There has been much speculation about why animals yawn. Prevailing theories include the ideas that yawning helps to regulate 02 and CO2 levels, or that it is a purely social activity. More recently, some researchers have proposed that yawning regulates brain temperature. For example, studies have shown that rats yawn following increases in brain temperature. The rapid influx of the cooler ambient air results in lower brain temperatures after the yawn.
Yawning to cool the brain would only be effective if the outside air temperature is lower than the desired brain temperature. Therefore, Andrew Gallup Of Princeton University and Omar Eldakar from the University of Arizona hypothesized that people would yawn more often during cooler weather. This is exactly what they found.
The researchers exposed pedestrians in differing weather conditions to pictures of people yawning, a technique known to elicit sympathetic yawns. At 22oC (71.6oF), 45% of people yawned back, whereas at 37oC (98.6oF) only 24% of people had contagious yawns. Air at the latter temperature would be counterproductive for cooling purposes.
Next time you see someone yawn on a cool day, you might want to offer them a cold compress rather than a pillow.