Taking a deep breath when you yawn moves hot blood from the brain and brings cooler air up from the lungs. Yawns are believed to be your body's natural radiator system. You might be surprised to see people at the gym yawn; it's just their body trying to cool things down. You're tired.
Does excessive yawning mean lack of oxygen?
This seems logical since yawning does bring in more oxygen with a deep breath and the expiration removes more carbon dioxide than the usual breath, but research by putting people in low-oxygen or high- carbon-dioxide environments does not cause yawning.