Altered sleep duration and sleep period time displacements: Effects on performance in habitual long sleepers

Physiology & Behavior, Vol. 16, pp. 177-184. Pergamon

Press and Brain Research Publ., 1976. Printed in the U.S.A.

Altered Sleep Duration and Sleep Period Time Displacements: Effects on Performance in Habitual Long S l e e p e r s 1': J O H N M. T A U B

University o f California, Los Angeles AND R A L P H J. B E R G E R

University of California, Santa Cruz (Received 14 April 1975) TAUB, J. M. AND R. J. BERGER. Altered sleep duration and sleep period time displacements: effects on performance in habitual long sleepers. PHYSIOL. BEHAV. 16(2) 177-184, 1976. - Performance was studied in 10 healthy young adult males who characteristically sleep 9 112 - 10 1/2 hr following an electroencephalographically (EEG) recorded habitual sleep night and 4 nights on which their customary sleep was altered by 3 hr as follows: extended (E), deprived (D), delayed shift (DS), and advanced shift (AS). In the E condition sleep was extended by advancing sleep onset 3 hr corresponding to the AS condition which had the same retiring time, but differing from it with awakening occurring 3 hr earlier. In the D and DS conditions time of sleep onset was delayed 3 hr and the subjects were awakened at their customary time in the D condition, but 3 hr later than usual in the DS condition. Subjects performed an auditory vigilance task 35 min after awakening, at midday and in the early evening. Throughout the day after both shifted sleep and altered sleep duration performance was significantly impaired to an equivalent degree as reflected by longer reaction time, increased misses and a dedine of intrinsic perceptual capacity. Changes in the vigilance measures did not correlate with sleep duration or any other specific alterations in the EEG patterns of sleep. The behavioral deficits which resulted from altered sleep schedules are discussed viewing sleep as a biological adaptive process with respect to the feature of its occurrence under natural conditions in a temporally rhythmic sequence. Behavior Circadian rhythms EEG Performance Reaction time Sleep deprivation Sleep extension Sleep stages Vigilance

AS a c o n s e q u e n c e of terrestrial e v o l u t i o n there are behavioral variations in m a n y biological systems of plant and animal life associated with geophysical and diurnal cycles. U n d e r natural conditions the h u m a n sleep-waking r h y t h m is entrained to about 24 hr by periodic environmental agents (Zeitgebers) as living routine and social cues which are the most p o t e n t o f such synchronizing agents for man [3]. When Zeitgebers are volitionally or involuntarily altered such that it is not possible for persons to follow a c u s t o m a r y circadian r h y t h m of sleeping and waking, behavioral deficits would seem to be a predictable consequence. Relatively neglected, b u t a significant q u e s t i o n is the e x t e n t to which o p t i m a l waking behavior is contingent u p o n stability in the individual normal sleep-activity cycle. Probably the popular c o n c e p t i o n of sleep as being primarily a restorative process has led to an emphasis o f research on its experimental elimination m o r e c o m m o n l y

Signal detection

k n o w n as sleep deprivation. A view [6] for which there is some supportive evidence is that sleep loss studies have quite successfully d e m o n s t r a t e d alterations in psychophysiology and behavior due to the i n t e r r u p t i o n of an adapted process, m a n ' s basic 24 hr sleep-wakefulness cycle. Performance deficits on tasks shown sensitive to the direct effects of total sleep loss were observed in the morning, but not in the a f t e r n o o n after a night of unrestricted sleep following 24 hr awake, a finding which indicated to Wilkinson [37] the disruption of diurnal physiological r h y t h m s by sleep deprivation. The regular recurrent t e m p o r a l occurrence of sleep may perhaps be equally i m p o r t a n t to any fixed a m o u n t of sleeping t i m e per se for peak behavioral efficiency. In a previous study we observed that p e r f o r m a n c e declined during the day following acute 2 - 4 hr advances and delays in the times w h e n regular 1 2 : 0 0 - 8 : 0 0 a.m. sleepers were allotted

1 Support for this research was provided by National Institute of Mental Health Research Grant MH18928 and by National Institute of Mental Health Interdisciplinary Training Grant MH06415. 2The authors wish to thank Douglas Clarkson, Donald Guthrie and Robert T. Wilkinson for their advice and comments. Results of this study were presented in part at the annual meeting of the Association for the Psychophysiological Study of Sleep, San Diego, California, May, 1973. 177

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TAUB AND BERGER

their usual 7 or 8 hr of sleep [27]. In another study of a similar group of sleepers [25] lengthening or shortening sleep by 3 hr caused similarly adverse behavioral deficits as advancing or delaying their regular sleep period by 3 hr. In both the above studies time spent asleep was similar for the shifted-sleep conditions and the 1 2 : 0 0 - 8 : 0 0 a.m. habitual condition, and averaged at least 7 hr. Furthermore, changes in the performance measures were unrelated to sleep duration or any specific changes in the electrophysiologically recorded sleep stages [20]. The findings from these studies and others [12,16] lend support to the hypothesis that alterations in a customary sleep-waking pattern may be more closely related to the efficiency of behavioral functions than total sleep duration or time spent in specific sleep stages [ 11,37]. Often underemphasized is the fact that individual members of any species, including a general population of humans of the same age, exhibit a wide range of individual differences in average sleep length [30,31]. If acutely altering the sleep-wakefulness rhythm produced similar defects of waking behavior in groups of subjects who differ in their habitual sleep durations, this might indicate the more significant behavioral adaptation conferred by maintenance of regular sleeping schedules than by the accumulation of any invariant amount of sleep. The purpose of the present investigation was to compare the behavioral effects of partial sleep deprivation and sleep extension with those following temporal shifts of the sleep period in a longer than normal group of subjects who characteristically sleep 9 1 / 2 - 1 0 1/2 hr per night. It was intended to determine whether similar findings of impaired performance would occur in this group of long sleepers as was observed to occur in habitual 7 - 8 hr sleepers [25] following 3 hr alterations in the length of timing of their accustomed sleep period. METHOD

Subjects Ten subjects were selected from 1,000 male respondents to an inventory distributed among students on 2 college campuses. The screening device was a modified version of the Cornell Medical Index [4[ which consisted of questions about medical and psychosomatic conditions, and sleep characteristics. Subjects were considered for further study only if their responses to the inventory were not indicative of sleep disturbance, medical problems, psychiatric disorders, and frequent alcohol or other drug usage; and if they reported having consistently slept 1 1/2 hr or longer over at least an immediately preceding period of 2 years. Charts similar in form to those used previously [28] were mailed to the 107 respondents who satisfied the above criteria with instructions to record for 2 weeks each 30 min period during which they were asleep. Subjects were eliminated if a discrepancy of more than 1 hr existed between their questionnaire estimate of sleep duration and average sleep during the 2 weeks. Sleep charts were returned by 54 subjects and 28 were selected for further study who showed that they almost always retired at an habitual time and had 9 1 / 2 - 1 0 1/2 hr of uninterrupted sleep nightly with no evidence of unusual fluctuations or daytime naps. In the final stage of screening, subjects were administered the MMPI and rejected if they scored 2 standard deviations above normal on any MMPI scale except Mf (since elevated Mf scores are quite common among male college students).

Ten subjects were randomly selected from 17 who were eligible and paid $75.00 for their participation in the experiment. The subjects ranged from 1 8 - 2 5 yr old with a mean age of 20 yr.

Measurement of Performance A 45 min. Wilkinson [ 39 ] auditory vigilance task was used to measure performance. Auditory vigilance has proven to be an aspect of behavior especially sensitive to moderate manipulations in sleep-waking patterns [9, 25, 27, 38 ]. During testing subjects sat in a sound attenuated cubicle and were presented the task binaurally through headphones. The auditory stimuli were 1/2 sec tones occurring at 2 sec intervals over 85 dB ambient white noise. Thirty tones were slightly shorter than the others (3/8 sec) and it was the subject's task to press a telegraph key immediately whenever he detected a short (critical) signal. The subject's reaction times to the critical signals were recorded in 1 msec units by a timer [29]. The critical signals occurred at irregular intervals such that they seemed random to the subjects. Ten different audiotapes were used so that the order of signals remained unpredictable to the subjects throughout the experiment. The number of signals missed (misses) and incorrectly detected (false reports) were scored, and further transformed to yield measures of intrinsic sensory capacity (6) and decision criterion for reporting signals O). These latter analyses were based on signal detection theory [24] and values of 6 and 0 were obtained from Freeman's [8] tables. Signal detection analysis separates factors of perceptual discrimination presumably from those of expectation and motivation which also affect the responses [23]. At a minimum of 2 days before the experiment, subjects practiced the task with at least 24 hr intervening between the 2 sessions. During the first practice a recorded 20 min preliminary instruction tape was presented which contained explanatory information about the vigilance task, delineated the signal from nonsignal stimuli and provided initial practice in detecting them. A 3 min familiarization period during which signals occurred at a relatively frequent rate to remind the subjects of their perceptual characteristics then preceded a 45 rain test tape. During the second practice session subjects were presented another vigilance test tape.

Design The experiment comprised 5 nights of sleep and an adaptation night of the subject's habitual 9 1 / 2 - 1 0 1/2 hr sleeping period preceded it by a week. The subjects were studied individually with sleep treatments spaced 1 week apart in a 10 x 5 balanced incomplete block design. The order of the sleep conditions was randomly assigned to each subject. The 5 experimental treatments were as follows: an habitual sleep condition; 2 conditions, 1 in which the period allowed for sleep was lengthened and 1 in which it was shortened by 3 hr; and 2 conditions, 1 in which the usual period for sleep was advanced and 1 in which it was delayed by 3 hr. In the habitual (H) condition subjects slept 9 1 / 2 - 1 0 1/2 hr at their accustomed times. Under the extended (E) sleep condition subjects accumulated extra sleep by having them retire 3 hr earlier than usual. In the advanced shift (AS) condition subjects were required to retire at the same time as in the E condition, but were awakened 3 hr sooner

ALTERED SLEEP PATTERNS AND PERFORMANCE IN LONG SLEEPERS than usual thereby obtaining their regular 9 1 / 2 - 1 0 1/2 hr of sleep. In the sleep deprivation (D) condition, subjects were required to remain awake 3 hr later than usual. In the delayed-shift (DS) condition subjects were required to remain awake also until the same time as in the D condition, but were awakened 3 hr later than usual thereby accumulating their regular 9 1 / 2 - 1 0 1/2 hr of sleep. For the D and E conditions of altered sleep duration, time of awakening was the same as in the H condition. The independent variables of sleep duration, retiring time, and awakening time for the 5 sleep conditions are depicted in Table 1. A common awakening time was chosen for the D and E conditions to control for the possible influence of circadian rhythms on behavioral measures taken in the morning. Three-hour manipulations of sleepwaking patterns were chosen to render comparable the magnitude of differences produced by the effects of shifting sleeping time and altering sleep duration between the long sleepers and regular 7 - 8 hr sleepers studied previously under an identical experimental design [25]. Table 2 shows the sleep habits of the subjects and times of testing. The sleeping times varied from subjects who retire at 10:00 or 10:30 p.m., and awaken at 7:30 or 8:00 a.m. to late sleepers who retire after midnight and wake up almost at noon. Testing times were scheduled relative to habitual sleep patterns as exemplified by subjects 9 and 10 who were awakened at 11:00 a.m. and were tested before lunch, at midday which was for them 4:30 p.m., and at 8:30 p.m. in the early evening.

Electrodes were placed for recording electroencephalographic, electromyographic, and electrooculographic activity during all conditions and the records were scored for sleep stages by 30 sec epochs according to standard procedures [20]. The 9 1 / 2 - 1 0 1/2 hr recordings from the H, AS and DS conditions were coded and scored blind with respect to treatment. Sleep records from the D and E conditions differed from these in duration so that it was not possible to employ a blind scoring procedure with them. Upon awakening subjects were served 120 ml of fruit juice as a partial nutritional control for blood sugar level. Thirty-five min later they were given the vigilance task which was administered twice again on the same day before lunch and in the evening at fixed clock times (Table 2) to control for possible effects of diurnal rhythms; and to determine the extent to which any effects of altered sleep patterns persist throughout the following day. Measures taken soon after awakening, however, were independent of the varying amounts of wakefulness preceding the tests taken at fixed times. Tests were not given at midday after the DS condition because postsleep testing for some subjects had finished almost exactly then or was in very close proximity to midday testing for others. The same procedure was, therefore, adopted as in earlier studies [25,27] in considering data obtained from the first DS test session twice in separate statistical analyses both as immediate postsleep and midday measures. There were no cues present in the experimental setting nor from the experimenter to inform subjects about their performance.

Procedure Before and during experimental sessions subjects were instructed not to nap or to drink caffeinated beverages, but to maintain their usual physical activity and food and fluid intake. They were told that the purpose of the experiment was to determine the relationship between sleep, biological time cycles, and personality functioning. All subjects reported to the laboratory at 6:00 p.m. (except as noted below) and sat in bed reading magazines or books. Time cues including daylight, chronometers and radios were absent until termination of postawakening testing. Since subjects 1 - 4 retired relatively early (see Table 1), in the AS and E conditions they were instructed to arrive at the laboratory by 4:00 p.m. allowing at least 3 hr of isolation from time cues and other periodic environmental factors before they were required to retire. The experimenter explained to these 4 subjects that the changes in their times for reporting to the laboratory were due to technical difficulties.

179

RESULTS Nonparametric statistics [22] were used in data analyses so as few assumptions as possible would have to be made concerning population distributions. Spearman rank correlation coefficients were computed to explore the possible relationship between the various parameters of sleep physiology and measures of vigilance. Two tailed values of the Wilcoxon matched pairs signed ranks test were used to evaluate effects of the sleep conditions on parameters of the vigilance task at each time of day. For each performance variable 4 main sets of comparisons were performed : H with D, DS, AS, and E; D with E; D with DS; and E with AS.

Main Effects of the Sleep Conditions There were statistically significant decrements in measures of both speed and accuracy on the vigilance task following each of the experimental treatments compared to

TABLE 1 TREATMENTVARIABLESFOR THE DIFFERENTCONDITIONS

Independent variables Sleep duration Retiring time Awakening time

Sleep deprivation (D)

Delayed shift ( D S )

6½-7½ hr 3 hr later than H Same as H

Same as H 3 hr later than H 3 hr later than H

Sleep Condition Advanced Habitual (H) shift (AS) 9½-10½ hr

Same as H 3 hr earlier than H 3 hr earlier than H

Extended sleep (E) 12½-13½ hr 3 hr earlier than H Same as H

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TAUB AND BERGER TABLE 2 SUBJECTS' HABITUALTIMESOF SLEEPAND SCHEDULEOF TESTING

Subjects 1 2,3 4 5-7 8 9 10

Habitual Sleep 10:00 p.m.-7:30 a.m. 10:30 p.m.-8:00 a.m. 11:00 p.m.-9:00 a.m. 12:00-9:30a.m. 12:30-10:00 a.m. 12:30--1h00 a.m. 1:00-11:00 a.m.

Postsleep

Time of Test Sessions Midday

Evening

12:00 p.m. 12:00 p.m. 12:30 p.m. 12:30 p.m. 4:30 p.m. 4:30 p.m. 4:30 p.m.

5:00 p.m. 5:00 p.m. 5:30 p.m. 5:30 p.m. 8:30 p.m. 8:30 p.m. 8:30 p.m.

8:00 a.m. 8:30 a.m. 9:30 a.m. 10:00 a.m. 10:30 a.m. lh30a.m. lh30a.m.

the condition of habitual sleep. Mean reaction time to critical signals on the vigilance task during postsleep, midday, and evening testing sessions for each experimental condition is shown in Fig. 1. Reaction times were shorter in the H condition compared to the D, DS, AS, and E (experimental) conditions (Ts< 3, ps<0.01 ) postsleep and at midday (Ts<5, ps<0.02), but not in the evening. Presented in Figure 2a is the mean number of misses for each sleep condition. Misses were fewer in the H condition compared to the D, AS, E (Ts<5, p s < 0 . 0 2 ) a n d DS (T = 6, p<0.05) conditions postsleep, and in the evening (Ts<3, ps<0.01). Mean scores on 8, the signal detection parameter are shown in Figure 2b. The trend for ~ was similar to that for misses, indicating that shifts in the length and timing of sleep decreased the subjects' perceptual capacity in responding to the critical signals. Levels of 8 were significantly higher in the postsleep test session of the H condition than in the D, DS, AS ( T s < 5 ps<0.02) and E conditions (T = 7, p<0.05); and were also higher in the evening after the H condition than following each of the experimental conditions (Ts<2, ps<0.01). Neither misses nor 8 at midday were significantly different across treatments. No statistically significant differences were revealed among the conditions for either false alarms or #, the signal detection measure presumed to reflect subjects' motivation or willingness to perform the vigilance task. There was only 1 statistically significant difference between the altered sleep duration and the shifted sleep conditions. At midday reaction time in the E condition was shorter than in the AS condition (T = 4, p<0.02, Figure 1). The findings presented above for the vigilance task are summarized in Table 3. The pattern of the results is clear in that statistically significant deficits in performance occurred following all of the 3 hr altered conditions of sleep compared with the H condition and were present during testing sessions 35 rain after awakening; and when testing times were kept constant at midday and in the evening.

l~Tectrophysiological Sleep-State Analyses In Table 4 are shown the electrophysiological parameters of sleep. Total time asleep did not differ significantly between the AS, DS, or H conditions and averaged approximately 9 hr. In the sleep deprivation condition subjects averaged 6.2 hr of sleep, and on nights of extended sleep they averaged 1 1.3 hr of sleep. The systematic changes in some sleep parameters that

580 570 560 ~ ~

~)

,v

[]postsleep ggmidday Wearly evening

550 540 s3c 520 510 490

48o 470 460 45O

61/27V2hr 9V2-10Y2hr 12Y2-13Y2hr D DS *-- I'-I ---'- AS E

SLEEP CONDITION FIG. 1. Reaction time to signals on the vigilance task after extended (E), advanced-shift (AS), habitual (H), deprivation (D), and delayed-shift (DS) sleep conditions. resulted from alterations in circadian placement indicates rhythmicity to be a critical factor involved in almost all biological systems. As the sidereal times of the sleep periods occurred earlier by 3 hr intervals within a 9 hr continuum in the H and shifted conditions the changes in sleep physiology included reduced amounts of Stage REM, and Stage 4 sleep; and increased amounts of Stage 2 and Stage 1 sleep. Advancing the time of retiring 3 hr earlier than usual was achieved with reasonable success inasmuch as the range of sleep onset times was 6.5 to 51.5 min in the sleep extension condition and 7 min to 36 min in the advanced shift condition. In the E condition extra sleep was accumulated at a time when the subjects would have ordinarily been awake. The surplus in terms of each stage of sleep above amounts in the H condition comprised 75 min of Stage 2, 28 min of Stage REM, 19 min of Stage 1, 9 min of Stage 3, and 4.6 min of Stage 4. The time spent awake on nights of

ALTERED SLEEP PATTERNS AND PERFORMANCE IN LONG SLEEPERS

~postsleep a

4.01 b

Imidday ~Jeorly evening

>- 3.9

11 tll

,,o t/") w

.iI

< u I-

10

LLI ill

9

.iI LLI

,:0

8

Z

7

181

Z< O ,

6

3.83.73.63.53.4 3.33.2

II

3.1 3,0

I I

6V2-7~/2hr 9V2-10~/2hr 121/2-131/2 hr D DS "-- H ---" AS E

6V2-71/2hr 91/2-10V2hr D DS " - H ---" A

12V2-13~/2hr E

SLEEP CONDITION FIG. 2. Misses and 8 on the vigilance task after extended (E), advanced-shift (AS), habitual (H), deprivation (D), and delayed-shift (DS) sleep conditions. than found for 7 - 8 hr sleepers [26] and 55 min less compared with the reported value in younger subjects ( 1 7 - 1 9 years of age) not as extreme in their patterns of long sleep [32]. The wide variability for reported values of Stage 4 [5] and the large ranges at all age groups quite possibly reflect as yet undelineated factors that fluctuate with age and among individuals [33]. It can be seen, however, from Table 2 that the amount of Stage 4 was of similar duration for the sleep deprivation and extended sleep conditions which differed in length by an aver,age of 5.1 hr, reflecting both its independence from total sleep time and its noncircadian distribution during the first portion of sleep.

extended sleep was divided between an average of 38.5 min wakefulness after failing asleep and a mean sleep latency of 23.3 min. The range of total sleep times in the E condition was 10.6-11.8 hr. In the habitual condition the means for total sleep time and sleep Stages 1, REM, and 2 are very close to those presented by Webb and Agnew [32] in a normative study of high school seniors, who characteristically slept 8 1/2 hr or more. Stage 4 was entirely absent in 5 of the nights involving 2 subjects and varied considerably between subjects as indicated by the large standard deviations. The mean number of min of Stage 4 reported in the present study for the habitual sleep condition is almost 30 min less

TABLE 3 SUMMARY OF COMPARISONS

ON VIGILANCE TASK MEASURES

Comparisons Reaction time was shorter under:

Misses were fewer under:

tSwas higher under:

Postsleep H versus D DS AS E E versus AS H versus D DS AS E H versus D DS AS E

0.01 0.01 0.01 0.01 ns 0.02 0.05 0.02 0.01 0.02 0.01 0.01 0.05

Significance levels Midday Evening 0.02 0.02 0.02 0.01 0.02 ns ns ns ns ns ns ns ns

ns ns ns ns ns 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01

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TAUB AND BERGER TABLE 4 - SLEEP PARAMETERS OF THE LONG SLEEPERS IN THE DIFFERENT CONDITIONS (MEAN -+

STANDARDDEVIATION) Condition

6~--7½ Sleep Deprivation Sleep Variables Total sleep (hr) Totaltimeawake(min) Sleep stage duration (min) Stage 1 Stage2 Stage3 Stage4 StageREM Time to sleep onset Terminal wakefulness No awakenings Transitions to Stage 1 Totaitransitions

Mean

SD

6.2 0.3 18.5 11.5 17.4 178.8 45.2 21.6 107.9 10.2 3.5 7.8 15.6 101.0

9½-10½ Delayed Shift Mean

SD

9.1 0.4 4 4 . 0 18.4

Habitual Mean

SD

9.1 0.3 3 1 . 7 16.0

Advanced Shift Mean

9.0 0.3 42.1 2 4 . 5

7.3 37.3 15.1 3 1 . 0 11.8 42.2 25.2 293.2 38.8 297.1 20.9 332.6 11.6 3 6 . 8 17.3 4 8 . 7 21.4 42.0 22.1 2 1 . 9 12.8 17.1 16.5 11.8 20.7 155.4 46.7 150.6 28.5 112.1 6.6 8.8 5.2 17.2 10.0 18.6 3.1 2 0 . 4 18.0 4.6 4.8 3.1 3.7 18.4 14.1 15.8 9.7 19.3 5.7 3 2 . 0 14.0 25.6 11.7 3 3 . 4 32.4 153.2 34.6 143.1 34.0 159.5

Correlational Analyses Total sleep time, wakefulness, time spent in each sleep stage, and the other variables extracted from the physiological records shown in Table 2 were correlated with vigilance performance measures of reaction time, 6, and misses. Two sets of within treatment correlation coefficients were calculated between the physiological sleep variables and the subsequent waking behavioral variables. One set was calculated using absolute amounts of time spent in the various sleep stages; the other set of correlations was computed for percentages of total time spent asleep for the various sleep stages. The correlations were generally low, the range of values for the coefficients was widely dispersed and the number of statistically significant correlations was no greater than expected by chance when a large series of statistics is computed [21 ]. DISCUSSION The findings of the present investigation indicate that in habitual long sleepers, a 3 hr advance or delay and a 3 hr extension on deprivation of an accustomed sleeping period all result in generally equivalent degrees of impaired performance. The present results coincide with findings of behavioral deficits which followed 3 hr alterations in the timing and duration of sleep in subjects who regularly retired at midnight and awakened at 8:00 a.m. [25]. Decreased behavioral efficiency was also observed in another study of habitual 1 2 : 0 0 - 8 : 0 0 a.m. sleepers following 2 - 4 hr advances and delays of their regular 8 hr of sleep [27]. The convergent evidence from these experiments on both the long and 7 - 8 hr sleepers indicate that optimal levels of certain behavioral functions as exemplified by vigilance are highly dependent upon maintenance of an established temporal rhythm of sleep and wakefulness. The vigilance decrement which followed manipulated sleep patterns is perhaps itself indicative of the brain's incapacity to subsequently maintain effectiveness in re-

SD

12½-13½ Extended Sleep M e a n SD 11.3 0.4 7 5 . 3 19.2

21.6 5 0 . 0 12.7 32.3 372.2 28.5 11.4 5 7 . 2 16.0 12.6 21.7 11.1 41.2 178.7 23.4 10.4 2 3 . 3 16.2 2.7 13.5 10.4 10.5 2 4 . 2 10.7 13.7 37.9 8.2 36.5 180.8 29.0

sponse to changes of environmental stimulation. Certain limitations on sustained performance are imposed by the inherent organization of the nervous system [7] and it can be reasonably inferred that behavioral efficiency is further limited by interference with the natural sleep-wakefulness sequence. Natural selection must then favor those adaptations that increase an individuals level of vigilance amongst which include a circadian sleep-waking pattern followed by the majority of avian and mammalian species. The disruption of human sleep cycles following rapid time zone displacement causes transient behavioral deficits and relatively more persistent desynchronization of circadian performance rhythms [ 13,t 4]. Even comparatively moderate modifications in the sleep-wakefulness continuum as in the present investigation further demonstrates the importance of maintaining equillibrium in biological rhythms for optimal levels of behavioral response capacities. Until relatively recently there has been little critical examination of the circadian or 24 hr character of the human sleep-waking cycle [10] with respect to its biological adaptive aspects. Yet perhaps the most ubiquitous property concerning the organization of mammalian sleep and other behavioral states is their 24 hr periodicity [2]. Overt behavioral activity occurs at those times of day which have proven most favorable for survival in the evolutionary history of a species. In accordance with evidence from the present investigation and previous findings of a similar nature [25,27] it can be reasonably assumed that for the young human adult peak behavioral efficiency is closely related to the daily occurrence of a regular time for sleep [1]. The detrimental effects produced by altering sleep duration or temporally displacing it into other than an accustomed time period emphasizes the importance of fixed sleeping schedules in assessing the human sleep requirement. Based upon the lack of clearly discernible relations and significant correlations between the electroencephalo-

ALTERED SLEEP PATTERNS AND PERFORMANCE IN LONG SLEEPERS graphic measures and waking behavioral variables, it seems unlikely that degraded performance following the altered sleeping conditions was a specific as opposed to general consequence of changes in sleep duration or any other electrophysiological parameter of sleep. The trend of the decrement in these functions did not covary to any great degree with sleep physiology. It is, however, possible that measures of sleep physiology would yield closer correlations with performance variables if subjects slept under modified sleeping regimes for many consecutive nights. But at present it appears reasonable to conclude that deficient functioning following acute shifts in the length and timing of sleep is largely the result of wakefulness being imposed upon a customary period for sleeping or sleep occurring during a state of normally being awake [30]. Behavioral effects of acute sleep pattern variation and perhaps of more chronically modified sleeping regimes [34], although possibly indpendent of changes in sleep physiology, might be more closely related to as yet u n k n o w n alterations in other biophysiological substrata. There are diurnal variations in adrenocorticotrophic hormone and cortisol [18,35] the secretion of which can be altered by acute reversal of the sleep-wakefulness cycle [36]. There is also evidence that brain (CNS) biogenic amines are closely involved in the regulation of sleep and

183

vigilance [19]. Studies more elaborate in nature than the present remain to be conducted with regard to elucidating the covariation between sleep cycle variables, waking psychophysiology, hormonal secretory rhythms, CNS biogenic amines and human performance. An assumption guiding the research of many Russian scientists [see 15] is conceptualization of the sleep-waking rhythm as a profoundly conditioned set of reflexes, largely habitual, such that integrity of wakeful functions requires perpetuation of the rhythm [17]. From what is known about the dysrhythmic effects due to time displaced sleep schedules such an assumption seems a plausible one. As previously shown by Taub and Berger [25,27] for 7 - 8 hr sleepers with stable sleeping routines, peak behavioral efficiency in long sleepers also appears continent upon a delicately maintained balance between sleep and wakefulness. That sleep-activity rhythmicity is a critical feature for behavioral adaptation remains an hypothesis with generality to a finite segment of the human population thus far studied. Research concerning this hypothesis is required on a much wider scale from evidence derived of experimentally altered sleep patterns in various age groups and in comparative studies between individuals with differing sleep characteristics.

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