Yawning is a reflex action and many of us want to know why we yawn and also to know whether it has any biological benefit. No body has been able to fully explain it but theories abound. Everybody including new born babies do it, and so do animals. Yawning is also partly an expression of feeling, since we do it when we are bored as well as when we are sleepy. One suggestion put forward is that it is a way of increasing the blood supply to the brain, since when we yawn we experience a temporary increase in heartbeat rate and also some constriction of peripheral blood vessels. But this is far from proven.
Oddly enough, yawning tends to be contagious. One yawner in a room can set everyone else off. Even reading about it can make it happen. Have you yawned yet? I bet you have.