Yawning is a reflex action, which causes us to open our mouth wide and inhale a large amount of air, which is then followed by its exhalation.Though yawning is more commonly associated with boredom, fatigue, sleepiness, overwork or stress, many of us also yawn when we wake up or simply when see others yawn.ut of the various theories regarding what causes a yawn, the most important ones are given below. A yawn is contagious, in the sense that it can easily spread to the person who is observing the other person yawning.