Why Do We Yawn, and Why Are Yawns Contagious?

Fair warning: you will probably yawn while reading this. That's not a sign the post is boring. It's science, and it's one of the strangest things your body does.

Everybody yawns, even before birth

People yawn. Dogs, cats, lions and birds yawn too. Doctors have seen babies yawning on ultrasound scans before they are born. A yawn is one of the oldest moves in the animal world.

A typical yawn is a long, deep breath in with the jaw stretched wide, a short pause at the top, then a quicker breath out. It lasts a few seconds and often comes with a stretch and watery eyes.

The famous answer that turned out to be wrong

For a long time people said we yawn because our bodies need more oxygen. It sounds logical, but when scientists tested it, it didn't hold up. People who breathed air with extra oxygen didn't yawn less. People who breathed air with extra carbon dioxide didn't yawn more.

So oxygen isn't the main reason. Then what is?

Idea 1: Yawning cools the brain

A leading theory says a yawn works like a radiator for your head. Stretching the jaw wide boosts blood flow around the skull, and the big gulp of air cools that blood. A slightly cooler brain may work a little more sharply.

There's evidence for it. People tend to yawn more when the air around them is cooler than their body, and less when it's very hot outside. Holding something cool against the forehead can make yawns less frequent.

Idea 2: Yawning helps us switch gears

Think about when you yawn: just after waking up, before bed, during a long car ride, in a slow lesson. These are all moments when the body is changing state, from asleep to awake, from alert to drowsy, or trying to stay alert when nothing is happening.

A yawn may be a little reset button that helps the brain shift gears. Athletes sometimes yawn before a big race, and that's not because they're bored.

Why one yawn spreads around the room

Here's the really strange part. You can catch a yawn by seeing someone yawn, by hearing a yawn, or, as you may have noticed by now, just by reading about one.

Scientists link contagious yawning to the parts of the brain that help us copy and connect with other people. Some clues:

• Babies yawn, but they usually don't catch yawns. Contagious yawning tends to start in early childhood, around the age kids begin to understand other people's feelings.
• People often catch yawns more easily from family and friends than from strangers.
• Dogs can catch yawns from their humans.

So when you yawn because your friend yawned, it may be a tiny sign that your brain is tuned in to them.

The short answer

Nobody knows the full answer yet, and that's part of the fun. Yawning probably isn't about oxygen. It may help cool the brain and help us switch between sleepy and alert. Yawns are contagious because our brains are built to connect with the people around us.

Did you yawn? Thought so.