The most scientifically supported explanation for yawning is brain temperature regulation. A 2014 study published in Physiology & Behavior found that people yawn less in winter than in summer — because inhaling cool air helps bring an overheated brain back to its optimal temperature. This is why you yawn when you’re tired: sleep deprivation raises brain temperature, and yawning acts as a cooling mechanism.
The older theory — that yawning increases blood oxygen — has been largely debunked. Breathing extra oxygen doesn’t reduce yawning, and yawning doesn’t reliably raise blood oxygen levels. Yawning also stretches the lungs and facial muscles, which may help increase alertness by forcing blood toward the face and brain.
And yes, yawning is contagious. A study from Baylor University found that people with higher empathy are more likely to catch a yawn from someone else. Even reading about yawning can trigger one — and you probably just did.
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