Why do we yawn?

Answer: Nobody is sure; the best-tested idea is that a yawn cools the brain

Nobody is sure; the best-tested idea is that a yawn cools the brain ✓ — Yawning is one of the oldest reflexes we have and still one of the least understood. The oxygen story was tested directly in 1987 and failed: breathing pure oxygen did not reduce yawning, and extra carbon dioxide did not increase it. The idea with the most evidence today is thermoregulation. Yawning rates rise and fall with brain and air temperature, and cooling the forehead or breathing through the nose cuts contagious yawns. Many researchers still call it unproven, so the honest answer is a leading hypothesis, not a settled fact.

To take in extra oxygen when blood oxygen runs low — This is the explanation most people give, and it was tested: Robert Provine's 1987 experiments found that neither extra oxygen nor extra carbon dioxide changed how much people yawned.

To stretch the jaw muscles before sleep — A yawn does stretch the jaw, but it also happens when we are bored, before exercise, and even in the womb, so muscle-stretching cannot be the reason.