Question

Why do you think we yawn when we are sleepy and drowsy?

Answer 1

When you feel sleepy..it is actually the brain cells which are in a sense of saying..going to fall asleep for lack of oxygen (hypoxia or hypoxemia). Also when we feel drowsy, our respiratory rate being controlled by reflex centres in our brain falls. This is a vicious cycle. The yawn breaks this cycle.

Answer 2

when we are sleepy we yawn because , it is actually the involuntary movement of our mouth . it is due to mainly that the brain cells which are in the senses  are going to fall asleep for the lack of oxygen (hypoxia or hypoxemia) . also when we feel drowsy , our respiratory rate being controlled by reflex centers in our brain falls . this is the viscous cycle .the yawn break this cycle