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20 févier 2002
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R. Provine
Yawning: Relation to Sleeping and Stretching in Humans
Robert R Provine Heidi B Bamernik & Barbara C Curchack
Department of Psychology University of Maryland Baltimore County Baltimore, Maryland 21228
Abstract : The temporal relations among yawning, sleeping, and stretching were investigated by having undergraduate college students record the occurrences of these behaviors during a one-week period. Yawning was most frequent during the hour before sleeping and after waking. In contrast, stretching was common only during the hour after waking. Concurrent yawning and stretching were common but stretching was accompanied more often by yawning than vice-versa. The temporal proximity of yawning to sleep and waking times is probably the basis for yawning as a paralinguistic signal for drowsiness. The role of contagious yawning in synchronizing the behavior and physiological state of a group is considered.

Introduction : Yawning is a common and probably universal human behavior that is performed throughout life (GESELL 1928). It is characterized by gaping of the mouth accompanied by a long inspiration followed by a shorter expiration. Yawning is a stereotyped behavior that is triggered by yet undefined physiological states or released by witnessing yawns (PROVINE 1986). Even yawn-related stimuli such as thinking about, or reading about yawns evoke yawns (PROVINE 1986). Yawning is of clinical importance because it is symptomatic of pathology including brain lesions and turnors, hemorrhage, motion sickness, chorea, and encephalitis (BARBIZET 1958; GRAYBIEL & KNEPTON 1976; HEUSNER 1946; JURKO1975). In addition, it is therapeutic in preventing post-operative respiratory complications (BARTLETT 1971; CAHILL 1978) and in adjusting the air pressure in the middle ear (LASKIEWICZ 1953). Despite the promirience of yawning in everyday behavior, yawning as recelved little attention in the scientific literature. There is much popular opinion but little empirical evidence about when we yawn, why we yawn, what function yawning may serve, and what environmental circumstances modulate yawning rate.

Yawning is commonly associated with drowsiness, boredom and low levels of arousal. Humans, for example, yawn when participating in lengthy, uninteresting or repetitive tasks (KATAOKA 1975; KisHiDA 1973; SAKAI & TAKAHASHI 1975), and yawn more when observing uninteresting Chan interesting stimuli (PROVINE & HAIMERNIK 1986). Beliefs about the relation between sleepiness and yawning are based upon folk wisdom and everyday observations to which science has had littie to add. Texts on sleep occasionally mention the prominence of yawning in drowsy people but typically cite only unrelated or general references on yawning such as BARBIZET (1958) or HEUSNER (1946). Stretches often accompany yawns in humans and other animals. Neurological evidence for a correlation between yawning and stretching comes from case reports of human hemiplegics who are unable to separate the behaviors. Such individuals often perform associated stretching movements of otherwise paralyzed body parts during yawns (WALSHE 1923). A pharmacological basis for a yawn-stretch relationship comes from the observation that drugs chat produce yawning also produce stretching in a variety of animais (DOURISH & COOPER 1987; GESSA CC al. 1967; YAMADA & FURUKAWA 1980).

The present report explored the relation between yawning, stretching, and sleeping. Particular attention was devoted to the concurrence of yawning and stretching and the correlation of both behaviors to bed and waking times. The study of humans permitted the use of a self-report procedure that is unusual in ethological rescarch. Each subject recorded his or her yawning, stretching, and sleeping times in log-books during one week. This procedure enabled a total of over one subject-year of data to be collected from 64 participants engaged in normal life activities. An alternative procedure, the direct 24-h per day observation of each subject would have required the long-term committrient of a team of observers whose presence may have biased the results; the social sanctions against yawning in the presence of others are well known. The self-report procedure has other advantages. Outside observers but not the behaving subjects have a problem in discriminating between the subject's own yawns, sighs, stretches, and ambiguous or shielded movements. The test-retest reliability of self-reported yawn frequency and duration in a laboratory setting is already documented (PROVINE 1986). To decrease the obtrusiveness of the record keeping procedure, subjects recorded yawns or stretches when they occurred instead of logging behavior every hour. This was also a precaution against yawns being cued hourly by the act of record keeping; even the thought of yawning is sufficient to trigger yawns (PROVINE 1986). To minimize possible individual differences in the accuracy of subject's record keeping and to furcher reduce the possible effects of cue-induced yawning, the results compiled from the log-books were reported as the probability, not the frequency, of yawning or stretching petrhourly interval.


Discussion : The present research confirms the folk wisdom that yawning is most frequent shortly before bedtime and after waking. Stretching, in sharp contrast to yawning, is frequent only shortly after waking. There are, thus, two peak periods per day for the performance of yawning, but only one for stretching. Only the morning peak is shared by yawning and stretching. The temporal patterning data suggest a possible basis for the emergence of the yawn, but not the stretch, as a gesture or paralinguistic signal for drowsiness. Yawns are related to the drowsiness experienced both before and after waking while stretching corresponds only to that after waking. The gestural significance of stretching, if any, is less obvious than that of yawning. However, the frequent stretching immediately after waking and the common practice of stretching before physical activicy is consistent with stretching as a signal for the preparation for action.

The neurological and pharmacological studies cited earlier suggest similar or shared mechanisms for yawning and stretching. The prescrit report of frequent, concurrent yawning and stretching and a similar peak time of occurrence during the hour after waking also indicate a relation. However, there is aiso evidence for at least partial autonomy: Whereas 47 % of stretches were accompanied by a yawn, only 11 % of yawns were accompanied by a stretch. Also, stretching shared the post-waking but not the pre-sleeping peak of yawn activity. The above evidence suggests that human yawning is not simply the facial manifestation of a generalized stretch response. Stretching after waking may be a useful warm-up for a person arousing from the immobility of sleep. Powerful stretching movements have major, widespread physiological consequences, including increases in the puise rate, blood pressure, and blood flow to the muscles and other body parts, and increases in the flexibility of the muscles and l'oints (McARDLF et al. 1981; SHEPHARD 1982).

The physiological consequences of yawning are largely unknown. Yawning shortly before sleeping and after waking may be either a maneuver to increase alertness or brain function of a drowsy organism or to depress alertness and hasten or otherwise prepare us for sleep. Only a few hypotheses about yawn function have been evaluated. No support has been found for the popular assumptions that yawning is either a response to, or somehow adjusts blood levels Of C02 or 02 (PROVINE et al. 1986, 1987). Yawning rate is neither facilitated nor depressed by breathing gases with elevated levelS of C02 or 02, yawning is also unaffected by vigorous exercise (PROVINE et al. 1986, 1987). In addition, the absence of a significant correlation between yawn duration and inter-yawn interval indicates that infrequent yawners do not cornpensate by performing longer than normal yawns and vice versa (PROVINE 1986).

Further evidence against the yawn as mainly a respiratory act comes from tests of the fixedness of the routes of inhalation and exhalation during yawning (PROVINE 1986; PROVINE et al. 1986, 1987). During a yawn, inhalation and exhalation occur primarily through the mouth and this basic pattern is highly inflexible. It is very difficult, if not impossible, for most people to perform a satisfying yawn with their lips taped shut even though they are free to respire through their nose. Yawning does not have the flexibility of normal breathing which can bc accomplished with equal facilicy through nose or mouth. However, oral inhalation by itself is insufficient to produce a satisfactory yawn if the jaws are immobilized. Subjects attempting to yawn with clenched teeth often report the unpleasant sensation of being stuck in mid-yawn and being unable to perform a satisfying yawn although they could inhale and exhale through their teeth. Perhaps the gaping of the jaws during yawning produces a vigorous facial stretch thar increases cerebral blood flow or alters some other intracranial activitv.

One of the most striking and curious features of yawning is the tendency to yawn after visually observing a yawn (PROVINE 1986). Even reading or thinking about yawning evokes yawns (PROVINE 1986). The range and potency of yawnproducing sensory stimuli contribute to the spread of yawning through a group. The present study indicated that yawns, and therefore the intrinsic and/or extrinsic stimuli that evoked them, were most common shortly before bedtime and after waking. The yawning that evolved as a response to the internal body state at such times has acquired a secondary signalling function that can synchronize the behavior of a group. The yawning of an individuail rnay trigger a chain reaction of yawning in witnesses. The resulting communal yawning may signal bedtime for a group as it did for the Bakairi of Central Brazil in the presence of their first European visitor. "If they seemed to have had enough of ail the talk, they began to yawn unabashedly and without placing their hands before their mouths. That the pleasant reflex was contagious could not be denied. One after the other got up and left until I remained with my dujour (STEINEN, cited in EIBL-EIBESFELDT 1975, p. 163). If yawning affects the physiology of the yawner, the contagion of yawning may synchronize the physiological as well as the behavioral state of a group.