Question

You might have noticed whenever you feel sleep or drowsy you start yawning. Why is it so?

Answer 1

because One of the more obvious reason for yawning is we do it when we are tired. The deep breath associated with yawning helps us to drawn in more oxygen and supply the much needed oxygen to the tired body organs including the brain. However,this theory has been challenged by some studies.

Answer 2

It's because of the lack of oxygen in our body.