mise à jour du
9 avril 2006
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Yawning gaps
Sleepfaring: The Secrets And Science of a Good Night's Sleep
Jim Horne
Oxford University Press, USA (13 avril 2006)


There are other apparent signs of sleepiness that can be quite deceptive, even to the trained observer, with the best example being yawning. Despite it being a universal human behaviour, no one really knows much about it, other than that it usually has something to do with boredom or sleepiness. Yawning can be seen in many zoo mammals, as well as in dogs, horses, and mice, for example, when it is usually accompanied by stretching. The ostrich also seems to do a lot of it, especially when drowsily sitting on its eggs.
Look up the word in a dictionary, and apart from learning that it cornes from the old English verb 'ganien' to gape, there usually follbws only bland statements of the obvious. Medical textbooks are little better, and often the subject is conspicuous by its absence - t should lie somewhere between 'yaba virus' and 'yaws'. Astute physians and scientists lost for an explanation for a phenomenon willoften give it an impressive new name, usually derived from Latin. Thus the term once commonly adopted for yawning was 'oscitation' from Oscitare' (to open the mouth wide), and this can be found in older medical texts. It implies that science and medicine knew all about it, and one ponderous explanation, providing an effective screen to conceal ignorance is along the lines of 'a deep inspircarried out with widely opened glottis, typically with open mouth, and frequently accompanied by movements of the arms, etc. It is caused by certain psychic influences.'
Another account comes from a medical textbook by Dr H. Russel, from New York, dated 1891, who proposed that yawning was produced by 'bad air in the lungs designed by Nature as a gymnastic to awaken the respiratory organs into activity'.' Similar views prevail today that somehow yawning 'aerates' the lungs and increases the oxygen supply to the brain. However, more enlightened research has clearly shown that yawning has little to do with 'oxygenating the blood', because breathing oxygen does not suppress the urge to yawn and, conversely, raising carbon dioxide levels in the blood does not increase yawning. If anything, yawning leads not to a rise in blood oxygen levels, but a fall, because breathing usually ceases for a while after a yawn. It is also caused by stress and fear, which may account for its association with injury and severe bleeding. In the First World War, yawning was commonly seen among troops in the trenches waiting for the whistle to blow, for going 'over the top'. Heroin addicts withdrawing from the drug and going 'cold turkey', which can be a frightening experience, can yawn extensively-so can migraine sufferers before an impending migraine, which may or may not be linked to fear of the migraine attack.
The Victorians had other various theories about yawning, many of which can best be described as 'imaginative'. For example, yawning stimulates arousal by boosting blood levels of the hormone thyroxine (which raises metabolism). The yawning of the lower jaw was supposed to squeeze the thyroid gland, which is located in the neck, to release more of this hormone into the bloodstream. Certainly, yawning can cause a momentary increase in heart rate, but this is a reflex associated with any deep inspiration and is followed by a slowing on expiration.
The peak of research into yawning was in the 1920s. Notable was a Dr Carl Mayer who devoted much time to precisely measuring yawning, including taking radiographs of his wretched subjects attempting to yawn, as well as having their throats probed by laryngeal mirrors and their necks palpated. He was at pains to note that his measurements did not interfere with yawning, and declared, 'the complete yawning complex, unmodified by inhibition, was allowed to
develop'. He confidently claimed that yawning could be divided into three precisely timed and distinct phases: 'initial inspiration', taking between 1.9 and 4.3 seconds, 'acme', lasting exactly 2.3 seconds, and 'expiration' of 4.5-7.8 seconds' duration. As to why we yawn, he dismissed it with two words-'cerebral fatigue'.
Despite common knowledge that yawning has 'something psychological about it', surprisingly few psychologists have investigated it. Many psychiatrists have taken a keen interest, however, but often with some strange concepts. For example, in patients with schizophrenia, yawning has been taken as a sign of a good prognosis, supposedly showing that the patient wants to maintain contact with the real world. Some antidepressant medicines can produce frequent yawning, not because of increased sleepiness, but through direct effects on yawning control mechanisms in the brain.
One of the most endearing accounts of yawning was based on a series of experiments performed in 1941, by Dr Joseph Moore from the George Peabody College, in Tennessee. His first study employed a stooge, who was able to yawn at will and sat in a nearby public library reading room, in full view of other readers. He yawned obtrusively every ten minutes while Moore sat unobtrusively in an overlooking gallery, recording the events in his notebook. Within a minute or so of each rendition almost half of the unwitting audience would follow suit. Another of Moore's studies was more blatant, with a short movie of a girl yawning, shown to an unsuspecting audience, which was soon followed by a doubling of the incidence of yawning among the onlookers. His last study was more imaginative: can yawning be stimulated simply by hearing it rather than by seeing the yawner? Gramophone records of yawning were played to college students, with little response, but when played to blind students yawning became most apparent. Indeed, yawning is highly suggestible-try it now!

Mayer C Physiologisches und pathologisches über das Gähnen Zeitschrift für Biologie 1921;73:101-114