十万个为什么·科学阅读30篇（2）

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人疲倦时为什么会打哈欠？而且它为什么会传染？

Yawning increases oxygen intake and cools the brain by drawing cool air over sinus membranes and blood vessels.
It often occurs before sleep or upon waking, helping regulate alertness and transition between mental states.
Contagious yawning is linked to empathy and social bonding, appearing strongly in humans and some primates.
Brain scans show that the same neural networks activate whether you yawn or just watch someone else yawn.
Children under five rarely catch yawns, suggesting the trait develops alongside theory-of-mind skills.
People with autism spectrum conditions yawn less contagiously, supporting the empathy connection.
Even reading or thinking about yawning can trigger it — a sign of strong mental simulation ability.
Yawning stretches jaw muscles and increases heart rate slightly, preparing the body for activity.
It’s not caused by low oxygen or high CO₂, as early theories claimed — controlled experiments disproved that.
This ancient, automatic behavior bridges physiology, psychology, and group survival in subtle ways.