Contrary to popular belief, we don't yawn because we need more oxygen. Studies have found no difference in yawning rates among people breathing air with high or low levels of oxygen. There is probably more truth to other common beliefs about the causes of yawning. We are more likely to yawn right after we rise in the morning, before we retire at night and when we are bored. On the other hand, we may yawn when we are very anxious or under stress.
Some psychologists think that yawns help regulate changes in alertness.
Other studies confirm that seeing or even thinking about another person yawning will make you yawn too. Even reading an article about yawning can make a person more likely to yawn. One possibility is that contagious yawning may have evolved as an unconscious way to synchronize behavior in a group, though the reason for this is unclear. In very rare instances, some nervous system disorders affect yawning. People with schizophrenia, for instance may yawn less, while people with brain lesions, concussions, tumors or hemorrhages may yawn more.