A few theories exist to explain human yawning:
• Do increased amounts of carbon dioxide in the blood induce yawning to bring in oxygen? This theory has been around since Hippocrates, but studies have since shown it to be either incorrect or flawed. Why? Because yawning may, in fact, reduce oxygen intake compared to normal respiration (breathing), not increase it.
• Is yawning the body’s way of controlling brain temperature? This theory suggests that yawning cools off the brain much like a fan cools the insides of a computer. Reptiles, for instance, might yawn to maintain their body temperature. Since most reptiles rely on ambient temperature to maintain body temperature, yawning might be a means for promoting rapid cooling down. This may also work for animals that don’t sweat. Pigs and dogs may yawn because they don’t sweat. Exposing the mouth and tonsils to the outside air may cool down the head, even if only by a few tenths of a degree.
• Does yawning satisfy the need to stretch your muscles?
• Is a yawn associated with nervousness?
• Do yawns help you to stay alert? Some studies have suggested that yawning--especially “contagious” yawning--may have developed as a way of keeping a group of animals alert. Anecdotal evidence suggests that yawning helps increase the state of alertness of a person. Paratroopers, for instance, have been noted to yawn in the moments before they exit the aircraft.
• Is yawning a sign of dominance? Observation of dog and primate behavior has shown that alpha males and alpha females tend to yawn more frequently than do their beta counterparts. Yawning may actually be considered aggressive in certain species, since it is a chance to display lots of pointy teeth!
None of these theories has been definitely proven, and these are just the tip of the theoretical iceberg.
More Answers from Michael Breus, PhD