Yawning is usually due to the following factors: 1, physical fatigue, usually when the body is fatigued very want to yawn, but some times the fatigue is not enough, yawning can not yawn, belongs to the normal phenomenon. 2, in the case of severe lack of oxygen, the body appears obvious signs of ischemia and lack of oxygen, due to excessive lack of oxygen will want to yawn, but, at this time, the body for oxygen transport capacity is not enough Therefore, there will be a transient situation of insufficient oxygen supply to the cerebral blood vessels. At this time, the neurological function of the brain decreases, the neurological reflexes of the brain decrease, and the brain reflexes are significantly weakened, so there is a yawning but not out, which is a pathological reaction.