Question

Why di we yawn when sleepy or drowsy?

Answer 1

One is that when we are bored or tired,we just don't breathe as deeply as weusually do. As this theory goes, our bodies take in less oxygen because our breathing has slowed. Therefore,yawning helps us bring more oxygen into the blood and move more carbon dioxide out of the blood.

Answer 2

we yawn when feel sleepy and drowsy because at that time we do not take more oxygen that are needed by our body and we take a deep breathe to fulfil the need of oxygen in our body.
hope this will help you.