the science behind why we yawn

Yawning: The Science Behind an Ancient Reflex

Yawning is one of the most familiar yet least understood behaviors in the animal kingdom. A yawn is a reflex in vertebrate animals characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax with muscle stretching, and a rapid expiratory phase with muscle relaxation, which typically lasts a few seconds. The study of yawning is called chasmology. Despite its universality, scientists have yet to reach a firm consensus on its primary purpose.

A Universal and Ancient Behavior

Almost all vertebrate animals, including mammals, birds, reptiles, amphibians, and even fish, experience yawning. In humans, the behavior begins remarkably early in development. A fetus starts yawning in the uterus about 12 weeks after conception, and a fetus yawns around 25 times per day, with the frequency of yawning tending to decrease with age. At the beginning of the third month, the embryo becomes a fetus with the occurrence of the first oral and pharyngeal motor sequences under the control of the neurological development of the brainstem. In adult humans, self-report studies indicate that people yawn between six and 23 times per day, depending upon an individual's circadian rhythm or chronotype.

The Physiology of a Yawn

Yawning most often occurs in adults immediately before and after sleep, during tedious activities, and as a result of its contagious quality. It is commonly associated with tiredness, stress, sleepiness, boredom, or even hunger. On a mechanical level, during a yawn, muscles around the airway are fully stretched, including chewing and swallowing muscles, and the airway dilates to three or four times its original size. Yawning is a complex neurological act, mainly orchestrated in the brainstem near the basic life centers for breathing, swallowing, mastication, and coughing.

Scientific Theories on Why We Yawn

Approximately twenty psychological reasons for yawning have been proposed by scholars, but there is little agreement on the primacy of any one. The leading hypotheses include:

Oxygen and Carbon Dioxide Regulation: One study states that yawning occurs when blood contains increased amounts of carbon dioxide and therefore becomes in need of the influx of oxygen that a yawn can provide. However, this theory has fallen out of favor: the frequency of yawning is not decreased by providing more oxygen or reducing carbon dioxide in the air.
Brain Thermoregulation: Another notion states that yawning is the body's way of controlling brain temperature. In 2007, researchers including a professor of psychology from SUNY Albany proposed yawning may be a means to keep the brain cool, since mammalian brains operate best within a narrow temperature range. Supporting this, subjects with cold packs on their foreheads yawned significantly less in controlled experiments.
Alertness and Group Signaling: At least one study suggests that yawning, especially contagious yawning, may have developed as a way of keeping a group of animals alert. If an animal is drowsy or bored, it will be less prepared to spring into action, and contagious yawning could be an instinctual signal between group members to stay alert. Anecdotally, paratroopers have been noted to yawn in the moments before they exit their aircraft.

Contagious Yawning and Empathy

In humans, yawning is often triggered by the perception that others are yawning — seeing a person yawn, talking to someone on the phone who is yawning, or even reading about yawning. This contagious yawning has also been observed in chimpanzees, dogs, cats, birds, and reptiles, and can occur between members of different species. In humans and certain social animals, contagious yawning is triggered by seeing or hearing another individual yawn, and it can be suppressed only with difficulty. The evolutionary value of collective yawning remains unknown, but may have something to do with the desire of an animal to protect the members of its tribe. Researchers have proposed that contagious yawning is linked to empathy and social bonding, driven by mirror neuron systems in the brain.

Excessive Yawning and Medical Significance

Healthcare providers consider excessive yawning as more than three yawns per 15 minutes several times a day. Excessive yawning can be a symptom of conditions such as narcolepsy, sleep deprivation, obstructive sleep apnea, opioid withdrawal syndrome, and certain medications like SSRIs. Many neurological conditions are also linked to excessive yawning, including amyotrophic lateral sclerosis (ALS), head trauma, migraine headaches, and multiple sclerosis. Yawning is a stereotyped physiological behavior that can represent a sign or symptom of several conditions, such as stroke, parkinsonism, Parkinson's disease, and epilepsy. Opioid peptides are known to inhibit yawning, and some psychotic disorders may be related to decreased yawning.

Conclusion

Yawning remains one of biology's most intriguing open questions. While it is clear that yawning is an ancient, evolutionarily conserved reflex present across virtually all vertebrates and even in fetuses, its precise adaptive function — whether to cool the brain, signal alertness, regulate breathing, or strengthen social bonds — continues to be debated by researchers worldwide.