Just because it’s interesting:

… According to psychologists and researchers who study such things, yawning has nothing to do with boredom, rudeness, or even fatigue. Quite the contrary. Yawning helps cool down our brains so they function better, explains Andrew Gallup, a researcher who specializes in yawning at New York’s University of Binghamton.
“Our brains are like computers,” says Gallup, who conducted yawning studies in 2007 with his father, Gordon, of the State University of New York at Albany. “They operate most efficiently when cool. Our research indicates that we yawn in response to increased brain or body temperature.”

Apparently, we associate yawning with tiredness because our body temperature is highest just before and just after sleep, but in general, it just means that our brains are overheating and need a boost of cool air.