Why Do We Yawn? The Science of Contagious Yawning

You may yawn while reading this. Just seeing the word "yawn" can be enough. Yawning is something we all do every day, yet science still has not fully explained it. Recent research has, however, turned up some fascinating clues.

What happens in the body when we yawn?

An average yawn lasts about six seconds and has three phases:

1. The mouth opens wide while you take a long, deep breath.
2. The muscles of the jaw, face and neck stretch hard for a moment — even your eardrums tighten slightly.
3. You breathe out more quickly and the muscles relax.

This brief routine noticeably changes blood flow inside the skull, and that is where the strongest clue about yawning’s purpose comes from.

The old theory: “not enough oxygen”

For a long time people assumed yawning raised oxygen levels in the blood or flushed out excess carbon dioxide. It sounded sensible: breathing slows when we are sleepy, so a yawn would “catch up”.

When the idea was tested, it fell apart. In psychologist Robert Provine’s experiments in the 1980s, volunteers breathed air with far more oxygen or carbon dioxide than normal, and their yawning rate did not change. People who exercise hard, and need much more oxygen, do not yawn more while doing so either.

The current favourite: cooling the brain

Today the best-supported explanation is the brain-cooling hypothesis. The idea goes like this:

• The deep breath draws cool air over blood vessels in the mouth and nasal cavity.
• The strong jaw stretch speeds up blood flow to and from the brain.
• Slightly warmer blood is replaced with cooler blood.

The brain is very sensitive to temperature and works best within a narrow range. Tiredness, sleep deprivation and boredom can all affect brain temperature — which may be exactly why we yawn more at those moments.

Several findings support the hypothesis:

• People who held a cold pack to their forehead, or breathed only through the nose, were less likely to catch a yawn.
• Yawning becomes more frequent in mild weather and less frequent when it is very hot or very cold. If the air is warmer than the body, a yawn cannot cool; if it is very cold, there is no need to.
• Across mammals, species with bigger brains and more cortical neurons yawn longer, and humans rank near the top.

All this suggests yawning is not just about sleep. It may help the brain stay alert and perform well, which could explain why people yawn before exams or before going on stage.

Why is yawning contagious?

Perhaps the strangest thing about yawning is that it spreads. Seeing, hearing or even reading about a yawn can trigger one. Roughly half of people are susceptible to contagious yawning.

Researchers have noticed a few intriguing patterns:

• Contagious yawning happens more often between family and friends than strangers.
• In children it usually becomes noticeable after the age of four or five — around the time they get better at understanding other people’s feelings.
• It has also been observed in social animals such as chimpanzees, bonobos and dogs.

That is why contagious yawning has been linked to empathy and social synchrony. One idea is that yawning together aligns the alertness of a group. The empathy link is debated, though: some large studies found it weaker than expected, so the question remains open.

Can you stop a yawn?

Suppressing a yawn completely is hard, and a stifled yawn rarely feels satisfying. If the brain-cooling hypothesis is right, these tricks may reduce the urge:

• Breathing deeply through your nose
• Drinking something cold or splashing cold water on your face
• Moving to a cooler room

In short

Yawning is most likely an ancient mechanism that cools the brain and helps keep us alert. The oxygen theory has not survived testing, while contagious yawning may be a trace of our social nature. If you yawned a few times reading this, your brain was probably just after a little fresh air.