A: Theory No.1: A yawn is your body’s operandi to take more oxygen into blood while yanking more carbon dioxide out of your fluid. It happens as an auto reflex, which means it’s implemented the same way the traffic enforcer executes traffic rules: you’re left with no choice. But sleepless nights of research have shown that neither more O2 nor breathing more CO2 affects your yawning frequency level. So junk that.
Theory No.2: yawning stretches the lung tissue to increase heart rate and wake up sleeping muscles and joints, which may be true.