Chances are you read this sentence about yawning and you yawned. Right? Well, a new study suggests that you may not really be yawning because it's contagious, or you're bored or tired.
You're actually yawning because your brain is too hot.
A group of researchers at the University of Vienna tested subjects in Austria and Arizona and tracked their activity, finding that that the only significant predicator of yawning was temperature: subjects were much more likely to yawn at higher temperatures. Other factors like sex, season, age, humidity, time spent outside, and hours of sleep the night before did not have a significant effect on the likelihood of a subject's yawning.
Ultimately, it appears that yawning is related to regulating brain temperature and creating a state where arousal in a yawner can be achieved. The idea is that if it's hot, but still cool enough so that a large intake of air will bring your body temperature down, you're in perfect storm territory for yawns.
The study builds on research that shows that in both rats and humans, yawns are preceded by intermittent rises in brain temperatures, and that brain temperature decreases immediately afterward.