We yawn less in hot weather.

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That’s because yawning helps cool the brain, so it can function at maximum capacity and help us stay alert. But when it’s hot out, we don't get the brain-cooling benefit from inhaling boiling hot air. In fact, researchers found people yawn most when it's around 68 degrees. And yes, being sleepy and stressed makes us yawn too because both of those physical states cause the brain to overheat.

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