Often when a person is tired he or she takes a deep breath, which we call yawning. The evolutionary reasons for yawns are not known definitively, but scientists have a few theories on why we take deep breaths when we get sleepy. One theory is that yawning allows us to quickly increase the amount of oxygen in the blood. Oxygen diffuses through the blood vessels inside the lungs directly into the blood stream. This increase in oxygen may help make us more alert.
However, there is no strong evidence in support of the theory that yawning increasing oxygen levels in the blood. Instead, some scientists now believe that yawning may be related to our evolution as social animals. They speculate that yawning can be a way of communicating to those around us about a change in environmental conditions. This could explain why yawns seem to be contagious- when we see someone else yawn we often feel the need to yawn ourselves. But exactly what kind of information are we conveying in our yawns?
One possibility is that yawning is an adaptation for thermoregulation. Our brains are hardworking organs that produce a large amount of heat. When you yawn, a rush of cool air enters the mouth and nose, which are adjacent to the forebrain. This cool air may serve to cool off the brain. It makes sense then, that we would yawn first thing in the morning, or late at night- times when we are likely to be very warm. Furthermore, when we see someone near us yawn this may indicate rising temperatures, triggering an instinctual yawning response.
Koren, M (2013). Why do we yawn and why is it contagious? Smithsonian Magazine, published online.
Wong, A., & Andrews, M. A. W. (2002). Why do we yawn when we are tired? And why does it seem to be contagious?. SCIENTIFIC AMERICAN, 287(5), 99-99.