Yawn

Why do we yawn? Because our body needs oxygen and there is a carbon dioxide buildup. That I know.

When do we yawn? when we feel sleepy. This I know.

So what happens when we feel sleepy that makes us yawn?

When we feel sleepy, the brain gives a signal to the body to begin the shut down process. So the lubrication in your eyes would go down, your breathing will become shallow, and your muscles will start relaxing.

But the body is still on. It's working. You are still awake, just feeling sleepy.

The shallow breathing that the brain induces fails to expel the carbon dioxide that your body produces, and also fails to provide the oxygen that the body needs. The result is your yawn, in which you first take in a lot of oxygen, and then exhale. Note that a yawn is so designed that your mouth and throat are opened to the maximum.

This means that a yawn is a full throttle overdrive for your body.

So the next time you want to watch your yawn, keep the breathing going...