Why Do We Yawn — and Why Is It Contagious?

You are reading about yawning, so there is a good chance you will yawn before the end of this page. That alone shows how strange yawning is.

Not about oxygen

The popular explanation — that we yawn because we need more oxygen — was tested decades ago. People breathing extra oxygen or extra carbon dioxide didn’t yawn more or less, so oxygen isn’t the main reason.

Leading theories

• Brain cooling: a deep yawn draws in air and increases blood flow, which may help cool the brain. Studies have found people yawn more when the air is cooler than body temperature and less in very hot conditions.
• Changing state: we yawn most around transitions — waking up, getting sleepy, getting bored, or before a stressful task (athletes and performers often yawn before they start).
• Stretching: yawning stretches muscles in the face, jaw and throat.

Why yawns are contagious

Seeing, hearing or even thinking about a yawn can trigger one. Contagious yawning appears in humans, chimpanzees, bonobos and dogs, and some studies find it is more likely between people who are close to each other — suggesting a link to social connection and empathy, though the evidence is debated.

Babies and yawning

Babies yawn even before birth, but contagious yawning usually develops only around the age of four or five.

More body oddities: why we get goosebumps.