We all do it, that involuntary universal gesture of fatigue in which your mouth slowly opens and you give a deep noisy breath.
Contrary to popular belief, a yawn does not significantly affect your blood’s carbon dioxide levels. What it does affect is your brain’s temperature. Why is this important? Your brain is the most metabolically demanding organ in your body, especially when you are awake. Metabolic activity generates heat, so over the course of a day your brain heats up until you go to bed. A hot brain doesn’t work as well. So, a yawn is like a fan for your brain.
When you yawn, you propel blood through the blood vessels in your head by stretching your jaw muscles. As cool peripheral blood replaces hot blood, you brain cools down. Yawns also force air into your upper nasal and oral cavities, which are replete with blood vessels that project to the parts of your brain responsible for motor functions and higher processing. The air cools the blood on its way to these systems, improving your ability to move and think.
So next time your professor catches you yawning during class just tell him, “I’m sorry, I was just focusing too hard on your lecture and my brain was overheating.”