Yawn

A yawn is a reflex in vertebrate animals characterized by a long inspiratory phase with gradual mouth gaping, followed by a brief climax (or acme) with muscle stretching, and a rapid expiratory phase with muscle relaxation, which typically lasts a few seconds. For fish and birds, this is described as gradual mouth gaping, staying open for at least three seconds and subsequently a rapid closure of the mouth. Almost all vertebrate animals, including mammals, birds, reptiles, amphibians, and even fish, experience yawning. The study of yawning is called chasmology.

Yawning (oscitation) 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. In humans, yawning is often triggered by the perception that others are yawning (for example, seeing a person yawning, or talking to someone on the phone who is yawning). This is a typical example of echopraxia and positive feedback. This "contagious" yawning has also been observed in chimpanzees, dogs, cats, birds, and reptiles and can occur between members of different species. Approximately twenty psychological reasons for yawning have been proposed by scholars, but there is little agreement on the primacy of any one.

During a yawn, muscles around the airway are fully stretched, including chewing and swallowing muscles. Due to these strong repositioning muscle movements, the airway (lungs and throat) dilates to three or four times its original size. The tensor tympani muscle in the middle ear contracts, which creates a rumbling noise perceived as coming from within the head; however, the noise is due to mechanical disturbance of the hearing apparatus and is not generated by the motion of air. Yawning is sometimes accompanied, in humans and other animals, by an instinctive act of stretching several parts of the body including the arms, neck, shoulders and back. In humans it is often visible that nostrils are dilating involuntarily during yawning.

Etymology

The English yawn continues a number of Middle English forms: yanen from Old English ġānian, and yenen, yonen from Old English frequentatives ġinian, ġionian, from a Germanic root *gīn-. The Germanic root has Proto-Indo-European cognates, from a root *g̑hēi- found also with -n- suffix in Greek χαίνω ('to yawn'), and without the -n- in English gap (compare the figura etymologica in Norse ginnunga-gap), gum ('palate') and gasp (via Old Norse), Latin hiō, hiatus, and Greek chasm, chaos.

The Latin term used in medicine is oscitatio (anglicized as oscitation), from the verb oscito ('to open the mouth'). Pandiculation is the act of yawning and stretching simultaneously.

Proposed causes

There are a number of theories that attempt to explain why humans and other animals yawn.

One study states that yawning occurs when one's blood contains increased amounts of carbon dioxide and therefore becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide. Yawning may reduce oxygen intake compared to normal respiration; however, the frequency of yawning is not decreased by providing more oxygen or reducing carbon dioxide in the air.

Animals subject to predation or other dangers must be ready to physically exert themselves at any given moment. At least one study suggests that yawning, especially psychological "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 alert than when fully awake and less prepared to spring into action. "Contagious" yawning could be an instinctual signal between group members to stay alert.

Nervousness, which often indicates the perception of an impending need for action, has also been suggested as a cause. Anecdotal evidence suggests that yawning helps increase a person's alertness. Paratroopers have been noted to yawn during the moments before they exit their aircraft.

Another notion states that yawning is the body's way of controlling brain temperature. In 2007, researchers, including a professor of psychology from the SUNY Albany (US), proposed yawning may be a means to keep the brain cool. Mammalian brains operate best within a narrow temperature range. In two experiments, subjects with cold packs attached to their foreheads and subjects asked to breathe strictly nasally exhibited reduced contagious yawning when watching videos of people yawning. A similar hypothesis suggests yawning is used for regulation of body temperature. Similarly, Guttmann and Dopart (2011) found that when a subject wearing earplugs yawns, the air moving between the subject's ear and the environment causes a breeze to be heard. Guttmann and Dopart determined that a yawn causes one of three possible situations to occur: the brain cools down due to an influx or outflux of oxygen; pressure in the brain is reduced by an outflux of oxygen; or the pressure of the brain is increased by an influx of air caused by increased cranial space.

One review hypothesized that yawning's goal is to periodically stretch the muscles of the throat, which may be important for efficient vocalization, swallowing, chewing, and also keeping the airway wide.

Yawning behavior may be altered as a result of medical issues such as diabetes, stroke, or adrenal conditions. Excessive yawning is seen in immunosuppressed patients such as those with multiple sclerosis. A professor of clinical and forensic neuropsychology at Bournemouth University has demonstrated that cortisol levels rise during yawning.

Social function

With respect to a possible evolutionary advantage, yawning might be a herd instinct. Theories suggest that the yawn serves to synchronize mood in gregarious animals, similar to howling in a wolf pack. It signals fatigue among members of a group in order to synchronize sleeping patterns and periods.

Research by Garrett Norris (2013) involving monitoring the behaviour of students kept waiting in a reception area indicates a connection (supported by neuro-imaging research) between empathic ability and yawning. "We believe that contagious yawning indicates empathy. It indicates an appreciation of other peoples' behavioral and physiological state," says Norris.

The yawn reflex has long been observed to be contagious. In 1508, Erasmus wrote, "One man's yawning makes another yawn", and the French proverbialized the idea to "Un bon bâilleur en fait bâiller sept" ('One good gaper makes seven others gape'). Often, if one person yawns, this may cause another person to "empathetically" yawn. Observing another person's yawning face (especially their eyes), reading or thinking about yawning, or looking at a yawning picture can cause a person to yawn. The proximate cause for contagious yawning may lie with mirror neurons in the frontal cortex of certain vertebrates, which, upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation, which lies at the root of much human learning, such as language acquisition. Yawning may be an offshoot of the same imitative impulse.

A 2007 study found that young children with autism spectrum disorders do not increase their yawning frequency after seeing videos of other people yawning, in contrast to non-autistic children. In fact, the autistic children actually yawned less during the videos of yawning than during the control videos.