It has traditionally been thought that yawning is an invol-untary reflex that draws more oxygen into our bloodstream and removes a buildup of carbon dioxide . This theory was fueled by the notion that when people are bored or tired, their breathing slows, resulting in a lack of oxygen, which causes them to yawn . However, research based on exercise sug-gests that this theory is incor-rect . In tests, it was discovered that people’s yawning rates were not altered during exercise, de-spite an increase in the breath-ing rate and levels of oxygen in the bloodstream . In addition, athletes often yawn before big events, which is unlikely to be as a result of boredom or a reduced level of breathing . It has also been found that fetuses yawn in the womb, even though they don’t breathe oxygen into their lungs until after birth .
It has been suggested that people yawn to stretch the lungs, jaw and facial muscles, which increases the heart rate and makes a person feel more awake, although this sugges-tion is largely posited on the fact that a stifled yawn that does not stretch the jaw is unsatisfying . Other theories are that yawning is used to regulate body temperature or is caused by a variation in certain chemicals, such as dopamine, in the brain . It is now accepted that the exact reasons why we yawn are unknown .
It’s also not known why yawning is contagious . One the-ory is that we have evolved to yawn when others around us do because our early ancestors used yawning to coordinate social behavior or to build rapport in a group . When one person yawned to signal something, such as it being time to sleep, the rest of the group also yawned in agreement and the members’ activities were synchronized .
Yawning might also have been used to bare the teeth to intimidate enemies, so that, when one member of the group yawned, the rest followed suit . This has carried through to modern times, when the suggestive power of yawning is still contagious . Lending weight to this theory is the fact that babies, who are unaware of social codes, don’t yawn conta-giously until they’re about one year old .