Psychophysiology, 38 ~2001!, 979–987. Cambridge University Press. Printed in the USA.
Copyright © 2001 Society for Psychophysiological Research
Effects of rapid versus slow accumulation
of eight hours of sleep loss
CHRISTOPHER L. DRAKE, TIMOTHY A. ROEHRS, ELENI BURDUVALI, ALICIA BONAHOOM,
MARK ROSEKIND, and THOMAS ROTH
Sleep Disorders and Research Center, Henry Ford Hospital, Detroit, MI, USA
Abstract
The present study assessed alertness, memory, and performance following three schedules of ;8 hr of sleep loss ~slow,
intermediate, and rapid accumulation! in comparison to an 8-hr time in bed ~TIB! sleep schedule. Twelve healthy
individuals aged 21–35 completed each of four conditions according to a Latin Square design: no sleep loss ~8-hr TIB
for 4 nights; 2300–0700!, slow ~6-hr TIB for 4 nights; 0100–0700!, intermediate ~4-hr TIB for 2 nights; 0300–0700!,
and rapid ~0-hr TIB for 1 night! sleep loss. On each day, participants completed a multiple sleep latency test ~MSLT!,
a probed-recall memory task, a psychomotor vigilance task, a divided attention task, and the Profile of Mood States.
“Rapid” sleep loss produced significantly more impairment on tests of alertness, memory, and performance compared
to the “slow” accumulation of a comparable amount of sleep loss. The impairing effects of sleep loss vary as a function
of rate, suggesting the presence of a compensatory adaptive mechanism operating in conjunction with the accumulation
of a sleep debt.
Descriptors: Multiple sleep latency test ~MSLT!, Sleepiness, Sleep deprivation, Performance, Memory, Sleep
restriction
Roth, Rosenthal, & Andreski, 1997!. Laboratory findings also
support the contention that a large proportion of individuals are
chronically sleep deprived ~Bonnet & Arand, 1995; Levine,
Roehrs, Zorick, & Roth, 1988!. In view of the high prevalence
of excessive daytime sleepiness and the associated adverse economic impact of accidents related to sleepiness ~$43–$56 billion
per year; Leger, 1994!, there is a need for additional research
directly comparing the course and consequences of varying degrees of sleep loss.
In addition to the noted effects of acute sleep loss, chronic
partial sleep restriction has been shown to adversely affect alertness and performance as a sleep debt accumulates across successive sleep episodes ~Carskadon & Dement, 1981; Dinges et al.,
1997; Friedman, Globus, Huntley, Mullaney, Naitoh, & Johnson,
1977; Johnson & Macleod, 1973; Tilley & Wilkinson, 1984; Webb
& Agnew, 1965!. Although sleep restriction to less than 4 hr per
night for even 1 night can produce impairment ~Rosenthal et al.,
1993!, even more modest reductions can significantly reduce alertness and performance ~Carskadon & Dement, 1981; Dinges et al.,
1997; Rosenthal et al., 1993; Taub & Berger, 1973!. In a recent
study, sleep restricted to '5 hr per night for 7 nights reduced mean
MSLT values from 11 to 3 min and produced a significant decline
in psychomotor performance ~Dinges et al., 1997!. However, as in
previous studies ~Carskadon & Dement, 1981!, no data were obtained during successive days with $8 hr of sleep per night.
Therefore, differences may have been due to the experimental and
performance regimen itself rather than sleep restriction per se. In
a study of sleep restricted to approximately 6 hr per night, no
impairment in alertness or performance was seen in comparison to
There is evidence that when isolated from external time cues and
social0occupational obligations and required to remain in bed for
10–14 hr nightly, humans naturally consume approximately 8 hr of
sleep per night, suggesting that sleep need may lie near 8 hr per
24-hr day ~Harrison & Horne, 1996; Roehrs, Shore, Papineau,
Rosenthal, & Roth, 1996; Roehrs, Timms, Zwyghuizen-Doorenbos,
& Roth, 1989; Wehr et al., 1993!. When one’s nightly sleep
consumption falls below this level, a sleep debt is hypothesized to
occur and accumulate across days, which can have a significant
negative impact on daytime functioning. It is well recognized that
loss of 1 night of sleep has impairing effects on performance and
consistently reduces multiple sleep latency test ~MSLT! latencies
below 5 min, well within the pathological range ~Bishop, Roehrs,
Rosenthal, & Roth, 1997; Carskadon & Dement, 1979; Wright,
Badia, Myers, & Plenzler, 1997!. In addition, reductions from an
8-hr sleep period impair alertness and performance in a “doserelated” manner ~Devoto, Lucidi, Violani, & Bertini, 1999; Harma
et al., 1998; Jewett, Dijk, Kronauer, & Dinges, 1999; Rosenthal,
Roehrs, Rosen, & Roth, 1993!.
Epidemiological findings estimate that the average individual
sleeps from 6–7 hr per night during weekdays, suggesting that a
large percentage of the population may be accumulating a chronic
sleep debt along with concomitant daytime sleepiness ~Breslau,
This work was supported by NIMH Grant RO1-MH59338-02 and
NASA grant NCC 2-878.
Address reprint requests to: Christopher L. Drake, Ph.D., Sleep Disorders and Research Center, Henry Ford Hospital, 2799 West Grand Blvd.,
CFP3, Detroit, MI 48202, USA. E-mail: cdrake1@hfhs.org.
979
980
a control group ~Horne & Wilkinson, 1985!. However, this was not
a laboratory-controlled study, practice effects were evident in both
groups, and control participants reported an average sleep length
below 8 hr during the study protocol. Additional studies using both
restricted ~,8 hr per night! and extended ~$8 hr per night! sleep
durations are clearly needed.
Although evidence for the impairing effects of different amounts
of sleep restriction is amassing ~Devoto et al., 1999; Harma et al.,
1998; Jewett et al., 1999; Rosenthal et al., 1993!, scarce data are
available directly comparing the effects of acute versus cumulative
sleep restriction. For example, no studies have compared the
alertness- and performance-impairing effects of an equal amount
of sleep loss experienced acutely versus cumulatively. Thus, it is
not known whether 4 consecutive nights of 6 hr of sleep per night
~accumulated sleep loss 5 8 hr! produces the same degree of
impairment in alertness and performance as that of a full night of
sleep loss ~acute sleep loss 5 8 hr!. Such information would be
useful in characterizing the behavioral morbidity associated with
various rates of accumulated sleep loss. To address this issue, the
present study directly compared the effects of different rates of
sleep loss on measures of memory, psychomotor performance,
mood, and physiological sleep tendency following three schedules
of 8 hr of sleep loss ~slow, intermediate, and rapid accumulation!
in comparison to 5 days of an “optimal” 8-hr time in bed ~TIB!
sleep schedule.
Methods
Participants
Twelve healthy individuals ~5 women, 7 men! aged 21–35 years
~mean 27.5 6 5.4 years! were studied. Participants were required
to be free from medical or psychiatric disease as determined by
physician assessment, within 10–25% of normal body weight, with
a total caffeine consumption of ,150 mg0day, alcohol consumption of ,14 drinks per week, and no history of drug or alcohol
abuse. Participants were also required to pass a urine drug screen
and a medical evaluation ~including urinalysis, hematology, blood
chemistries, pregnancy test! prior to experimental procedures. Individuals using drugs acting on the central nervous system or
scoring outside the normal range ~T score . 70! on the Minnesota
Multiphasic Personality Inventory ~clinical scales excluding M-F!
or Cornell Medical Index were excluded. In addition, participants
were required to have nocturnal sleep times of 7–8 hr per night,
sleep latencies of ,30 min, consistent bedtimes and rise times ~not
varying night to night by .2 hr over a 1-week period!, and no
habitual napping. Actual self-reported mean total sleep time was
7.9 6 0.9 hr per night. Participants were instructed to maintain
their regular bedtimes and rise times before and throughout the
study period ~including between laboratory intervals!. All had
normal sleep on an 8-hr nocturnal polysomnogram ~see criteria
below! and an MSLT of 8–14 min. Henry Ford Hospital’s Institutional Review Board approved study procedures. All participants provided written informed consent and were paid for their
participation.
A total of 28 individuals who were initially recruited for participation were discontinued for the following reasons: 10 had
MSLT latencies below 8 min, 5 were found to have a positive drug
screen, 4 did not pass the medical evaluation, 1 had a sleep
efficiency of ,85%, 6 discontinued the study without reason
following the medical evaluation, and 2 individuals canceled their
participation due to time constraints.
C.L. Drake et al.
Procedures
Screening. Participants reported to the laboratory at 2200 hr and
were screened within 1 month of beginning the study using an 8-hr
nocturnal polysomnogram scheduled consistent with self-reported
bedtimes and waketimes ~i.e., 2300–0700!. Recordings included
electroencephalograms ~C3 , C4 , and OZ referenced to mastoid!,
two electrooculograms ~EOG; bilateral horizontal!, submental electromyogram EMG, and electrocardiogram ~V5 lead! and were
scored in 30-s epochs according to standard procedures ~Rechtschaffen & Kales, 1968!. In addition, leg movements were
monitored with a tibialis EMG and respiration with a nasal0oral
thermistor. Recordings were made using Grass model 78-D or
Nihon Kohden ~models 4312 and 4212! polygraphs. The Grass
polygraphs were calibrated with a pen deflection of 50 mV 5
7.5 mm for the EEG and EOG and 50 mV 5 10.0 mm for the EMG.
The 0.5-amp low frequency filter was set at 0.3 Hz with sensitivity
at 5 mV0mm for the EEG and EOG and at 10 Hz with sensitivity
at 1 mV0mm for the EMG. The 0.5-amp high frequency filter was
set at 90 Hz. The Nihon Kohden machines were calibrated at
50 mV 5 10.0 mm for the EEG and EOG and 50 mV 5 16.5 mm
for the EMG. The 0.5-amp low frequency filters were set at 0.3 Hz
with sensitivity at 5 mV0mm for the EEG and EOG and at 0.003 Hz
with sensitivity at 1 mV0mm for the EMG. The 0.5-amp high
frequency filters were set at 70 Hz. All electrode impedances were
,10,000 ohms and paper speed was 10 mm0s. There was no
evidence of clinically significant apnea0hypopnea ~AHI . 100hr!
or periodic leg movements during sleep ~.100hr! for any of the
participants.
At screening, participants were tested using a five-trial ~0930,
1130, 1330, 1530, 1730! MSLT that was administered and scored
according to standard criteria ~Carskadon et al., 1986!. In accordance with standard procedures, sleep latency was scored as the
time from the start of the MSLT to the first epoch of any stage of
sleep. Each sleep latency test was discontinued following three
consecutive epochs of stage 1 sleep or one epoch of any other stage
of sleep. Each participant was required to have a sleep efficiency
.85% at screening and an average daily MSLT sleep latency of
8–14 min. The MSLT range was used to exclude individuals in the
upper and lower quintiles of MSLT latency in order to maintain a
more representative study sample with respect to physiological
sleepiness. All participants were trained on each of the performance tasks at screening in order to minimize the possibility of
practice effects.
Time-in-bed conditions. A schematic diagram of the study
protocol for a typical participant is provided in Figure 1. Immediately prior to each of the four experimental conditions, participants completed one 8-hr nocturnal polysomnogram and MSLT as
a baseline assessment. Participants completed each of the following TIB conditions presented according to a Latin Square design:
no sleep loss ~8-hr TIB for 4 nights; 2300–0700 hour!, slow ~6-hr
TIB for 4 nights; 0100–0700 hour!, intermediate ~4-hr TIB for 2
nights; 0300–0700 hour!, rapid ~0-hr TIB for 1 night! sleep loss.
Thus, individuals in each sleep loss condition ~slow, intermediate,
and rapid! accrued a total sleep loss of 8 hr relative to the control
condition but at different rates. To reduce carryover effects, each
condition was immediately preceded by 8-hr baseline night and
followed with a night of recovery sleep in the laboratory ~ad lib up
to 18 hr!. The conditions were separated by approximately 1–2
weeks. Compliance with each sleep restriction protocol was monitored through strict behavioral observation. Participants reported
to the laboratory at 2200 on each study night. No sleep was
Effects of rapid versus slow accumulation of sleep loss
981
Figure 1. Following a baseline night with 8 hr time in bed, multiple sleep latency test and performance was measured daily for each
condition. Nightly time-in-bed allotments for each of the restricted conditions produced a total sleep debt of 8 hr.
permitted outside the allotted TIB period and MSLT trials and no
caffeine or other stimulants were allowed during the study. Sleep
was recorded on each night during each condition as previously
described.
Physiological sleep tendency (MSLT). On each baseline day
and each experimental day, participants completed a five-trial
MSLT ~0930, 1130, 1330, 1530, 1730! with procedures and scoring identical to that used for screening. To minimize possible “end
of day” expectancy effects, only the first four MSLT trials were
scored.
Performance assessments. The Psychomotor Vigilance Task
~PVT! was administered to participants during a 30-min block
beginning at 1000, 1200, 1400, and 1600. The PVT is a simple
reaction time task with a duration of 10 min that has been shown
to be sensitive to the effects of sleep deprivation ~Dinges et al.,
1997!. The dependent measures were median reaction time, number of lapses ~response times . 500 ms!, fastest 10%, and slowest
10% reaction times. Participants used the dominant index finger
for this task.
During the probed-recall memory ~PRM! task, participants
were exposed to a written list of four word-pairs for 30 s. Fifteen
minutes later participants were provided with one of the words in
each pair and asked to recall the paired words within a 30-s time
limit. Performance was measured as number correct per trial. A
previous study has found this task to be sensitive to the effects of
acute sleep deprivation ~Dinges, Kribbs, Bates, & Carlin, 1993!.
Participants completed a divided attention task at 1030, 1230,
1430, and 1630 hr on each testing day. This 15-min task required
participants to track a moving target across a video screen using a
joystick while simultaneously responding with a button press ~nondominant index finger! to the appearance of stimuli in the center of
the target or periphery of the screen. A total of 52 stimuli were
presented at random intervals throughout each task period. Dependent measures for this task were reaction time ~in milliseconds! to
central and peripheral stimuli and tracking deviations measured in
pixels.
Subjective measures. Ratings of “effort” ~Likert scale 1– 4!
were made following each performance battery to assess differential effects due to changes in motivation between restriction
conditions.
The Profile of Mood States ~POMS! was administered at 0800,
1230, and 1630 hr during baseline and each experimental day to
assess subjective sleepiness, concentration, and mood effects. The
POMS is a previously validated measure that has been used in
numerous studies evaluating the effects of sleep loss and pharmacological challenges on mood and performance ~Bishop et al.,
1997; Roehrs, Papineau, Rosenthal, & Roth, 1999; Roehrs, Pedrosi,
Rosenthal, Zorick, & Roth, 1997; Wright et al., 1997!. Mean T
scores for each standard POMS scale ~fatigue0inertia, vigor0
activity, confusion0bewilderment, tension0anxiety, anger0hostility,
depression0dejection! were calculated for each study day.
Statistical Analyses
To compare the effects of different rates of sleep loss, change
scores were calculated ~last day of each condition minus the
respective baseline! for each dependent variable and submitted to
separate one factor ~condition, 4 levels! repeated-measures analysis of variance ~ANOVA! using the General Linear Model function
of Systat, version 9.0 for PC ~SPSS, Inc., Chicago, IL!. Planned
comparisons were performed to evaluate differences between conditions. Single degree of freedom polynomial contrasts were used
to identify significant trends across increasing rates of sleep loss
~none, slow, intermediate, rapid! and across successive days for
each condition.
As both the 8-hr TIB condition and the 6-hr TIB condition
included a baseline day and 4 experimental days, the complete
982
C.L. Drake et al.
dataset from these conditions were also compared using a separate
two-factor Condition ~2 levels! 3 Day ~5 levels! repeated measures
ANOVA. Similarly, the first three days of the 8-hr and 4-hr TIB
conditions were compared using a two-factor Condition ~2 levels! 3 Day ~3 levels! repeated measures ANOVA. Logtransformations were performed where appropriate ~MSLT and
PVT!, and for each ANOVA the Greenhouse–Geisser epsilon was
used to correct for violations of sphericity. Polysomnographically
recorded sleep during each condition was compared as above using
ANOVA.
Results
Average daily sleep latencies on the MSLT across each experimental day during each TIB condition are presented in Table 1. For
change scores from baseline to the last day of each condition, there
was a significant main effect of condition, F~3,33! 5 13.58, p ,
.001. Planned comparisons revealed that reductions from baseline
were greater for the 0-hr TIB condition compared to the 8-hr TIB
condition, F~1,11! 5 28.90, p , .001, indicating that the MSLT
was sensitive to the effects of 8 hr of acute sleep loss ~see Figure 2a!. Reductions from baseline were also significantly greater
following the 6-hr, F~1,11! 5 12.61, p , .01, and 4-hr, F~1,11! 5
10.88, p , .01, TIB conditions compared to the 8-hr TIB condition, indicating that the MSLT was also sensitive to the effects of
8 hr of slow and moderate cumulative sleep loss, respectively.
Importantly, rapid ~0-hr TIB, 1 night! accumulation of sleep loss
produced greater reductions from baseline compared to slow ~6-hr
TIB, 4 nights! accumulation, F~1,11! 5 4.29, p 5 .06. Additionally, the intermediate ~4 hr TIB, 2 nights! rate of sleep loss
produced reductions in MSLT latency that were midway between
the slow and rapid rate conditions ~see Figure 2a! although neither
comparison was significant.
One individual was excluded from the Psychomotor Vigilance
Task analyses due to missing data from several of the test sessions.
PVT reaction time across each experimental day during each TIB
condition is presented in Table 1. There was a significant main
effect of condition, F~3,30! 5 4.10, p , .05. As a change from
baseline, reaction times slowed ~increased! significantly for the
0-hr TIB condition compared to the 8-hr TIB condition, F~1,10! 5
7.71, p , .05, indicating that the PVT was sensitive to acute sleep
loss. In addition, rapid ~0-hr TIB, 1 night! accumulation of sleep
loss produced significantly slower reaction times compared to
intermediate ~4-hr TIB, 2 nights!, F~1,10! 5 11.29, p , .01, and
slow ~6-hr TIB, 4 nights!, F~1,10! 5 6.95, p , .05, accumulation
of sleep loss ~Figure 2b!.
The number of words recalled on the probed-recall memory
task across each experimental day during each TIB condition is
presented in Table 1. There was a significant main effect of condition, F~3,33! 5 3.82, p , .05. Pairwise comparisons revealed
that there was a greater decline from baseline for the 0-hr TIB
condition compared to the 8-hr TIB condition, F~1,11! 5 11.98,
p , .01, indicating that the PRM was sensitive to acute sleep loss
~Figure 2c!. In addition, rapid ~0-hr TIB, 1 night! accumulation of
sleep loss produced a significantly greater decline in memory
compared to slow ~6-hr TIB, 4 nights! accumulation, F~1,11! 5
10.09, p , .01.
No significant differences between conditions were found for
the Divided Attention Task ~ p . .05 for all!.
No significant differences between conditions were found for
the effort ratings ~ p . .05!, indicating that subjects’ perception of
effort expended towards task completion was not affected by sleep
deprivation or the rate of cumulative sleep debt.
Profile of Mood State scale scores across each experimental
day during each TIB condition are presented in Table 2. For change
scores from baseline to the last day of each condition, there was a
significant main effect of condition for the fatigue, F~3,33! 5
18.70, p , .001, vigor, F~3,33! 5 9.46, p , .001, and confusion,
F~3,33! 5 4.17, p , .05, scales. For fatigue, pairwise comparisons
revealed that reductions from baseline were greater for the 0- and
4-hr TIB conditions compared to the 8-hr TIB condition, F~1,11! 5
56.37, p , .001, F~1,11! 5 9.08, p , .05, respectively, indicating
Table 1. Means (6 Standard Deviation) for Sleep Latency on the Multiple Sleep Latency
Test (in Minutes), Median Reaction Time on the Psychomotor Vigilance Task
(in Milliseconds), and Number of Words Recalled During the Probed
Recall Memory Task Across Days of Each Time-in-Bed Condition
Baseline
day
MSLT
8-hr
6-hr
4-hr
0-hr
PVT
8-hr
6-hr
4-hr
0-hr
PRM
8-hr
6-hr
4-hr
0-hr
Experimental
Day 1
Experimental
Day 2
Experimental
Day 3
Experimental
Day 4
TIB
TIB
TIB
TIB
8.14
9.73
9.54
10.18
~6.18!
~2.82!
~5.31!
~4.03!
8.32
6.99
6.34
4.25
~5.52!
~3.40!
~4.60!
~2.55!
8.93 ~5.53!
7.17 ~5.47!
4.63 ~3.08!
—
10.35 ~4.92!
7.78 ~5.67!
—
—
11.18 ~5.27!
6.90 ~5.85!
—
—
TIB
TIB
TIB
TIB
292.55
285.46
273.30
266.30
~74.89!
~65.83!
~56.22!
~54.01!
274.36
298.36
284.71
310.07
~46.55!
~77.18!
~64.17!
~68.90!
279.93 ~57.54!
290.5 ~71.25!
291.14 ~60.39!
—
286.14 ~72.95!
292.14 ~72.53!
—
—
291.39 ~93.66!
288.25 ~61.93!
—
—
TIB
TIB
TIB
TIB
3.13
3.29
3.63
3.42
~.79!
~.95!
~.29!
~.76!
3.13
3.27
3.23
2.83
~.98!
~.78!
~.77!
~.84!
3.38 ~.78!
3.13 ~.86!
3.06 ~.87!
—
3.31 ~.92!
3.44 ~.61!
—
—
3.42 ~.65!
3.27 ~.91!
—
—
Note: MSLT: multiple sleep latency test; PVT: psychomotor vigilance task; PRM: probed memory
recall; TIB: time in bed.
Effects of rapid versus slow accumulation of sleep loss
983
Figure 2. Mean change ~6SEM! from baseline to last day of each condition for ~a! sleep latency on the multiple sleep latency test,
~b! reaction time on the psychomotor vigilance task, ~c! number of words recalled on the probed recall memory task, and ~d! fatigue
scale of the profile of mood states.
that these measures were sensitive to the effects of rapid and
intermediate rates of sleep loss ~see Figure 2d!. Similar results
were obtained for the vigor scale @rapid sleep loss: F~1,11! 5
13.78, p , .01, intermediate sleep loss: F~1,11! 5 5.16, p , .05#.
For the confusion scale, only rapid sleep loss was significantly
different from the control 8-hr condition, F~1,11! 5 13.49, p ,
.01. Reductions from baseline were not significantly greater following the 6-hr TIB condition compared to the 8-hr TIB condition,
indicating that the POMS scales did not show any effect of ;8 hr
of slow cumulative sleep loss, p . .05 for all. For each of these
scales, rapid ~0-hr TIB, 1 night! accumulation of sleep loss produced greater reductions from baseline compared to slow ~6-hr
TIB, 4 nights! accumulation: fatigue, F~1,11! 5 33.81, p , .001;
vigor, F~1,11! 5 13.88, p , .01; confusion, F~1,11! 5 7.91, p ,
.05. Rapid sleep loss also produced greater impairment than intermediate sleep loss: fatigue, F~1,11! 5 6.99, p , .05; vigor,
F~1,11! 5 10.22, p , .01; approached significance for confusion,
F~1,11! 5 4.46, p 5 .06. The intermediate ~4-hr TIB, 2 nights! rate
of sleep loss produced reductions in each scale that were midway
between the slow and rapid rate conditions ~Figure 2 and Table 2!.
Linear and Polynomial Trend Analyses
Trend analysis using change from baseline to the last day of each
condition ~8 hr of sleep loss! was used to compare among the three
rates of sleep loss. These analyses revealed significant linear effects for the MSLT, F~1,11! 5 37.11, p , .001, PVT, F~1,10! 5
8.36, p , .05, memory, F~1,11! 5 19.45, p , .01, and POMS
@Fatigue, F~1,11! 5 46.68, p , .001; Vigor, F~1,11! 5 15.26, p ,
.01; and Confusion, F~1,11! 5 13.12, p , .01#, indicating a linear
dose-response relationship between rate of sleep loss and impairment. A second-order polynomial trend was present for the MSLT,
F~1,11! 5 5.07, p , .05. Third-order polynomial trends were not
significant for any of the measures. As can be seen clearly in
Figure 2, as the rate of accumulation of sleep loss is increased,
alertness ~subjective and objective!, performance, and memory are
concurrently impaired. It should be emphasized that data are compared on changes from baseline to the last day of each condition so
that each sleep loss condition represents a total sleep debt of ;8 hr
~actual sleep loss deviated somewhat from 8 hr due to differences
in sleep efficiency: 6-hr TIB 5 9.18 hr, 4-hr TIB 5 8.32 hr, 0-hr
TIB 5 8 hr!. Thus, trends represent differences due to rates of
accumulation of the sleep debt rather than total amount of sleep
loss, as the latter was similar for all restriction conditions.
The fact that both the 8-hr and 6-hr TIB conditions included
data for 5 days permitted a comparison including both conditions
across days. Significant Condition 3 Days interactions were followed by trend analyses ~across days within each condition!. The
comparison across days within the 8-hr and 6-hr TIB conditions
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C.L. Drake et al.
Table 2. Mean T Scores (6 Standard Deviation) for Fatigue, Vigor, Confusion, Tension,
Anger, and Depression Scales Across Days for Each Time-in-Bed Condition
POMS
scale
Fatigue
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Vigor
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Confusion
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Tension
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Anger
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Depression
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Baseline
day
Experimental
Day 1
Experimental
Day 2
Experimental
Day 3
Experimental
Day 4
36.94
37.18
37.1
37.51
~3.0!
~2.4!
~1.24!
~3.96!
36.93
38.17
43.19
51.53
~1.89!
~4.56!
~6.95!
~7.72!
36.89 ~2.82!
38.58 ~5.16!
45.56 ~8.49!
—
36.39 ~2.01!
37.89 ~4.69!
—
—
36.92 ~2.68!
39.5 ~6.32!
—
—
58.94
56.68
56.74
59.72
~13.93!
~13.81!
~10.08!
~13.39!
57.01
52.32
48.0
44.61
~14.02!
~13.49!
~10.92!
~9.55!
57.69 ~14.27!
55.65 ~13.87!
48.92 ~12.93!
—
59.26 ~14.23!
54.14 ~15.51!
—
—
57.11 ~13.57!
54.57 ~15.82!
—
—
33.83
35.11
34.38
33.74
~2.83!
~2.96!
~2.76!
~2.28!
33.5
34.07
35.61
36.65
~1.82!
~2.64!
~3.56!
~3.64!
33.42 ~2.64!
34.42 ~3.33!
35.42 ~4.14!
—
33.53 ~2.32!
34.33 ~2.96!
—
—
33.69 ~2.88!
35.1 ~3.86!
—
—
30.56 ~.93!
31.31 ~1.98!
30.97 ~1.81!
31.0 ~1.98!
30.83
31.64
31.5
32.42
~1.04!
~1.9!
~1.92!
~1.83!
31.11 ~1.55!
31.83 ~2.79!
31.75 ~2.22!
—
31.36 ~2.37!
31.86 ~2.77!
—
—
31.53 ~2.60!
32.14 ~3.0!
—
—
37.25 ~.43!
37.31 ~.52!
37.69 ~1.13!
37.58 ~1.35!
37.67
37.61
37.83
38.67
~2.0!
~1.36!
~1.24!
~2.33!
37.25 ~.68!
37.64 ~.96!
38.06 ~1.55!
—
37.44 ~.99!
37.72 ~1.06!
—
—
37.53 ~1.15!
38.0 ~1.84!
—
—
~1.66!
~1.94!
~1.45!
~1.87!
33.83
34.0
34.28
34.31
~1.63!
~1.66!
~1.60!
~1.71!
33.03 ~1.88!
34.06 ~1.72!
34.19 ~1.51!
—
33.83 ~1.51!
33.92 ~1.56!
—
—
33.78 ~1.66!
34.11 ~1.68!
—
—
34.14
34.22
34.03
34.13
Note: TIB: time in bed; POMS: profile of mood states.
yielded a significant Condition 3 Days interaction for the MSLT,
F~4,44! 5 4.74, p , .05. Mean MSLT latency across each experimental day for each condition is shown in Figure 3. Pairwise
comparisons revealed a significant decrease in sleep latency after
the first day of sleep restriction in the 6-hr TIB condition, F~1,11! 5
10.64, p , .01. This effect continued without a further decrease for
the remaining days, p . .05 for all. A linear trend was present
Figure 3. Mean ~6SEM! sleep latency on the multiple sleep latency test
across days for each of the conditions.
across the 6-hr TIB condition, F~1,11! 5 7.54, p , .05. In contrast,
latency increased ~more alert! linearly throughout the 8-hr TIB
condition, as indicated by a significant linear trend across days for
this condition, F~1,11! 5 7.44, p , .05. However, pairwise differences from baseline did not become significant until the fourth
day of the 8-hr TIB schedule, F~1,11! 5 7.44, p , .05. No main
effects or interactions were found comparing the 8- and 6-hr TIB
conditions across days for the PRM or divided attention task, p .
.05. A main effect for TIB was present for the POMS confusion,
F~1,11! 5 10.48, p , .01, and tension F~1,11! 5 7.15, p , .05
scales, indicating greater tension and confusion for the 6-hr compared with 8-hr TIB conditions.
Similarly, the initial 3 days of the 8-hr and 4-hr TIB conditions
were examined using repeated measures ANOVA. A Condition 3
Days interaction was found for MSLT latency, F~2,22! 5 8.08, p ,
.01, PVT median reaction time, F~2,20! 5 4.22, p , .05, and the
POMS fatigue, F~2,22! 5 7.53, p , .05, and vigor, F~2,22! 5
3.85, p , .05, scales. Trend analysis revealed significant linear
trends for each of these measures across the 4-hr TIB condition,
F~1,11! 5 18.70–8.79, p , 0.05 for all. A secondary quadratic
trend was present for the vigor scale, F~1,11! 5 16.71, p , .01. No
significant trends were found for the same time period ~first 3 days!
across the 8-hr TIB condition ~ p . .05 for all!. Pairwise comparisons across days revealed significant differences between baseline
and Day 2 of the 4-hr TIB condition for the MSLT, F~1,11! 5
15.99, p , .01, and PVT, F~1,10! 5 8.78, p , .05. Baseline versus
Day 1 comparisons were significant only for the MSLT, F~1,11! 5
Effects of rapid versus slow accumulation of sleep loss
6.30, p , .05. Day 1 versus Day 2 comparisons were not significant, p . .05. Results for the POMS fatigue and vigor scales
paralleled those of the MSLT and PVT with regard to baseline
versus Day 2 differences: fatigue, F~1,11! 5 11.72, p , .01; vigor,
F~1,11! 5 18.70, p , .01. In addition, baseline versus Day 1
comparisons: fatigue, F~1,11! 5 8.97, p , .05; vigor, F~1,11! 5
21.94, p , .01, were significant. However, Day 1 versus Day 2
comparisons were significant only for fatigue, F~1,11! 5 8.84,
p , .05.
Means and standard deviations for polysomnographically recorded sleep during each of the conditions is shown in Table 3
along with the results of the statistical analyses comparing baseline
to subsequent days across each condition.
Discussion
As expected, each sleep restriction regimen produced a degradation in alertness compared with the 8-hr TIB condition; greater
impairment was observed when sleep loss ~total sleep debt ;8 hr!
was experienced rapidly ~1 night! compared to cumulatively ~across
2 or 4 nights!. In addition, clear dose–response relationships were
apparent between the rate of sleep loss and impairment. These
results were consistent across nearly all measures and suggest that
some adaptation occurs when sleep loss is amassed slowly rather
than rapidly. Also consistent with this view, is the finding that in
985
the 6-hr TIB condition, alertness decreased after the first night
only and no further reductions were observed during the subsequent 3 days of restriction. Importantly, in the 6-hr TIB condition, alertness levels remained substantially reduced throughout
the restriction period in comparison to the 8-hr TIB schedule.
Thus, even in the face of some adaptation across 4 nights ~i.e.,
absence of cumulative deficits! sleep reductions to 6 hr per night
impair alertness relative to an 8-hr TIB schedule.
Although there are currently no data available regarding the
effects of different rates of sleep loss, our data are in agreement
with that of a previous study where an “adaptation” to moderate
sleep restriction became evident within 2– 6 days following approximately 5 hr of sleep per night for 7 nights ~Dinges et al.,
1997!. Despite no clear evidence for adaptation to sleep restriction
in the data from Carskadon and Dement ~1981!, as in the present
study, the largest decrease in latency ~reduced alertness! followed
the first restriction night whereas an increase in latency followed
the third restriction night. Moreover, both subjective measures
assessed in that study followed similar patterns of an initial increase in sleepiness with minimal changes thereafter. However,
beyond 4 nights of restriction further decreases in latency were
observed indicating that any adaptation that may take place is
relatively short-lived.
Although the present results do provide evidence for adaptation
to a modest rate of sleep loss ~6 hr per night!, we did not observe
Table 3. Means (6 Standard Deviation), and ANOVA Values for Polysomnographically Recorded Sleep Across Nights
for Each Time-in-Bed Condition
Sleep
measure
SE %
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
SWS %
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
REM %
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Stage 1 %
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Stage 2 %
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Latency
8-hr TIB
6-hr TIB
4-hr TIB
0-hr TIB
Baseline
nights
Experimental
Night 1
Experimental
Night 2
Experimental
Night 3
Experimental
Night 4
Repeated-measure
ANOVA ~omnibus F!
92.63
87.31
92.45
88.64
~3.10!
~11.27!
~6.51!
~7.10!
91.84 ~5.20!
94.20 ~5.92!*
93.81 ~5.18!
—
94.17 ~2.31!
95.36 ~3.31!*
98.14 ~1.14!*
—
90.19 ~7.90!
95.16 ~3.19!*
—
—
88.06 ~9.69!
95.56 ~1.69!*
—
—
F~4,44! 5 1.72, p 5 .20
F~4,44! 5 4.66, p , .05
F~2,22! 5 5.31, p , .05
11.51
9.42
11.75
15.62
~5.56!
~8.70!
~5.70!
~9.44!
10.25 ~6.95!
17.35 ~10.13!*
23.00 ~11.23!**
—
14.98 ~5.92!
19.70 ~12.71!*
31.29 ~11.50!***
—
13.38 ~9.37!
19.21 ~8.79!*
—
—
18.77 ~11.65!
32.09 ~17.25!*
—
—
F~4,44! 5 2.41, p 5 .09
F~4,44! 5 8.05, p , .01
F~2,22! 5 10.56, p , .01
19.16
17.93
19.66
17.25
~7.31!
~7.38!
~5.57!
~7.71!
18.75 ~7.08!
20.14 ~7.16!
15.99 ~6.27!*
—
20.43 ~4.45!
19.94 ~6.56!
13.38 ~10.07!*
—
20.41 ~6.03!
19.66 ~6.40!
—
—
16.01 ~8.27!
13.67 ~6.24!
—
—
F~4,44! 5 1.68, p 5 .20
F~4,44! 5 4.64, p , .01
F~2,22! 5 3.84, p 5 .06
18.17
14.63
17.50
16.45
~10.12!
~8.58!
~9.14!
~9.48!
14.62 ~6.37!
15.09 ~9.51!
15.27 ~12.38!
—
15.07 ~8.72!
14.19 ~10.26!
5.53 ~3.25!*
—
13.45 ~7.94!
17.57 ~9.91!
—
—
10.09 ~5.73!*
9.64 ~4.81!
—
—
F~4,44! 5 3.43, p , .05
F~4,44! 5 3.18, p , .05
F~2,22! 5 8.45, p , .01
51.15
57.73
51.08
50.66
~8.75!
~10.35!
~7.65!
~9.44!
56.39 ~13.29!
47.42 ~14.06!**
45.74 ~9.87!
—
49.52 ~9.15!
46.33 ~11.13!**
49.79 ~12.51!
—
52.77 ~11.45!
44.68 ~8.42!***
—
—
55.06 ~16.95!
44.66 ~15.09!*
—
—
F~4,44! 5 0.78, p 5 .51
F~4,44! 5 3.98, p , .05
F~2,22! 5 0.87, p 5 .40
10.40
37.96
13.23
24.91
~6.47!
~50.15!
~14.96!
~15.67!
17.86 ~11.67!*
7.08 ~4.20!
8.50 ~7.46!
—
14.45 ~9.19!*
7.86 ~10.18!
1.94 ~1.16!*
—
19.64 ~17.66!
8.55 ~6.47!
—
—
23.39 ~20.13!*
7.07 ~5.72!
—
—
F~4,44! 5 3.64, p 5 .05
F~4,44! 5 4.18, p 5 .06
F~2,22! 5 5.66, p , .05
Note: SE: Sleep efficiency ~total sleep time0time in bed*100!, SWS: Slow wave sleep stages 3 1 4, REM: Rapid eye movement sleep.
Significant difference from baseline night *p , .05, **p , .01, ***p , .001 ~single degree of freedom comparisons!. All omnibus p values shown
using Greenhouse–Geisser correction.
986
a similar pattern with a faster rate ~4 hr per night!. Indeed, the 4-hr
TIB condition produced an accumulation of a sleep debt across
two successive days as evidenced by primary linear trends for both
sleepiness and performance measures.
The mechanism~s! responsible for the cognitive and behavioral
adaptation that appears to take place with cumulative sleep loss is
not known. However, previous studies have shown that, in humans, compensatory increases in slow wave sleep, EEG slow wave
activity, and rapid eye movement sleep ~REM! occur in response
to sleep restricted below 5 hr across 4 nights ~Balkin & Badia,
1988; Brunner, Dijk, & Borbely, 1993; Carskadon & Dement,
1981!. Similar to previous studies, we found increases in both slow
wave sleep percentage and sleep efficiency. Such alterations in the
relative composition of sleep during cumulative sleep restriction
may, as other researchers have posited ~Dinges et al., 1997!, lead
to adaptation such as that observed in the present study. The
positive correlation between slow-wave sleep duration during daytime naps and mean MSLT scores following sleep restriction is
consistent with this possibility ~Lumley, Roehrs, Zorick, Lamphere, & Roth, 1986!. Finally, the differential effects of REM
versus NREM sleep deprivation on MSLT latency ~Nykamp et al.,
1998! is also in accord with the notion of specific alterations in the
composition of sleep affecting daytime function. Although the
present methodology does not allow for causal implications, both
slow wave sleep and sleep efficiency are candidates for a putative
mechanism~s! that may facilitate adaptation to sleep restriction;
however, further research must be undertaken before definitive
conclusions can be drawn.
Although evidence for adaptation to cumulative sleep loss in
the present study rests on the assumption that sleep need for the
individuals studied was approximately 8 hr per night, several
aspects of the present study support this contention. First, individuals selected for participation were not particularly “short” or
“long” sleepers, as screening criteria required an average 7–8-hr
sleep period per night. In addition, MSLT screening criteria ~8–
14 min! would likely have screened out many excessively “long”
~low MSLT! or “short” ~high MSLT! sleepers. Also, it seems
unlikely that “short” sleepers would have had progressively increasing alertness levels across days of an 8-hr TIB schedule, as
was demonstrated. Finally, a previous study has shown that individuals consume an average of 8.2 hr of sleep nightly with minimal
variance when given 14 hr TIB once preexisting sleep debt has
dissipated ~Wehr et al., 1993!.
No changes in psychomotor vigilance or memory occurred
across days in the 6-hr TIB condition. This stability is not likely to
be due to the presence of a practice effect ~counteracting impairment!, as no trends were found during the 8-hr TIB condition. In
two previous studies, impairment was found following only 2 days
of restriction '5 hr of sleep per night with evidence for cumulative
impairment across 7 nights ~Carskadon & Dement, 1981; Dinges
et al., 1997!. Thus, as sleep is restricted below 6 hr, cumulative
impairment in functioning becomes apparent both objectively and
subjectively.
Although our results provide evidence for some degree of
adaptation to moderate sleep loss ~6 hr per night for 4 days!,
adaptation may dissipate following longer periods of accumulated
sleep debt and greater amounts of nightly sleep restriction may
prevent any adaptation ~Carskadon & Dement, 1981!. The strategy
of the present study was to evaluate the effects of a fixed degree of
sleep loss ~8 hr! accrued slowly compared to rapidly. Further
studies are needed to clarify the precise threshold of nightly sleep
that will produce evidence for adaptation to a sleep debt as well as
C.L. Drake et al.
characterize the specific functions associated with various degrees
of nightly sleep loss accumulated across successive days. That is,
adaptation will occur in response to what degree of sleep loss and
for what sustained period of time?
Similar to previous studies, subjective measures of sleepiness
~“fatigue” and “vigor”! increased with intermediate and rapid rates
of sleep loss. However, minimal changes were observed across
4 days with 6 hr of TIB ~slow rate of sleep loss!. Thus, individuals
were apparently unable to detect the increase in physiological
sleepiness ~decreased MSLT latency! resulting from a slow rate of
moderate sleep loss. This inability to detect reductions in alertness
is consistent with numerous studies that have failed to show consistent relationships between subjective and objective measures of
sleepiness and may contribute to the neglect of appropriate sleep
hygiene practices by the general population.
Consistent with results from previous studies ~Rosenthal et al.,
1993; Wright et al., 1997!, total sleep deprivation for 1 night
produced marked reductions in psychomotor performance, alertness, and memory. In addition, restricting TIB to 6 hr for 4 nights
decreased MSLT latency by 3 min, whereas maintaining 8 hr TIB
for 4 nights improved MSLT latency by 3 min. Consistent with our
results are findings from a recent study where MSLT latency was
reduced from 11 to 3 min following 5 nights of sleep restricted to
approximately 5 hr per night ~Dinges et al., 1997!. Although
slightly greater reductions in latency were observed in that study,
the results are in line with the present findings given the differences in duration and amount of cumulative sleep loss. An earlier
study found that MSLT latency was reduced from 17 to 10 min
following sleep restricted to 5 hr per night for 4 nights ~Carskadon
& Dement, 1981!. Again, larger reductions in latency could be
attributed to differences in the amount of sleep restriction and the
higher overall mean values are likely due to the 3-day, 10-hr TIB
baseline provided in that study.
Regarding sleep duration, maintenance of an 8-hr TIB period
for 5 nights significantly increased sleep latency ~improved alertness!. This improvement in alertness may have been due to the
presence of a preexisting sleep debt that may not have fully
dissipated during this period. In a previous study, 2 weeks with an
average of 8 hr sleep0night did not improve MSLT latency ~Harrison & Horne, 1996!. However, participants selected for that
study were functioning well within the optimal range of alertness
~mean MSLT 5 16.2! prior to sleep extension. Thus, individuals in
that study were likely sleep satiated following fulfillment of a
biological sleep need and, consequently, one would not expect
further increases in alertness with extended sleep duration. Considering recent data indicating that the average amount of sleep
obtained by individuals in the population is less than 8 hr per night
~Breslau et al., 1997!, the present results suggest that maintaining
a nightly schedule of 8 hr TIB may result in a significant improvement in alertness for many individuals.
This study replicates previous results demonstrating the cumulative effects of sleep restriction ~Carskadon & Dement, 1981;
Dinges et al., 1997!. In addition, the present results extend previous findings by demonstrating that the development of a sleep debt
appears to be rate sensitive. The impairing effects of 8 hr of sleep
loss on alertness and performance appear to be more severe when
experienced acutely as compared to cumulatively suggesting the
presence of a compensatory mechanism operating in conjunction
with the accumulation of a moderate sleep debt. Furthermore, the
current results support previous recommendations and provide
solid objective evidence for the benefit of maintaining an 8-hr per
night sleep period.
Effects of rapid versus slow accumulation of sleep loss
987
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~Received May 26, 2000; Accepted July 5, 2001!