Question

why yearning is coming form our mouth during we felt sleepy

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.When you yawn, it is simply a big volume cycle of inspiration (of oxygen) and expiration (of carbon di oxide). This makes up for the defecient oxygen reaching the brain and washes out excess carbon di oxide. Brain gets more oxygen, the vicious cycle is broken and brain cells are happier.

Hope it helps u.