bone. Everyone yawns, from before birth (seen on ultrasound from about 11 weeks) to old age, roughly 5 to 10 times a day.
Why? Nobody is sure
Grown-ups used to say yawning gives you more oxygen. Scientists tested that, and it turned out to be wrong. The best guess now is that a yawn cools your brain a little, like opening a window, and helps wake you up when you are sleepy or bored. You yawn more when the room is warm, and less when your head is cool.
The oxygen idea was tested in 1987 (Provine and colleagues): breathing extra oxygen or extra carbon dioxide did not change how often people yawned. Two hypotheses lead today. Brain cooling: yawning goes up when the brain and the air are warm and drops when the forehead or the air is cool, and the deep breath and stretched jaw pull cooler blood through the head. Arousal: yawns cluster at the switch between sleep and waking and before demanding tasks. The pattern is driven from the hypothalamus (its paraventricular nucleus) and the brainstem, and it is found across vertebrates, fish included.
Catching a yawn
Seeing someone yawn, hearing one, or even reading about yawning can make you yawn. That is a contagious yawn. It happens more between friends and family than between strangers, and dogs can catch yawns from their owners. Scientists think it is linked to how we feel what other people feel.
Contagious yawning appears at around 4 to 5 years of age and is strongest between people who are close. In lab tests about half of adults yawn after watching videos of yawns. Brain imaging links it to areas for seeing and hearing others act and to the motor cortex that then moves the jaw; a tie to empathy is often proposed but debated. Chimpanzees and dogs catch yawns too. The signal is weaker in some conditions, which is one reason it is studied.