Yawning is a normal stretching mechanism. It's similar to stretching other muscles in the body. Normally we stretch when we feel the need to do so.
Our muscles contract and squeeze the blood in the veins which are poor in
oxygen and have it replaced by newly oxygenated blood giving us a feeling of
renewal. That is not the only reason why we yawn, though, obviously. Its also a
contagious phenomenon, which hints at a strong psychological effect. Many
theories have been proposed, but it remains a mystery why we should yawn to
start with.
When there is too much carbon dioxide and too
little oxygen in the blood, though, yawning helps in establishing the balance
of oxygen.
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