An involuntary act, or reflex action, usually associated with drowsiness or boredom. The mouth is opened wide and a slow, deep breath is taken through it in order to draw air into the lungs. The air is then slowly released. Yawning is accompanied by a momentary increase in the heart-rate, and, in many cases, watering of the eyes. The purpose of yawning is unknown, but one theory suggests it is triggered by raised levels of carbon dioxide in the blood; thus, its purpose could be to reduce the level of carbon dioxide and increase that of oxygen in the blood.


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