Researchers are starting to unravel the mystery surrounding one of the most common behaviors: the yawn. WSJ’s Jonathan Rockoff and evolutionary psychologist Dr. Andrew Gallup explain new studies suggesting it’s a way of keeping your brain sharp. Science has disproved the theory that it’s the body’s way of sending oxygen to your brain when deprived. Says Rockoff, one of the leading theories is that yawning is a “cranial air conditioner.” In times of stress and anxiety, we experience a slight rise in brain temperature. So the body steps in to cool it off.