The scientific reason is that when blood supply becomes less the jaw muscles strech, and increse the blood flow towards the head.  
            The deep intake of breath during a yawn forces downward flow of spinal fluid and blood from the brain.
     Cool air breathed into the mouth cools these fluids.
1 3 1
  • Brainly User
When we are tired our lungs don't work properly, so our body receives less oxygen. Then the brain sends signals to yawn or to widely open mouth.
1 5 1
plz mark mine as best n click thnk u