Title says it all…
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One explanation is that you yawn when the lungs have low oxygen capasity. The yawn fills your lungs with fresh air. This happens more often when you’re sleepy and you breathing gets more shallow.
But there is evidence that fetuses do this too and it doesn’t serve this function for them.
Some say it has more to do with your heart rate. A yawn quickly increses your heart rate when your brains feels like it’s getting too low. This would also explain why you yawn when tired, because heart rate is lower when you rest/sleep.
Some say it has to do with bodytemperature and yawning cools it down. And there is some evidence that people yawn more during summer than during winter. And at night the body might be trying to prepare you for a good night sleep.
Also some say that yawning is a form of primal communication. And I’m sure there are other theories. It also could be a mixture of all these and more. Science hasn’t found an exact answer.
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