Yawning is reflex action in which large amount of air is inhaled and then exhaled. Some of the causes considered behind yawning are fatigue, overwork, sleepiness, stress or boredom.
There are different theories behind the causes of yawning but experts are not unanimous about these. It can be said that a combination of some of these factors causes yawning.Some consider yawning as an exercise similar to stretching that flexes the muscles and joints of the body. It also raises the blood pressure and pulse rate.