十万个为什么·科学阅读30篇（4）

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为什么疲倦时打哈欠，甚至看到别人打哈欠也会跟着打？

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