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2 septembre 2002
 Physiology and Behavior 1991;50(5):1067-1069
 Effects of yawning and related activities on skin conductances and hearte rate
Monica Greco and Ronald Baenninger
Department of psychology and biology, Temple University,
Philadelphia Pennsylvania


Until recently, the function of yawning has been the subject of very little empirical research, but there has been no dearth of speculation. The most widely held hypothesis is that yawning serves as a form of respiration induced by high levels of carbon dioxide in the blood. Barbizet surveyed the existing literature on yawning and found no evidence for this view. The fact that fish yawn also makes the respiration hypothesis less tenable. Lehman pointed out that yawning is unlikely to increase oxygen levels because the deep inspiration is normally followed by a period of apnea.

The strongest evidence against a simple respiration hypothesis was provided by Provine, Tate and Geldmacher who systematically varied carbon dioxide levels in compressed air that human subjects were breathing. They found that increasing the percentage of carbon dioxide did not cause the subjects, to yawn more, but did increase their rate of breathing; when their subjects breathed 100% oxygen their rate of yawning did not change.

We hypothesized that yawning is likely in situations where arousal is low, but in which there is some reason to remain awake and alert. The function of yawning would be to increase arousal when the environment provides inadequate stimulation, but where the consequences of low arousal could be hazardous. One implication of our hypothesis is that organisms in unstimulating or boring environments should yawn in anticipation of important events, a prediction previously confirmed.

When human subjects are alone they are accurate in reporting their own yawns, and their rate of yawning is dramatically higher than in the presence of an observer. In the present study, we used self-reported yawns to examine effects of yawning on galvanic skin response (skin conductance) and heart rate, two widely-used correlates of arousal.


Subjects : A total of 50 male and female undergraduate psychology students served as subjects.

Apparatus : Data were recorded using HRM Biofeedback Microlab software with appropriately attached sensors for skin conductance (SC), electromyogram (EMG), and heart rate (HR). These measures were portrayed before, during. and after each yawn.

Procedure : Subjects in the Yawn group participated in three separate trials, each of which lasted 15 minutes. In each trial, the subject was seated alone in a room and recorded his or her own yawns on an Esterline Angus recorder while GSR, HR, or EMG was recorded (in counterbalanced order) for 15 min by equipment in an adjacent room. Electromyogram was recorded using electrodes on the subject's masseter muscle and was used to verify that they \vere actually yawning when they so indicated, and that movement artefacts were not producing any spurious changes; skin conductance was recorded by electrodes on the fingertips, and heart rate was recorded using a gel-free sensor clipped to the ear.

Subjects were told that they were participating in a study concerning the physiological correlates of yawning. They were instructed to relax, to think about yawning, and to depress the button in front of them when they began to yawn, keeping it depressed until the yawn was finished. After a 3-min baseline reading was taken, subjects recorded their yawns for 12 min for each trial.

Subjects in the Control group each participated in two 15-min trials. In one trial they were instructed to take deep breaths and in the other trial they were instructed to open their mouths wide while breathing normally through the nose. Skin conductance and heart rate were recorded in different trial segments.


Experimental Group Data : Of the 30 subjects in the Yawn group, 24 emitted at least one yawn for a total of 418 yawns, a rate of 18.58 yawns/person-hour, considerably higher than most rates reported by Baenninger. The mean number of yawns emitted by a subject in a 15-min trial was 4.59, with a mean duration of 6.53 seconds. The skin conductance measure increased in all 21 subjects who yawned at least once. A Friedman ANOVA performed on the first yawn emitted by subjects showed a significant difference between the mean pre-, during, and postyawn skin conductance values (Fr= 14.90, p<0.01). Post hoc analysis indicated that the mean during and postyawn conductance values were significantly higher than the mean preyawn values. This pattern was consistent for the conductance data from the second yawns as well (Fr= 11.54, p<0.01). By the third yawns, only the postyawn conductance was higher than the preyawn value (Fr=7.09, p<0.05), and by the last yawn given by subjects, the three conductance values were not significantly different from each other. But there was a clear increase in the summed conductance values between the first and last yawns (sign test, p<0.01), indicating that successive yawns were accompanied by increased arousal. Thus the pattern of skin changes both within and between yawns showed some changes over successive yawns.

During the 15-min heart rate (HR) trial, 20 subjects yawned at least once, A Friedman ANOVA performed on their first yawns indicated that there were no significant changes in heart rate during the course of a yawn. This lack of effect was also true for the second, third, and fourth yawns emitted during the HR trial; no effects became apparent over successive yawns, from first to last yawns.

During the electromyogram (EMG) trial, 22 subjects emitted at least one yawn. The EMG values during and after yawns were significantly higher than the mean preyawn EMG value (Fr=33.09, p<0.01).

Control Data : In both the first and second mouth opening, the mean conductance values during and after were higher than the conductance value before opening the mouth, indicating an increase in arousal associated with opening the mouth (F,=9.10, p<0.05; F,=9.79, p<0.01, respectively). No differences were apparent by the third or final mouth opening.

Heart rate control data were also consistent with this pattern. There was a significant difference between the mean pre-, during. and postopen mouth values during the HR trial, but only for the first mouth opening (F,=9.70, p<0.01). Post hoc analysis indicated a decrease in HR as a result of wide opening of the mouth.

A Friedman ANOVA performed on the mean pre-, during and postbreathe skin conductance values indicated significant differences for all three deep breaths performed (F,=22.30, p<0.0 1; Fr = 12.48, p<0.0 1; F, = 13.58, p<0.0 1, respectively). Post hoc analysis showed that the postbreathing conductance values were increased. The HR values for the first deep breath performed also showed a significant difference between the three values (F,=9.10. p<0.05). Post hoc analysis revealed that the HR values belote and during a deep breath were higher than the postbreath HR value. There were no significant differences of these values for the second. or final deep breaths performed. The overall skin conductance and HR associated with the first mouth opening and deep breath did not differ froin the t%vo subsequent mouth openings and deep breaths.

Discussion : Some progress is being made in understanding hormonal and neurotransmitter mechanisms of yawning. The present study indicates that both yawning and two of its major components (opening the mouth and taking a deep breath) initially increased skin conductance, partially supporting our arousal hypothesis. Successive yawns accompanied progressively increased skin conductance only, and had only a slight cumulative effect on HR. Opening the mouth or taking a deep breath also was followed by skin conductance and HR changes consistent with increased arousal. Thus it appears that these two components of yawning might increase arousal at least as much as yawning itself.