Why Do I Yawn When I’m Not Tired?Your body's built-in air conditioning for your brain.

From the show

Science Around You

That contagious reflex might be more than a sign of boredom or fatigue. Yawning is a powerful physiological mechanism that helps regulate the temperature of your brain. Stretching your jaw and taking a deep breath increases blood flow and introduces cooler air, effectively giving your brain a blast of AC to maintain optimal function. So next time you yawn, you'll know your body is just trying to keep a cool head.