Cortisol levels increase after yawning and may increase alertness and the need for action.

Psychologists think yawning helps improve blood flow to the brain. The premise is that stretching the jaw increases blood flow to the face, head, and neck, while the deep breath of a yawn forces blood and spinal fluid to flow downward. This may explain why people yawn when they are stressed.

Scientists studying this reflex have proposed several reasons for the phenomenon. A yawn is a reflex that may be triggered by sleepiness, stress, boredom, or seeing another person yawn.

There are several reasons for yawning. snakes yawn to realign their jaws after eating and to aid respirat...