Sleep,5:S67-S72
© 1982 Raven Press, New York
The Multiple Sleep Latency Test: What
Does It Measure?
Mary A. Carskadon and William C. Dement
Stanford University Sleep Research Center, Stanford, California
In his 1961 review, Kamiya (1) described four types of measures that could be
used to assess drowsiness. Three of these categories had been (and stilI are) used
by many investigators to examine sleepiness and its concomitants. These
categories included measures of reduced activity, attenuation of "best effort" on
tests of performance, and introspective reports of sleepiness on subjective rating
scales. The fourth type of sleepiness measure, readiness to faIl asleep, was described by Kamiya in the following way:
Certainly the drowsier the person is, the more rapidly he should fall asleep under favorable conditions. This idea of drowsiness is close to its everyday meaning, and the method
to be used in applying it is self-evident, assuming some previously established criterion of
sleep onset. One of its difficulties as an assessment procedure, at least at the human
level, is that subjects must be made comfortable and be thoroughly familiarized with the
test setting. Moreover, the method obviously is not well suited to continuous or frequent
determinations of drowsiness. It has some utility as a measure of that drowsiness which
just precedes normal sleep at the usual hours, but otherwise it is not very practical.
The Multiple Sleep Latency Test (MSLT), despite Kamiya's pessimistic appraisal, falls within this fourth category.
Development of the MSLT
The MSLT evolved as a necessity from the bourgeoning clinical interest of the
early 1970s. During the years that saw the beginnings of a formalized approach to
sleep disorders medicine, it became evident that a measure was needed to
characterize the complaint of sleepiness in an effective manner. Previous measures included many from the first three categories outlined above, but these were
generally idiosyncratic measures that varied among sleep centers and for which
few norms had been established. None stood apart as a measure that clinicians
could readily identify and utilize.
At about this time, Eric Hoddes at Stanford developed the Stanford Sleepiness
Scale (SSS) in direct response to the clinical problem (2-4). In addition, the
Stanford group, as well as others, used single daytime nap recordings (5-8) and
24-h monitoring (9-11) in an attempt to characterize the sleepy patient. Each of
Address correspondence and reprint requests to Mary A. Carskadon, Ph.D., Sleep Research Center,
Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, California 94305.
Key Words: Sleep tendency-Sleepiness.
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M. A. CARSKADON AND W. C. DEMENT
these approaches was successful to a degree, but each had problems that made it
less than satisfactory as a useful clinical tool. As we have noted elsewhere, for
example, the SSS is inexpensive and easy to use, but its reliability in chronically
sleepy patients is questionable (12). Twenty-four-hour monitoring is certainly quite
effective, but very laborious and expensive. The chief difficulty of a single daytime nap recording, which had been used primarily in patients with narcolepsy,
arises from the relative "hit-or-miss" nature of this approach.
Nonetheless, introduction of the SSS reduced the urgency of the clinical problem, and our focus shifted to experimental studies. A crucial link in the development of the MSLT was the examination of sleep and wakefulness on a 90-min
schedule (13-15). In these studies, subjects were given 16 daily opportunities to
sleep for 30 min. Each of these "nights" was preceded and followed by the SSS,
so that the introspective measure was intimately associated with brief episodes of
sleep. The repeated association of SSS ratings and these naps, along with the
predictability of one from the other (15), led us to conclude that repeated determinations of sleep latency throughout the day might provide an objective, reproducible measure of daytime sleepiness.
Our first study concerning this hypothesis was performed in the spring of 1976,
when we attempted to validate the measure in a traditional sleep loss paradigm
(16,17). In this study, the measure achieved essentially its final form (as it is used
experimentally). The testing format was devised as much in response to practical
contingencies as to any theoretical considerations. Thus, sleep latency was measured at 2-h intervals across the day because we wished to provide time for
performance testing; each test was limited to 20 min, because we did not want
subjects to become bored if they could not fall asleep; and the amount of sleep was
limited to the 1st min of stage 1, so that subjects would not accumulate significant
amounts of sleep during the sleep deprivation procedure.
The initial success of the MSLT in this study and our growing concern about the
reliability of the SSS in patient groups led us to use the MSL T to evaluate patients
with narcolepsy (18,19). With a slight change in format-permitting patients to
sleep as long as 10 min-the MSLT became a reliable tool for diagnosing this
disorder. Subsequent clinical studies, by our group and others, have shown that
the MSL T may be useful in separating diagnostic categories of sleep disorders
patients (20,21), evaluating treatment (12,22), and even assessing the carryover
effects of hypnotic medications in patients with insomnia (23). The effectiveness
of the measure in the evaluation of treatment is not entirely clear at this time (24);
however, the diagnostic utility of the test, particularly for narcolepsy, appears to
be fairly well established (25).
We have also continued to pursue the MSLT as a dependent variable in a
number of nonclinical populations under several settings. These have included our
longitudinal evaluation of adolescence (26,27), the assessment of sleepiness produced by various sleeping schedules in adolescents and young adults (28- 30), and
the evaluation of daytime sleepiness in noncomplaining elderly volunteers (31).
Each of these studies has reaffirmed our conclusion that the MSLT provides an
objective measure of daytime sleepiness.
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THE MULTIPLE SLEEP LATENCY TEST
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What does the MSL T measure?
In the 6 years since we began using the MSLT, we have formulated a hypothetical framework concerning the function that it measures. This construct relies upon
the concepts of manifest and physiological sleepiness or sleep tendency. The
MSLT is standardly administered in a setting intencled to promote sleep: subjects
are in bed in a quiet, darkened room; they are instructed to close their eyes and try
to fall asleep; they have been given the opportunity to use the restroom before the
tests, and so forth. Given this setting, we feel that the MSLT measures physiological sleep tendency in the absence of alerting factors. That is, the MSLT is a
direct measure of the underlying tendency for sleep to occur.
Manifest sleep tendency, on the other hand, is more akin to introspective or
behavioral measures of sleepiness/alertness, which change on a moment-tomoment basis depending upon a whole host of factors. These factors may include
such diverse stimuli as light, noise, room temperature, activity level, motivation,
recumbency, anxiety, bladder fullness, hunger, thirst, excitement, attention, and
many others. When a person is asked, "How sleepy are you?" or when observational assessment is made, all of these factors will influence the response. Thus,
even a very sleepy person will not feel or appear sleepy when he is on a tennis
court or at Disneyland. Yet, the same sleepy person driving a car through Kansas
on a warm summer's afternoon would exhibit greatly augmented subjective and
behavioral sleep tendencies.
We do not feel that this type of environment augmentation actually causes
sleepiness to appear. Rather, the reduction of impinging stimuli serve to unmask
the physiological sleep tendency, Thus, we believe that a truly alert individual
does not feel or appear to be sleepy when placed in a low-stimulus environment.
Visualize two businessmen at a postluncheon meeting, for example. Both have
had the regulation two martinis at lunch and are well sated; the room is warm; the
speaker is imparting relevant information, but his delivery is monotonous. One of
these gentlemen has slept well at night for the last week; the second has been
burning the candle at both ends preparing for next week's sales meeting. The first
man is alert, seizing on the important data, making plans and formulating ways to
incorporate the information into his work. The second man yawns, nods off,
struggles to make sense of the presentation, squirms in his chair, pinches himself
to stay awake, and ultimately forgets everything he has heard. The crucial difference between these two men is not that one cannot tolerate liquor, or that the
other is more intelligent or more committed to his job. One is simply sleepy, and
as each layer of environmental input has fallen away, his underlying physiological
sleep tendency has become manifest as irresistible sleepiness.
In this view of sleepiness, the manifest and physiological sleep tendencies are
seldom equivalent. Manifest sleepiness approaches physiological sleepiness as
alerting factors are stripped away. The MSLT environment is designed to remove
as many of the alerting factors as possible and to enable the underlying sleep
tendency to be measured. If a person is not sleepy, he will not fall asleep or will
experience a protracted delay to sleep onset in this setting; the sleepy person will
fall asleep quickly.
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M. A. CAR5KADON AND W. C. DEMENT
The significance of the MSL T
When viewed in this perspective, the MSLT provides ready access to the underlying arousal state. What has not always been clear is the relationship ofMSLT
to other measures of behavior-especially performance testing-for which there is
a rather more obvious link to daily activities. A number of our studies showing
clear deficits or improvements on the MSLT have failed to reveal comparable
changes in the ability to perform (29,32). Only when the MSLT scores are very
low, indicating a pathological degree of sleepiness, have we been able to document a relationship to performance (17,28,32). In these instances, the sleep tendency has been so great that subjects actually fell asleep during the performance
tests.
Wilkinson's (33) appraisal of the types of performance tasks most likely to be
responsive to the effects of sleep loss reveals that decrements occur in a particular
type of test administered in a particular setting. The tests are more susceptible to
sleep loss effects if they are long, monotonous, lacking incentive, complex but
uninteresting, and given repetitively. Thus, performance decrements appear when
the test setting is least conducive to arousal, in other words, when those factors
masking physiological sleepiness are withdrawn.
In this sense, the MSLT is a measure that can gauge the likelihood that an
individual will have difficulty maintaining arousal in a nonarousing environment.
The relevance of the MSLT is thus to a certain extent dependent upon the nature
of the individual's life circumstances. At the same time, we feel that sleep tendency on this measure may reflect the likelihood of a person's becoming actively
involved with his environment. If physiological sleep tendency is very high, we
feel that sleep itself becomes a powerful motive and one tends to withdraw or has
difficulty overcoming the sleep-promoting inertia.
Sleep tendency versus wake tendency
The clinical interpretation of the MSLT, particularly in terms of assessing
treatment of disorders of excessive sleepiness, is somewhat difficult. A case in
point is the study of Roth et al. (24), in which patients with upper airway sleep
apnea were evaluated before and after surgery. The MSLT showed improvement
in only one of nine patients who reported clinical improvement of sleepiness.
Hartse et al. (22) have proposed that a more critical measure of improvement in
such cases might be possible with a slight modification of the MSLT procedure.
This modification involves placing patients in the sleep-promoting environment
but instructing them to try to stay awake. In a study of normal controls (22), sleep
latency was prolonged using this modification.
We suspect that, as sensitive as the MSLT appears to be in most situations,
there may be a level that is reached in sleep pathologies at which the test is no
longer sensitive to changes in arousal state. For example, in patients with narcolepsy
or sleep apnea, who tend to fall asleep almost instantaneously on the MSLT, there
may be subtle variations in ability to resist sleep that are not evident on the test.
On the other hand, we believe that the validity of the MSLT is not reduced by the
seemingly contradictory finding that subjective reports differ from the objective
data (24). Treatment may affect factors relating to manifest sleepiness without
Sleep. Vol. 5 (Suppl. 2). 1982
THE MULTIPLE SLEEP LATENCY TEST
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influencing the physiological sleep tendency. The type of test proposed by Hartse
et al. (22) may be better for assessing this type of difference in therapeutic response, but we feel that the MSLT remains a valid assessment of arousal.
Acknowledgments: This work was supported in part by National Institute on Aging
Grant AG 02504. W. C. Dement is supported in part by National Institute of Mental
Health Research Scientist Award MH 05804.
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