When do we yawn?
When we are tired: we open the mouth so more oxygen enters into and more blood is produced.
When we get sleep......
When we are sleepy or tired.
When your brain needs more blood. That's why people stay your tired and need to lay down when you lay down the blood can get to the brain faster.
We yawn because of lack of oxygen to the brain.Other reasons are nerves,nervous or when we notice others yawn.