Cooling and catching
Why Do We Yawn?
Yawning seems to cool the brain, and it spreads between people through the same circuits as empathy.
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Step 1 of 5
It is not about low oxygen.
People breathing pure oxygen yawn just as much, and people breathing air low in oxygen do not yawn more. The old explanation fails the test.
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Step 2 of 5
It seems to cool the brain.
The deep breath and the wide jaw stretch move cooler blood towards the brain. Brains work worse when slightly too warm.
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Step 3 of 5
Temperature predicts it.
People yawn more in mild warmth than in cold, and yawn less when holding something cold to their forehead. Both fit the cooling idea.
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Step 4 of 5
It clusters around sleep transitions.
Yawning peaks just before sleeping and just after waking — the moments when the brain is switching states and needs to sharpen up.
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Step 5 of 5
It is contagious.
Seeing, hearing or even reading about yawning triggers it. It spreads most strongly between people who are close to each other.
Dogs catch yawns from their owners, and are more likely to do it with a familiar person than a stranger.
The short version
Yawning appears to cool the brain rather than fetch oxygen, and it spreads contagiously between people who are close.
Try it yourself
Read this paragraph slowly while thinking about yawning. Many people yawn within a minute.
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