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Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal

mise à jour du
25 septembre 2010
Sleep Med
Sleepiness is not always perceived before falling asleep
in healthy, sleep-deprived subjects
Herrmann US, Hess CW, Guggisberg AG
Roth C, Gugger M, Mathis J
Department of Neurology, Inselspital, Bern University Hospital
and University of Bern, Switzerland.


OBJECTIVE: To test whether subjects spontaneously signal sleepiness before falling asleep under monotonous conditions.
METHODS: Twenty-eight healthy students were deprived of sleep for one night and then underwent a "maintenance-of-wakefulness test" (MWT) consisting of four 40-min trials. They were told to give a signal as soon as they felt sleepy and to try to stay awake as long as possible. In a first series of tests, the subjects were given no reward (nr); in a second series, monetary rewards (wr) were given both for an accurate perception of sleepiness and for staying awake longer.
RESULTS: Seventeen of the 28 subjects (60.7%) did not signal sleepiness before a sleep fragment occurred in at least one of the four MWT trials. Women were more reliably aware of sleepiness than men in the nr trials (p=.02), while the men's performance improved in the wr trials (p<.02), becoming equivalent to the women's performance.
CONCLUSIONS: Our results cast doubt on the general assumption that one cannot fall asleep without feeling sleepy first. If similar results can be obtained in monotonous driving or working situations, this will imply that accidents caused by sleepiness or by falling asleep cannot necessarily be attributed to an individual's negligence.

1. Introduction
Work accidents and motor-vehicle accidents (MVAs) are often caused by sleepiness or by falling asleep. Drivers involved in MVAs of this type usually deny having fallen asleep at the wheel or having felt sleepy before the crash. These subjective, retrospective judgments are generally considered false and attributed to the faulty recollection of sleepiness, rather than to the true absence of sleepiness or of feeling sleepy, J.Maiiy authors suggest'thatit' is nearly impossible to fall asleep without being aware of sleepiness beforehand, because certain physiological and cognitiv&indicators of sleepiness, such as straining, drooping eyelids, yawning, and wandering thoughts invariably precede the onset of sleep. Others, however, argue that sleepiness impairs driving performance in part by lessening the driver's insight into performance deficits, even though the warning signs of sleepiness are, in fact, perceived. A driver might also underestimate the grave risk of sleepiness at the wheel because of previous episodes of nodding off for a few seconds without incident. Driving while drowsy and transiently falling asleep at the wheel are known to happen far more often than sleepiness-rela crashes.
Driving-simulator studies ha repeatedly shown that drivers who fall asleep at the wheel and drift out of their driving lane do, in fact, perceive their momentary level of sleepiness fairly well, although they may underestimate the likelihood that they will fall asleep. Other studies, however, have found only a weak or moderately strong correlation between subjective sleepiness ("sleep complaints") and objective measures such as microsleep. We suggest that these conflicting results are due to differences in the experimental task conditions. In most studies, the researchers asked drivers about their subjective sleepiness before or after testing or else at regular intervals during the test. This differs from the actual situation before an MVA, in which the driver can become aware of his or her own sleepiness only by spontaneous introspection. Repetitive, externally timed questioning can itself provide a sleepy person with continuous feedback information about his or her alertness and thereby increase the accuracy of subjective judgments of sleepiness. Repetitive questioning also directly makes the subject more alert. Moreover, many studies that compared subjective sleepiness with objective measures were performed in driving simulators. We suggest that interaction with the environment, by providing continuous feedback on performance errors, helps the subject become aware that sleep is imminent.
The main goal of this study is to determine whether healthy subjects can fall asleep without spontaneously signaling sleepiness (SPS) before a sleep fragment lasting longer than 3 s when the trials are carried out under very monotonous conditions, with no interaction with the environment or feedback information on performance allowed. The subjects' later recollection of sleepiness played no role in this study; subjects were precisely instructed how to signal their imminent subjective sleepiness just oqe before the start of each trial of the maintenance-of-wakefulness test (MWT). They could not interact with the experimenters during the trial, and their perception of sleepiness was thus based purely on spontaneous introspection.
These MWT conditions are admittedly more passive than an average driving situation, and it is thus unclear to what extent MWT results can be extrapolated to driving situations. Subjects might fall asleep more easily under the MWT conditions, but in real-life situations the risk of falling asleep might be higher in general because of the longer exposure times. MWT is widely available in clinical sleep laboratories and is generally accepted as the gold standard for measuring maintenance-of-wakefulness. It thus seems to be a reasonable compromise solution to the problem of assessing both the subjects' ability to stay awake and their subjective perception of sleepiness (SPS).
As the risk of an accident in a real-life driving or working situation motivates people to behave responsibly, it would seem desirable to have a penalty for bad performance in the experimental setting as well. Because punishing experimental volunteers is not feasible, we introduced, in a second series, a monetary reward for goal achievement to improve the subjects' motivation.
We recruited men and women in roughly equal numbers to test whether women might be better than men at recognizing their sleepiness, which, if so, might explain their lower risk of accidents at work or behind the wheel.
4. Discussion
The main finding of this study is that sleep-deprived healthy young subjects often did not spontaneously signal sleepiness (SPS) before falling asleep under the monotonous MWT conditions. This late, rather than appropriate, SPS was seen in 37.5% of all trials when defined as the absence of signaling before any sleep fragment lasting longer than 3 s. The risk of late SPS was highest in subjects who suffered the greatest daytime sleepiness from sleep deprivation, which at first glance seems paradoxical. The finding makes sense, however, if we assume that spontaneous introspection and reporting of sleepiness are themselves impaired by progressing sleepiness. Hardly any late SPS was found with longer sleep latencies of 20 min or more. This corroborates the commonly used criterion of a mean sleep latency of 15-20 min or longer in the MWT for the ability to drive.
Our results differ from those of earlier studies, in which subjective sleepiness was found to be closely correlated with objective measures of sleep pressure or impaired performance. We argue that these differences can be explained largely by differences in the experimental methods used.
First, many previous studies. employed the sleep-onset criteria of Rechtschaffen and Kales, in which episodes of microsleep shorter than 15 s were disregarded. If we had used this criterion for overt sleep (OS) and disregarded all sleep fragments shorter than 15 s, we would have observed late, rather than appropriate, SPS in only 10.7% of all trials (12.5% of nr trials and 9.4% of wr trials). It is clear, however, that in demanding tasks requiring a high degree of wakefulness, such as surveillance tasks or driving, sleep fragments as short as 3 s or less could have disastrous consequences, and microsleep is known to be much more common than overt sleep during daylight hours.
Second, unlike in previous studies, the subjects in this study were not repeatedly questioned during the trials. Instead, they were instructed only at the beginning of the MWT trials to signal imminent SPS. Thus, no abrupt changes of alertness were induced by external interaction, and sleepiness was perceived purely by spontaneous introspection.
Third, in contrast to the situation in experiments on driving simulators or other performance tasks, our subjects had no feedback information on their performance.
We hypothesize that our subjects might have assessed their own sleepiness more accurately than they did in our SPS protocol if they had been allowed to interact continuously with examiners and if they had received feedback information about their performance.
Kaplan et al. also found that predictions about falling asleep were particularly poor for the very first sleep fragment, and they also suggested that their subjects misjudged the relevance of their sleepiness indicators.
In our study, sleepiness was not signaled at all during the full 40 min in only 7.1% of trials: according to the KSS obtained after each trial, sleepiness was undetected in only 2%. Thus, our subjects signaled sleepiness in almost all MWT trials,, but they often did so a few seconds up to a few minutes after a short sleep fragment of greater than 3 s duration. Thus, our results do not contradict the trivial fact that one is sleepy afthr a night of sleep deprivation: Rather, they suggest that the timing of the subjective judgment of sleepiness is not precise in the dynamic situation of a rapid fluctuating alertness level on a second-by-second basis.
To our knowledge, this is the first systematic study of the awareness of spontaneously perceived sleepiness under MWT conditions. It challenges the doctrine that one cannot fall asleep without prior perception of sleepiness, at least under monotonous MWT conditions without external interaction and without feedback about performance errors.
These results cannot automatically be generalized to everyday situations, to other age groups, or to sleepy persons suffering from a clinical disease. If, however, future studies reveal that similar results are generally applicable in a wide variety of driving or working situations, there could be major consequences both with respect to the forensic aspects of sleepiness-induced motor-vehicle or work accidents and with respect to the instructions that physicians should give to their sleepy patients. Until now, from a legal point of view, sleepiness-induced work or motor-vehicle accidents have been attributed to reckless behavior and punished accordingly, because it was assumed that these drivers continued to drive despite having perceived sleepiness. Our results, however, imply that this approach might be questionable under certain conditions such as absence of external interaction and feedback information of performance errors.
The main limitation of this exploratory study is the small number of subjects, all of whom were young. Further studies should be performed in older subjects, in persons who have undergone less severe sleep deprivation, and in sleepy persons with clinical diseases. The results in any of these groups might differ from the ones obtained here. Clinical experience suggests that persons who are chronically sleepy because of illness often underestimate their sleepiness.
Moreover, our results only permit limited conclusions as to why the subjects did not signal their sleepiness before the first sleep fragment, even though video recordings of the face regularly showed obvious indicators of sleepiness, such as yawning, straining, and drooping eyelids. Our interpretation (that the subjects did not, in fact, perceive sleepiness before falling asleep) was supported by the subjects' statements at the end of the test day. Only one subject explained her failure to appropriately signal sleepiness by saying that she had waited to become even sleepier in order to be more certain about the reason for signaling. A learning effect was not observed, as the frequency of late SPS was independent of the trial number over the course of the test day.
The sex difference found here requires cautious interpretation in view of the small number of subjects. An earlier study also showed that women perceive sleepiness better than men. Theoretically, this might explain why young men are more commonly involved than young women in sleepiness-induced accidents at work and at the wheel.
Further studies under different conditions, including surveillance tasks or driving simulators, are needed to elucidate the role of subjectively perceived sleepiness in the causation of accidents. We also propose including and assessment of SPS-and, even more importantly, of micros leep-in the routine MWT, in order to gain a better understanding of the impact of late SPS in patients at risk of work or motor-vehicle accidents, despite a normal MWT latency.
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