According to Osho, yawning is a mechanical reflex that occurs as sleep approaches: breathing slows, carbon dioxide builds up, and—when we resist sleeping—the body expels this CO2 with a sudden wide-mouthed breath, drawing in fresh oxygen. Yawning thus momentarily counteracts drowsiness and resets the gas balance, signaling that rest is needed and helping maintain wakefulness if we insist on staying up.
When you’re sleepy but keep yourself awake, a yawn dumps extra CO2 and pulls in oxygen to give you a brief wake-up signal.