Why do we yawn when we are tired? And why does it seem to be contagious?

Yawning appears to be not only a sign of tiredness but also a much more general sign of changing conditions within the body. Studies have shown that we yawn when we are fatigued, as well as when we are awakening and during other times when our state of alertness is changing. Yawning is characterized by a single deep inhalation (with the mouth open) and stretching of the muscles of the jaw and trunk. It occurs in many animals and involves interactions between the unconscious brain and the body. For years it was thought that yawns served to bring in more air when low oxygen levels were sensed in the lungs by nearby tissue. We now know, however, that the lungs do not necessarily detect an oxygen deficit. Moreover,fetuses yawn in utero, even though their lungs are not yet ventilated. In addition, different regions of the brain control yawning and breathing. Low oxygen levels in the paraventricular nucleus (PVN) of the hypothalamus of the brain can induce yawning. Another hypothesis is that we yawn because we are tired or bored. But this, too, is probably not the case—the PVN also plays a role in penile erection, an event not typically associated with boredom. It does appear that the PVN of the hypothalamus is, among other things, the “yawning center” of the brain. It contains a number of chemical messengers that can induce yawns, including dopamine, glycine, oxytocin, and adrenocorticotropic hormone (ACTH). ACTH, for one, surges at night and prior to awakening and elicits yawning and stretching in humans. Yawning also seems to require production of nitric oxide by specific neurons in the PVN. Once stimulated, the cells of the PVN activate cells of the brain stem and/or hippocampus, causing yawning. Yawning likewise appears to have a feedback component: If you stifle or prevent a yawn, the process is somewhat unsatisfying.
It is correct to say that yawns are contagious: Seeing, hearing, or thinking about yawning can trigger the event, but there is little understanding of why. Many theories have been presented over the years. Some evidence suggests that yawning is a means of communicating changing environmental or internal body conditions to others, possibly as a way to synchronize behavior. If this is the case, yawning in humans is most likely a vestigial mechanism that has lost its significance.