Sometimes when your body is very tired, your lungs and the rest of your respiratory system may slow down until there’s too little air in your lungs. As soon as this happens, your body sets off a quick movement, or spasm, in the muscles of your mouth, throat, and chest.
This spasm forces you to take in a deep breath of air, a reaction that we call a yawn. You yawn, then, not to stretch your jaw muscles or to relax or anything like that, but to take in more air.
Yawns can be contagious. Sometimes if you see a person yawning, you may yawn too!
Yawning can even be so contagious that simply by reading about yawning, a person will yawn! (Did you yawn while reading this article?)