Everyone yawns. Even our pets. Scientists studying this reflex have proposed several reasons for the phenomenon. In humans, yawning seems to be caused by both physiological and psychological factors.
Physically, yawning involves opening the mouth, inhaling air, opening the jaw, stretching the eardrums, and exhaling. It can be triggered by fatigue, boredom, stress or seeing someone yawn.
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As a reflex, yawning involves an interaction of neurotransmitters associated with tiredness, appetite, tension and emotions. These chemicals include nitric oxide, serotonin, dopamine and glutamic acid.
Scientists know that certain medical conditions (e.g. multiple sclerosis, stroke cerebral palsy and diabetes) alter yawning frequency and cortisol levels in saliva after yawning.
Because yawning is a matter of neurochemistry, there are several possible reasons it can happen. In animals, some of these reasons are easily understood. For example, snakes yawn to realign their jaws after eating and to help with breathing.
Fish yawn when their water doesn't have enough oxygen. Determining why humans yawn is more difficult to pinpoint. As cortisol levels increase after yawning, this can increase alertness and signal the need for action.
Psychologists Andrew Gallup and Gordon Gallup believe that yawning helps improve blood flow to the brain. The premise is that stretching the jaw increases blood flow to the face, head, and neck, while the deep breath of a yawn forces blood and spinal fluid to flow to low. This physical basis for yawning may explain why people yawn when they are anxious or stressed. Paratroopers yawn before exiting the aircraft.
Gallup research also indicated that yawning helps to cool the brain, while cooler, inhaled air cools the blood forced to flow during yawning. Gallup's studies included experiments with parakeets, rats, and humans.
The team found that people yawn more when the temperature is lower. Rat brains cooled slightly when the animals yawned.
To date, more than 20 psychological reasons for yawning have been proposed. However, there is little agreement in the scientific community on which hypotheses are correct.
Yawning can serve a social function, particularly as a herd instinct. In humans and other vertebrates, yawning is contagious. Yawning in captivity can communicate fatigue to members of a group, helping people and other animals synchronize wakefulness and sleep patterns.
Alternatively, it could be a survival instinct. The theory, according to Gordon Gallup, is that contagious yawning can help members of a group become more alert so they can detect and defend against attackers or predators.
In his book The Expression of the Emotions in Man and Animals, Charles Darwin observed baboons yawning to threaten enemies. Similar behavior has been reported in fish and guinea pigs. At the other end of the spectrum, Adelie penguins yawn as part of their courtship ritual.
A study by Alessia Leone and her team suggests that there are different types of yawns to convey different information (e.g. empathy or anxiety) in a social context. Leone's research involved a type of ape called a Gelada, but it's possible that human yawns also vary according to their function.
It is clear that yawning is caused by physiological factors. Fluctuations in neurotransmitter levels trigger a yawn. The biological benefits of yawning are clear in some other species, but not so obvious in humans.
At the very least, yawning quickly increases alertness. In animals, the social aspect of yawning is well documented. While yawning is contagious in humans, researchers have yet to determine whether the psychology of yawning is a leftover from human evolution or if it still serves a psychological function today.
