I am so sleepy,I yawn.I wakes up, I stretch and yawn.Why we yawn?It is an old topics and full of controversy.
Some people believe that we yawn,because it can cool the brain and then keep your brain healthy and alert. The people in fatigue will cause the brain temperature rises,so yawning is a good way to cool the brain.
Some people think theory of tired is the account,but this point has been denied.
Some scientists shed light on this problem from evolution perspective.The snake crouching motionless in the grass,before action,they often yawn first; The hippo in the water yawn first when they are out of water. Due to the development of human beings have entered the civilization society, issued a warning to others by yawning is outdated.If so,yawning is likely to be a evolution of monuments which has lost the significance of the existence.
There is a theory has approved by most of the scientists.Yawning is a reaction which can promote the body to wake up when people think they must keep awake.
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