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Why Do We Yawn When We’re Tired—or Even When Others Do?
为什么疲倦时打哈欠，甚至看到别人打哈欠也会跟着打？
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Yawning cools the brain by increasing blood flow and drawing in cooler outside air through the mouth.
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Brain temperature rises slightly before sleep and during mental fatigue, triggering this thermoregulatory reflex.
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Contagious yawning likely evolved as a social cue to synchronize group alertness and rest cycles.
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Studies show people with higher empathy scores yawn more readily when observing others yawn.
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Children under four rarely catch yawns, suggesting the trait develops alongside theory-of-mind abilities.
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Even dogs yawn contagiously after watching familiar humans—especially their owners—yawn.
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Yawning isn’t caused by low oxygen or high carbon dioxide, as older theories claimed.
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Stretching during a yawn also stimulates facial nerves linked to wakefulness and attention.
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In classrooms or meetings, shared yawning may subtly coordinate group focus and transition timing.
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While mostly harmless, excessive yawning can sometimes signal neurological conditions needing evaluation.