You and I, on average, will each yawn eight times today [source: Koren]. While one or two of those yawns may be to pop your ears after a flight or another change in air pressure, scientists can't yet fully explain for certain why we yawn — or why yawning is contagious. But they have ruled out the theory that yawns mean you're tired. Yawning also won't help wake you up by increasing the oxygen levels in your bloodstream.
Hippocrates theorized that yawning was the body's way of releasing noxious air when body temperature began to rise. Later theories emerged that yawning was the body's way of increasing the level of oxygen in the blood supply [source: Robson]. Currently, there are two leading theories as to why we yawn. One is a social explanation: Yawning is a primitive form of communication (although what we're trying to communicate to each other remains unclear). The other explanation is that yawning is a thermoregulatory function that cools the brain when its temperature rises as little as 0.18 degree Fahrenheit (about 0.1 degree Celsius). Humans do yawn more frequently when they're in a warm environment and when feeling anxious or stressed [source: Gallup].