Why does a person yawn? Well, actually, sleepiness doesn't play a key role here. Yawning is a reaction to the lack of oxygen in the blood, which is a deep breath. Why then do we yawn after a sleepless night or a long nap? Again, due to oxygen deprivation - the body is already working "above the norm". Therefore, in order to stay awake, we need a sharp rush - a yawn.