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Why do humans yawn?
asked Aug 11, 2010 by triuser (13,156 points)

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because when they are tired,their body becomes restless and is an indication of sleep ,so yawn is like a signal to go to bed and makes our mind to rest
answered Aug 11, 2010 by trianswer (334,515 points)
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The idea behind this (the most popular) theory is that yawning causes you to take a deep breath when you need some extra oxygen - perhaps because you are tired are have not been breathing enough. These are MANY problems with this idea, but the main problem is that yawning occurs only when you are tired and does not occur in the many other instances of hypoxia. For example, if you exercise hard you need more oxygen - but your body makes you breath harder, not yawn! If you are brought from, say, sea-level to an elevation of 20,000 feet within a time period of 5 min, you will immediately faint due to severe hypoxia, but you will not yawn.
answered Oct 6, 2010 by trianswer (334,515 points)

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