A yawn is an involuntary reflex where the mouth is opened wide, and the lungs take in a lot of air. The air is then exhaled slowly. During this time, the eardrums stretch, and the eyes may also close tight, causing them to become watery.
No thought or action has to be taken to produce a yawn, and the process is similar for everyone. Yawning commonly occurs either before or after sleep, which is why it is usually considered a sign of being tired. Yawning also occurs frequently in people who are doing boring or tedious things.
There is also a social aspect to yawning. Yawning appears to be contagious among humans and other animals, and the contagiousness of a yawn is well documented but hardly understood.
Originally thought to be a social cue, a series of experiments actually suggests a different reason for this bodily function. As Andrew C. Gallup, PhD, a postdoctoral research associate at Princeton University, told WebMD, the reason for yawning might be that it cools the brain.
The stretching of the jaw to yawn increases blood flow in the neck, face, and head. In conjunction with that, the deep intake of breath forces a downward flow of spinal fluid and blood from the brain. The air breathed into the mouth cools these fluids.
Source: medicalnewstoday.com, and bestlifeonline.com