There was a report a couple days ago on the morning news and they had a scientist who thinks he found out why people yawn. He says it's a way to cool off the brain, which can get very warm if you don't get enough sleep. He even did an experiment where he had people watching videos of people yawning. Some of the people held cold packs on their forehead, some people held hot packs on their forehead, and some people held nothing on their forehead. He found that the people with the hot packs yawned a lot more and the people with the cold packs yawned a lot less. I don't know if that's really the explanation, but it's better than anything I can come up with.