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| Stewart-yawn (Photo credit: Wikipedia) |
The simple answer is, to take in more air. Often we yawn when we feel sleepy. The body is then relaxed and the breathing is shallow. Opening the mouth wide and taking in extra air in a yawn is a reflex action. lt is something you cannot control (though you may be able to stifle it, with great effort). The extra oxygen speeds up the circulation and stretches the muscles. Perhaps,long ago,when our ancestors had to stay alert out in the wild,they yawned to keep themselves from falling asleep when there might be danger.
Sometimes we yawn when we are bored,which is almost the same as being sleepy the senses are not being stimulated,we relax,and our breathing becomes shallow. The lungs demand more air, and our reflex action is to yawn.
