Why do we yawn, and why is it contagious?

The best-supported explanation is that yawning helps regulate brain temperature. The deep inhalation and the stretch of the jaw increase blood flow to the head and pull in cooler air, cooling the brain slightly — and a slightly cooler brain is a more alert one.

That is why yawning clusters around tiredness, boredom and the moments just after waking — exactly when alertness is dropping. The older explanation, that yawning tops up low oxygen, has not held up: giving people extra oxygen does not reduce it.

The contagious part is separate and social. Catching a yawn is tied to empathy and mimicry — the closer you are to someone, the more catching their yawn is. Very young children generally do not catch yawns at all, because that capacity has not developed yet.

Dogs catch yawns from humans too, which is strong support for the social account. And trying to suppress one rarely works — it is an involuntary reflex rather than something willpower governs.