Physiological reasons for yawning in humans

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.