text: why we r yawning? when? how our body act? it's a mechanizm the body uses to regulate the o2 levels in the body tissues. if you are in a confined room you are more likely to yawn because of the decreased o2 levels in that room. when you are tired, you yawn because your body has decreased o2 levels in the tissue therefore when you yawn you actually take in a deep breath of air which helps elevate your o2 saturation rate.