Yawning

Yawning

All humans yawn. So do most vertebrate animals. Surely it serves some useful function. But what that might be has puzzled scientists throughout the ages.

Now a series of experiments suggests a surprising reason for yawning. It cools the brain, says Andrew C. Gallup, PhD, a postdoctoral research associate at Princeton University.

When you start to yawn, powerful stretching of the jaw increases blood flow in the neck, face, and head.
The deep intake of breath during a yawn forces downward flow of spinal fluid and blood from the brain.
Cool air breathed into the mouth cools these fluids.

Together these processes may act like a radiator, removing too hot blood from the brain while introducing cooler blood from the lungs and extremities, thereby cooling brain surfaces,
The theory predicts that colder outside air should cool the brain better than hot air. The body should therefore yawn more when the air is cool, and yawn less when the air is hot.

 Once in the winter, when it was a cool 71.6 degrees F outside, and once in early summer, when it was 98.6 degrees F.
Sure enough, in the cooler weather 45% of people yawned when they looked at the pictures. But in hotter weather, only 24% of people yawned. Moreover, people yawned more if they'd been outside longer in the cool weather, and yawned less if they'd been outside longer in the hot weather.

The cooling theory of yawning is the only theory that explains all these experimental results. But he has not yet convinced those who prefer another theory.

University of Geneva physician Adrian G. Guggisberg, MD, agrees with Gallup that changes in room temperature can trigger yawning. But he's wary of the brain cooling theory. 

There are other ways to regulate body temperature, such as sweating, and it is unclear why we would need another regulator which fails when it matters.