[5][6][7] Yawning (oscitation) most often occurs in adults immediately before and after sleep, during tedious activities and as a result of its contagious quality.

[9][10] This "contagious" yawning has also been observed in chimpanzees, dogs, cats, birds, and reptiles and can occur between members of different species.

[14] Due to these strong repositioning muscle movements, the airway (lungs and throat) dilates to three or four times its original size.

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.

The Germanic root has Proto-Indo-European cognates, from a root *g̑hēi-[17] 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.

[27][28] In 2007, researchers, including a professor of psychology from the SUNY Albany (US), proposed yawning may be a means to keep the brain cool.

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.

[14] Yawning behavior may be altered as a result of medical issues such as diabetes,[31] stroke,[32] or adrenal conditions.

[34] A professor of clinical and forensic neuropsychology at Bournemouth University has demonstrated that cortisol levels rise during yawning.

[37] Theories suggest that the yawn serves to synchronize mood in gregarious animals, similar to howling in a wolf pack.

[23][41][42] 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.

[43] Mirror neurons have been proposed as a driving force for imitation, which lies at the root of much human learning, such as language acquisition.

[44] The relationship between yawn contagion and empathy is strongly supported by a 2011 behavioural study, conducted by Ivan Norscia and Elisabetta Palagi (University of Pisa, Italy).

The study revealed that—among other variables such as nationality, gender, and sensory modality—only social bonding predicted the occurrence, frequency, and latency of yawn contagion.

[45] As with other measures of empathy, the rate of contagion was found to be greatest in response to kin, then friends, then acquaintances, and lastly strangers.

[45] Strangers and acquaintances showed a longer delay in the yawn response (latency) compared to friends and kin.

This finding makes it unlikely that visual attentional biases are at the basis of the social asymmetry observed in contagious yawning.

In their small-scale, informal study they concluded that yawning is contagious,[51] although elsewhere the statistical significance of this finding has been disputed.

In certain neurological and psychiatric disorders, such as schizophrenia and autism, the patient has an impaired ability to infer the mental states of others.

[citation needed] The results of various studies have showed a diminished susceptibility to contagious yawn compared to the control group of typically developing children.

[citation needed] Similarly, patients with neurological and psychiatric conditions, such as schizophrenia, have shown an impaired ability to empathize with others.

[citation needed] The Canadian psychiatrist Heinz Lehmann claimed that increases in yawning could predict recovery in schizophrenia.

[60] Empathy is a notoriously difficult trait to measure, and the literature on the subject is confused, with the same species sometimes displaying a connection between contagious yawning and social closeness, and sometimes apparently not.

Charles Darwin's book, The Expression of the Emotions in Man and Animals, mentions that baboons yawn to threaten their enemies, possibly by displaying large canine teeth.

Dogs are very adept at reading human communication actions, so it is unclear if this phenomenon is rooted in evolutionary history or a result of domestication.

[70] Exorcists believe yawning can indicate that a demon or possessive spirit is leaving its human host during the course of an exorcism.

For example, in his commentary on Al-Bukhari's hadith collection, Ibn Hajar, an Islamic theologian, mentions that yawning, in addition to its risks of letting demons enter or take hold of one's body, is unbefitting for humans as it makes them look and sound like dogs by crooking men's upright posture and making them howl: He [Prophet Muhammad] likened excessive yawning to the howling of dogs to deter people from it and to make it seem repulsive, for when a dog howls, it raises its head and opens its mouth widely, and the yawner resembles it when he yawns too much.