Researchers at the University of Vienna found that the amount of yawning we do increases when our brains are hot. (Via ScienceDirect)
"Much like a computer, your brain works best at a certain temperature and tries to avoid overheating. And it turns out, yawning increases your heart rate, blood flow and the use of muscles in your face, which are all essential to cooling the brain." (Via YouTube / AsapSCIENCE )
So, why is yawning associated with being tired? Well, exhaustion and sleep deprivation are known to increase overall brain temperature. (Via Flickr / twob / nate2009)
But as the Daily Mail points out, researchers found people yawn less when it's hotter than normal body temperature outside, which makes sense — you would be breathing in hot air. Not exactly cooling.
The research says people also yawn less at the other extreme. When it's super-cold outside — like, say, frozen — brain cooling may not be necessary. (Via Walt Disney Studios / "Frozen")
Time notes this study builds on research that showed yawns are preceded by sporadic rises in brain temperatures and are followed immediately by decreases in brain temps.
We still don't know why yawning is contagious, but the researchers argue it might be an evolutionary trait to improve overall alertness.