The yawning is a reflex caused due to excess of carbon dioxide and lack of oxygen in the blood. The brain stem detects this and triggers the yawn reflex, through its non vital nuclei in medulla oblongata. Recently it has been suggested by some scientists that yawning is used for regulation of body temperature, too. The yawns are also caused by the same chemicals produced in human nervous system often called neurotransmitters like serotonin, dopamine, glutamic acid and nitric oxide. When their quantity is above their optimum level in the brain, it causes yawning.
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