It's a question as old as man itself: why do people yawn? People yawn all the time! You're probably yawning right now as you read this, just because of yawning's awesome power. People yawn in the womb, for crying out loud, and it's contagious, too (once you're out of the womb and all). But finally, Science has stepped in with an answer: yawning cools the brain.
Apparently, your brain can become overheated from watching too many episodes of Unsolved Mysteries or playing Sodoku or whatever, so yawning helps "lower the brain's thermostat" by taking in cool air from its surroundings. And sleep deprivation raises your brain temperature, so that's why you yawn when you're tired. Appropriately, people in warm climates yawn less, though tropics-dwellers can take comfort by breathing through their nose and putting a cold pack on their forehead [pdf] instead. Or: just chug six Five Hour Energies and call it a day before your liver falls out.
