Everybody yawns. So do our pets. While you can suppress or fake a yawn, there's really nothing you can do to control the reflex. So, it makes sense yawning must serve some purpose, but why do we yawn?
Scientists studying this reflex have proposed several reasons for the phenomenon. In humans, yawning appears to be caused by both physiological and psychological factors.
Key Takeaways: Why Do We Yawn?
- A yawn is a reflex in response to sleepiness, stress, boredom, or seeing another person yawn.
- The process of yawning (called oscitation) involves inhaling air, stretching the jaw and eardrums, and then exhaling. Many people stretch other muscles when yawning.
- Researchers have proposed many reasons for yawning. They can be categorized as physiological reasons and psychological reasons. In either case, the underlying stimulus alters neurochemistry to elicit the response.
- Medications and medical conditions can affect the rate of yawning.
Physiological Reasons for Yawning
Physically, a yawn involves opening the mouth, inhaling air, opening the jaw, stretching the eardrums, and exhaling. It may be triggered by fatigue, boredom, stress, or seeing someone else yawn. Because it's a reflex, yawning involves an interplay of neurotransmitters associated with tiredness, appetite, tension, and emotions. These chemicals include nitric oxide, serotonin, dopamine, and glutamic acid. Scientists know certain medical conditions (e.g., multiple sclerosis, stroke, and diabetes) alter yawning frequency and the levels of cortisol in saliva following a yawn.
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 aid respiration. Fish yawn when their water lacks sufficient oxygen. Determining why humans yawn is harder to pinpoint.
Because cortisol levels increase after yawning, it may increase alertness and indicate a need for action. Psychologists Andrew Gallup and Gordon Gallup believe yawning helps improve blood flow to the brain. The premise is the 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 downward. This physical basis for yawning may explain why people yawn when they are anxious or stressed. Paratroopers yawn prior to exiting aircraft.
Gallup and Gallup's research also indicated yawning helps cool the brain, as the colder inhaled air chills the blood forced to flow during the yawn. The Gallup studies included experiments on parakeets, rats, and humans. Gallup's team found people yawn more when the temperature is cooler and yawns are more likely to have a chilling effect than when the air is hot. Budgie parakeets also yawned more in cooler temperature than hot temperatures. Rat brains cooled slightly when the animals yawned. However, critics point out that yawning seems to fail just when an organism needs it most. If yawning cools the brain, it makes sense it would function when body temperature would benefit from regulation (when it's hot).
Psychological Reasons for Yawning
To date, over 20 psychological reasons for yawning have been proposed. However, there is little agreement in the scientific community regarding which hypotheses are correct.
Yawning may serve a social function, particularly as a herd instinct. In humans and other vertebrates, yawning is contagious. Catching yawns may communicate fatigue to members of a group, helping people and other animals synchronize waking and sleeping patterns. Alternatively, it may be a survival instinct. The theory, according to Gordon Gallup, is that contagious yawning may 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 Siamese fighting fish and guinea pigs. At the other end of the spectrum, Adelie penguins yawn as part of their courtship ritual.
A study conducted by Alessia Leone and her team suggests 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 monkey called a gelada, but it's possible human yawns also vary according to their function.
Which Theories Are Correct?
It's clear 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 a minimum, yawning briefly 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 whether it still serves a psychological function today.
Sources
- Gallup, Andrew C.; Gallup (2007). "Yawning as a brain cooling mechanism: Nasal breathing and forehead cooling diminish the incidence of contagious yawning". Evolutionary Psychology. 5 (1): 92–101.
- Gupta, S; Mittal, S (2013). "Yawning and its physiological significance". International Journal of Applied & Basic Medical Research. 3 (1): 11–5. doi:10.4103/2229-516x.112230
- Madsen, Elanie E.; Persson, Tomas; Sayehli, Susan; Lenninger, Sara; Sonesson, Göran (2013). "Chimpanzees Show a Developmental Increase in Susceptibility to Contagious Yawning: A Test of the Effect of Ontogeny and Emotional Closeness on Yawn Contagion". PLoS ONE. 8 (10): e76266. doi:10.1371/journal.pone.0076266
- Provine, Robert R. (2010). "Yawning as a Stereotyped Action Pattern and Releasing Stimulus". Ethology. 72 (2): 109–22. doi:10.1111/j.1439-0310.1986.tb00611.x
- Thompson S.B.N. (2011). "Born to yawn? Cortisol linked to yawning: a new hypothesis". Medical Hypotheses. 77 (5): 861–862. doi:10.1016/j.mehy.2011.07.056
