Yawning is the act of opening the mouth wide, or gaping. The usual yawn is due to drowsiness or fatigue. It is a sign that the body needs sleep. Yawning is an involuntary reflex. After the act has started, it is almost impossible to stop it. The mouth can be held closed, but the yawning muscles still contract.
People and animals yawn when oxygen is slowly cut off from them, and when the muscles are thoroughly relaxed. People who yawn often are probably not getting enough oxygen. They may need better ventilation, or exercise. They will generally stop yawning if they drink a beverage, or bathe their face with cold water. Scientists are not sure what part of the nervous system controls yawning, but it may be the mesencephalon (midbrain) in the brain. One purpose of yawning may be to awaken a person by stretching the muscles, helping the blood to circulate, and by increasing the amount of inhaled air.
