Pierre Gagnon, Joseph De Koninck, and Roger Broughton. Schools of ..... delta chez l'humain soit gouvernee par un rythme ultradien approximatif de 12 h.
Sleep. 8(2):118--128
© 1985 Raven Press, New York
Reappearance of Electroencephalogram Slow Waves in Extended Sleep with Delayed Bedtime Pierre Gagnon, Joseph De Koninck, and Roger Broughton Schools of Psychology and Medicine, University of Ottawa, Ottawa, Ontario, Canada
Summary: A three-part study using prolonged nights of sleep was undertaken to verify Broughton's hypothesis of an approximate 12-h ultradrian rhythm of human slow wave sleep (SWS). Part I consisted of2 8-h adaptation nights followed by a prolonged 15-h night of sleep with bedtime at midnight. A significant return of SWS occurred 12 hand 32 min after the first appearance of SWS. In part II, after 1 adaptation night, subjects were asked to sleep for 15 h but bedtime was delayed until 0400 h. A two-peak return of SWS was observed with a first significant return at 1228 h and a second significant return at 1745 h (i.e., 13 h and 32 min after the first appearance of SWS). In part III, bedtime was again delayed to 0400 h; but subjects were given 3 nights to adapt before the 15 h extended sleep. A single significant return of SWS was then observed at 1656 h, i.e., 12 hand 24 min after the first appearance with no peaks around 1200 h, thus exhibiting the same pattern as in part I. These results suggest that the return of SWS seen normally between 1200 and 1500 h is relatively well entrenched since it remained present in the extended night following sudden bedtime delay. It appears, moreover, that SWS does follow a bimodal 12-h rhythm, which is seen immediately upon extended delayed sleep and can be fully phase-shifted with habituation. Key Words: Slow wave sleep-Extended sleep.
In an article on biorhythmic variations in consciousness and related psychological and physiological functions, Broughton (I) hypothesized a functional link between the periods from about midnight to 0300 h and from 1300 to 1500 h that involved slow wave sleep (SWS) mechanisms. The proposal was based on two groups of observations. On the one hand, the early post onset sleep period is characterized by intense SWS, that is, "stage 3 and 4 generally exhibit a maximum (' acrophase' or statistically fitted peaks) every 24 hours at 1:00-3:00 a. m. , ... " (ref. 1, p. 227). This early night period is also characterized by performance deficits in perceptual tasks (2), psychomotor tasks (3), and in work parameters (4,5), as well as by lowering of psychological functions exemplified by confusion, disorientation, and re-
Accepted for publication February 1985. Address correspondence and reprint requests to Dr. P. Gagnon at Centre Hospitalier Pierre-Janet, 20 rue Pharand, Hull, Quebec, Canada J9A lK7.
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EEG SLOW WAVES IN EXTENDED SLEEP
119
trograde amnesia (6,7). On the other hand, the early to mid afternoon period is also characterized by a decrease in performance and psychological functions usually referred to as the "postlunch dip" (5,8-12). This decrease appears to be independent of food intake (Blake, as cited in ref. 9) or of core body temperature (13). Napping also occurs preferentially in this afternoon period. Some studies have also indicated an increase in electroencephalogram (EEG) delta activity during daytime sleep recording at this time (14,15). Broughton (1) noted that this diurnal decrease in vigilance was about 1800 out of phase with the lowest nocturnal level of vigilance found at around midnight to 0300 h. He proposed the existence of a 2/day biorhythmic human sleep tendency and concluded, "The findings considered together suggest the persistence of a bipolar 12-hour ultradian NREM sleep mechanism for reduced vigilance ... the postlunch dip might be considered a subharmonic of the normal circadian distribution of delta sleep" (ref. 1, p. 229). This notion would predict that, if sleep were prolonged beyond the usual 7 to 8 h and extended into the postlunch dip period (i.e., exceeded 12 h of sleep), SWS should reappear. Such a reappearance of SWS after an extended period of sleep has more recently been noted by researchers studying spontaneous termination of sleep (16) or in sleep under free-running conditions (17). We decided to examine this phenomenon more systematically by selecting normal nonsleep-deprived subjects who could extend their sleep by up to 100%. It was found that extended sleep to 15 h showed SWS to follow a bimodal distribution (18). We observed in five young adults that SWS appeared in the first hours past midnight, then gradually disappeared and then returned a little over 12 h later. With bedtime at midnight, reappearance of SWS occurred at around 1200-1500 h the next day. This finding lent support to the proposed 12-h ultradian NREM sleep mechanism superimposed on the 90-min basic rest activity cycle (BRAC) (1). The present experiment was designed to replicate the previous finding and to test more completely Broughton's notion by clarifying whether the SWS return is (a) related to a fixed circadian periodicity of delta activity normally in the period between l300 and 1500 h, or (b) is attributable to the elapsed sleep time of 12 h between the two SWS peaks. For this purpose, prolonged nights of 15 h of sleep were again used but with the added feature of a delayed bedtime. It was reasoned that by delaying bedtime by 4 h, one of the following three events would occur: (a) the SWS return would be moved immediately and fully to 4 h later, hence supporting the notion that SWS returns after 12 h of prior SWS; (b) the return of SWS would still manifest itself between 1300 and 1500 h, thus supporting the hypothesis of a fixed circadian biorhythm of afternoon delta activity; or (c) the return of SWS would be delayed by the duration of the shift, but only after subjects had had time to habituate themselves to their new delayed bedtime. The first outcome would provide the best support to Broughton's notion while the third would still be compatible with its predictions.
METHODS Subjects Subjects were chosen from a pool of 450 undergraduate students who had answered a questionnaire on their sleep habits. Preference was accorded to those who reported usual sleep durations between 7 and 9 h, had no difficulty in falling or remaining asleep, and who were occasionally able to remain asleep for;:,: 12 h. No subject suffered from hypersomnia and none was taking central nervous system active medication. Sleep, Vol. 8, No.2, 1985
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The study was divided into three parts. All subjects slept alone in single dark room devoid of any time cues. Design Part I. Ten subjects (8 women, 2 men), aged 18-20 years, slept in the laboratory for 3 consecutive nights. The first night served for adaptation, and the second was the baseline night. On these 2 nights lights were turned off at midnight and subjects were allowed to sleep 7-8 h. On the third night they were asked to sleep 15 h consecutively; again, lights were turned off at midnight. Part II. The same 10 subjects returned to the laboratory after about a week and slept for 2 consecutive nights. On the first night, they went to sleep at midnight and were again allowed to sleep 7-8 h. On the second night, bedtime was moved to 0400 h and subjects were asked to extend their sleep until 1900 h. Part III. Ten different subjects (6 women, 4 men), aged 17-22 years, were selected. They slept in the laboratory for 4 consecutive nights. On the first 3 nights subjects went to bed at 0400 h and were allowed to sleep 7-8 h until 1100--1200 h. A prolonged night of 15 h of sleep then followed, with bedtime remaining at 0400 h. Physiological measures The EEG C3-Al, electrooculogram (EOG) , and submental electromyogram (EMG) were monitored using standard Rechtschaffen-Kales procedures (19). Records were later analyzed using the same manuals' scoring criteria. Reliability was assessed by analysis of the independent blind scoring of two judges over 3-h segments of records, which were randomly ordered and presented to the judges without information as to their time of occurrence. Correlations for all measurements reported here were > 0.80. Psychological measures At each recording session, subjects answered a modified version (20) of the Nowlis mood adjective checklist both before sleep and upon awakening. In addition, each morning they answered a questionnaire on the subjective quality of their previous night's sleep (21). The Minnesota Multiphasic Personality Inventory (MMPI) and the Eysenck Personality Inventory (EPI) were administered to each subject. Personality measures were within normal limits on all scales of both the MMPI and the EPI. When compared with other types of sleepers (22-25), these subjects, by their ability to extend sleep, could be described as good long sleepers with a tendency to be of the evening type. RESULTS
Part I It can be seen from the hypnograms in Fig. 1 that 8 of 10 subjects were able to extend their sleep for the full 15 h, and all experienced a return of SWS. Comparisons by analysis of variance between the first 7 h of both the baseline and the prolonged nights showed no significant difference on any sleep dimension. This suggested that the extended sleep patterns of the subjects after the first 7 h represent a true reflection of the extra hours spent asleep and were not an effect of prior partial sleep deprivation. Table 1 shows the average length of time spent in each sleep stage by 3-h units of sleep. Time spent in stages 3 and 4 showed a consistent drop from the first to the fourth unit, with an increase for both slow wave stages in the fifth unit. SWS first appeared at the beginning of the night on the average at 29 min past midnight (0029 h ± 7.12 min). The reappearance of SWS was, on Sleep. Vol. 8. No.2. 1985
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EEG SLOW WA VES IN EXTENDED SLEEP
00
01
02
03
04
06
07
HOURS FIG. 1.
08
09
10
"
12
13
14
15
Hypnograms of the extended sleep of eight subjects in part I (bedtime at midnight).
average, at 3 min past 1300 h (1303 h ± 47.04 min), i.e., 12 hand 32 min after its first appearance. Analysis of variance showed this return of SWS to be significant at p < 0.001. A polynomial trend analysis was performed on the distribution of SWS across 10 90min segments of the 15 h. Table 2 presents the analysis of variance summarizing these trends. Only three components significantly contributed toward the explanation of the distribution of SWS throughout the prolonged night of part I: quadratic, 47% of variance (p < 0.001); linear, 45% (p < 0.001); and cubic, 4% (p < 0.004). Together, they accounted for 96% of the variance of the polynomial trend. The linear component indicated a tendency to have less SWS with increasing sleep duration. The quadratic component also indicated a progressive diminution of SWS with prolonged sleep but showed an increase Sleep, Vol, 8, No.2, 1985
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TABLE 1. Average duration" of each stage by 3-h units of sleep in parts I, II, and III
Stage Stage Stage Stage Stage Stage
W 1 REM 2 3 4
Unit 1
Unit 2
II
III
II
III
II
0 2 43 72 119 16 10 S3 6
1 1 60 93 16
0 5 57 99 14 5
1 2 5 5 70 48 94 102 7 9 2 13
11 10 5
17 69 25 53
4 25 60 24 58
9 5 25
11
Unit 4
Unit 3
Unit 5
II
III
17 23 42 13 10 7 S7 4S 42 85 100 82 6 1 4 1 0 4
60 17 27
III
72 2 0
67 7 28 68 6 11
II
III
59 7 28 70 4 13
51 9 33 65 7 15
"Duration measured in minutes.
at the end ofthe prolonged night. The cubic component again indicated an increase of SWS at the end of the night. When the last 3 h of the prolonged night were taken out of the analysis, 70% of the variance of the polynomial trend was explained by the linear component (a 25% increase over the total night), whereas the quadratic component was reduced from 47 to 24% (a drop of 23%). It can be seen from this calculation that the return of SWS explained at least 23 to 25% of the trend. Figure 2 illustrates the third-order polynomial trend distribution of SWS and illustrates clearly the return of SWS. Part II In part II, bedtime was delayed by 4 h to observe the effect of this shift on the timing of SWS reappearance. Again, 8 of 10 subjects were able to extend their sleep beyond 12 h (Fig. 3). However, SWS reappearance exhibited a mixed pattern with a first return at 28 min past noon (1228 h ± 33.59 min) and a second return at 1745; i.e., 13 hand 32 min after the first appearance of SWS (0432 h ± 9.89 min). Analyses of variance showed both returns to be significant at 0.001. The polynomial trend analysis (Table 3) showed four components to contribute significantly towards explaining the distribution of SWS throughout the prolonged night: linear, TABLE 2. Polynomial trend analysis of the distribution of slow wave sleep over 15 h of sleep in part I Source
df
Intersegment (n = 9) Linear component Deviation Quadratic component Deviation Cubic component Deviation Quartic component Deviation Quintic component Deviation Intrasegment (n = 70) Total (n = 79)
1 8 1 7 1 6 1 5 1 4
MS
F (ratio)
F (probability)
16,831.01
1,870.11
20.09
0.001
7,653.41 9,177.59 7,865.45 1,312.13 823.86 488.27 60.44 427.82 14.23 413.59
7,653.41 1,147.19 7,865.45 187.44 823.86 81.37 60.44 85.56 14.23 103.36
82.21 12.32 84.49 2.01 8.85 0.87 0.64 0.91 0.15 1.11
0.001 0.001 0.001 0.06 0.004 0.518 0.423 0.473 0.695 0.358
6,515.87
93.08
SS
23,346.88
SS, sum of squares; MS, mean square. Sleep, Vol. 8, No.2, 1985
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EEG SLOW WAVES IN EXTENDED SLEEP b
Part I
Part ][
Part ][
FIG. 2. Graphic representation of the distribution of slow wave sleep (SWS) through polynomial trend analysis. a, time; b, SWS.
54% (p < 0.001); quadratic, 31% (p < 0.001); cubic, 4% (p < 0.007); and quartic, 2% (p < 0.03). Together, they account for 91% of the variance of the polynomial trend. The linear component indicated a tendency to find less SWS with increased sleep time. The quadratic component also showed a progressive diminution of SWS with prolonged sleep but showed an increase at the end of the night. The cubic component indicated a high amount of SWS in the first part of the night followed by a drop, and then an increase at the end of the night. Finally, the quartic component indicated a high amount of SWS around the middle of the prolonged night, i.e., around noon. This polynomial equation adequately describes the bimodality of SWS return found in the prolonged night of this protocol. Indeed, when SWS occurring between 1130 and 1300 h was withdrawn in a second analysis, the quartic component ceased to be significant (p = 0.73). The first return of SWS around noon was then responsible for the quartic component being significant in the original analysis. Similarly, when the second return of SWS was removed, 74% of the variance was explained by the linear component (an increase of 20%), whereas the quadratic term dropped from 31 to 23%. The second return of SWS, then, influenced the trend by diminishing its linearity. Both returns of SWS can clearly be seen in Fig. 2, which represents this fourth-order polynomial trend distribution.
Part III To clarify the mixed pattern of SWS found in part II, 10 new subjects were selected. The bedtime was again moved to 0400 h, but this time subjects were given 4 consecutive phase-delayed nights to habituate. Six subjects of 10 were able to extend their sleep (Fig. 4). Sleep stage comparisons between the first 7 h of the last habituation night and the first 7 h of the prolonged night showed no significant difference. This indicates that by then subjects had fully habituated to their delayed sleep schedule. Table 1 presents the average time (min) spent in each stage by 3-h units of sleep. Time spent in stages 3 and 4 showed a constant drop from the first to the fourth unit with an increase for both stages in the fifth unit. On the average, SWS first appeared at 0432 h (± 12 min) while the reappearance occurred on the average at 1656 h (± 36 min), i.e., 12 hand 24 min after the first appearance. An analysis of variance showed this return to be significant at p < 0.001. In the polynomial trend analysis (Table 4), three components significantly contributed towards explaining the distribution of SWS throughout the prolonged night of part III: linear, 44% (p < 0.001); quadratic, 40% (p < 0.001); and cubic, 3% (p < 0.05). Together, they accounted for 87% of the variance of the polynomial trend. As in part I, the linear Sleep, Vol. 8, No.2, 1985
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"~l fllllJ~~L/\~.1I-/~-llLL__1_/-~\_ w
04
05
06
07
08
09
10
"
12
13
14
15
16
17
18
19
HOURS FIG. 3. Hypnograms of the extended sleep of eight subjects in part II (bedtime acutely delayed to 0400 h, no adaptation).
component of part III indicated a tendency to find less SWS as sleep time increased; the quadratic component also indicated a progressive diminution of SWS with, however, an increase at the end of the night. And the cubic component, as well, pointed toward an increase of SWS at the end of the night. When the last 3 h of the prolonged night were taken out of the polynomial trend analysis, 79% of the variance of the polynomial trend was explained by the linear component (an increase of 25%), whereas the quadratic component was reduced from 40 to 17% (a drop of 23%). From this manipulation, it can be seen that the return of SWS explained at least 23-25% of the trend. Again, the return of SWS can clearly be seen in Fig. 2. Sleep, Vol. 8, No.2, 1985
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EEG SLOW WAVES IN EXTENDED SLEEP TABLE 3. Polynomial trend analysis of the distribution of slow wave sleep over 15 h of sleep in part II Source
df
Intersegment (n = 9) Linear component Deviation Quadratic component Deviation Cubic component Deviation Quartic component Deviation Quintic component Deviation Intrasegment (n = 70) Total (n = 79)
1 8 1 7 1 6 1 5 1 4
SS
MS
F (ratio)
F (probability)
0.001 0.001 0.001 0.001 0.001 0.007 0.002 0.03 0.004 0.09 0.005
16,015.51
1,779.50
22.93
8,593.24 7,422.27 5,013.82 2,408.44 585.41 1,823.03 340.46 1,482.57 222.76 1,259.80
8,593.24 927.78 5,013.82 344.06 585.41 303.83 340.46 296.51 282.76 314.95
110.74 11.95 64.61 4.43 7.54 3.91 4.38 3.82 2.87 4.05
5,431.87
77.59
21,447.38
SS, sum of squares; MS, mean square.
HOURS
FIG. 4.
Hypnograms of the postadapted extended sleep for subjects in part III (bedtime delayed to 0400 h, 4 nights of adaptation). Sleep, Vol. 8, No.2, 1985
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TABLE 4. Polynomial trend analysis of the distribution of slow wave sleep over 15 h of sleep in part III Source
df
lntersegment (n = 9) Linear component Deviation Quadratic component Deviation Cubic component Deviation Quartic component Deviation Quintic component Deviation Intrasegment (n = 70)
1 8 1 7 1 6 1 5 1 4
ss
MS
9,319.68
F (ratio)
F (probability)
1,035.52
12.15
0.001
4,076.39 5,243.28 3,722.33 1,520.94 333.41 1,187.52 305.32 882.19 79.25 802.94
4,076.39 655.41 3,722.33 217.27 333.41 197.92 305.32 176.43 79.25 200.73
47.85 7.69 43.69 2.55 3.91 2.32 3.58 2.07 0.93 2.35
0.001 0.001 0.001 0.025 0.053 0.046 0.065 0.084 0.339 0.066
4,259.16
85.18
Total (n = 79) SS, sum of squares; MS, mean square.
For each part of the study, subjective assessments of quality of sleep were generally lower following the prolonged nights than following the baseline nights. Subjects mainly complained of feeling less "in shape" (part I, p < 0.04; part II, p < 0.03; part III: p < 0.01) and experiencing more awakening (part I, p < 0.03; part II, p < 0.03; part III, p < 0.01). On the other hand, the results on the mood adjective checklist indicate no postsleep difference between normal and prolonged nights. DISCUSSION
The results confirm the phenomenon of the reappearance of SWS in sufficiently extended sleep periods, which was first observed in our earlier study (18). They also indicate that the return of SWS generally between 1200 and 1500 h is relatively well entrenched, since it remained present in the single extended night following sudden delay of sleep onset. It appears also, however, that SWS follows a bimodal 12-h ultradian distribution which can be shifted with habituation. This supports Broughton's notion of an -12-h bipolar ultradian rhythm of SWS. The results from parts I and III indicate that we might more precisely speak of a 12.5rather than a 12-h rhythm. In fact the return of SWS occurred on the average 12 hand 32 min and 12 hand 24 min after the first appearance of SWS in part I and part III, respectively. These figures are consistent with the findings of our previous study (18). This extra half hour may be responsible for the lengthening of the circadian sleep-waking period to 24.525 h in the free-running state. Alternatively, it is possible that the delay between peaks of the -12-h SWS rhythm is 12.5 h because this rhythm is allowed to free-run within each day, whereas the endogenous 24.5-h circadian sleep/wake rhythm remains entrained at 24 h (26). The return of SWS also adds new dimensions to previously held beliefs. First, it does not support the contention of a simple decrease with the amount of time spent asleep (27). Second, the return of SWS after about 12 h cannot be explained by the concept that the amount and timing of SWS is a simple reflection of the amount of prior wakefulness (27). Rather, the return indicates the presence of an -12-h biorhythm of SWS, which may be conceived of as either a harmonic or an integral part of the circadian SWS biorhythm. It Sleep, Vol. 8. No.2, 1985
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may also represent one period of an apparent propensity of biological rhythms to show multiple integer ratios within the basic circadian 24-h periodicity (26). Finally, these findings lead us to believe that, in normal entrained conditions, the phenomenon of afternoon napping may reflect a biological propensity to re-enter the psychobiological state that accompanies SWS. As well, the postlunch dip may be thought of as a behavioural expression of physiological sleep tendency changes relating to pressure toward that state. Although less common in North America, midday sleep is quite prevalent elsewhere. Webb (ref. 28, p. 21) rightly states, "How natural is the uniphasic pattern under which we 'typically' operate and certainly assume to be the norm? .. in many subtropical and tropical regions the noonday siesta and resulting biphasic pattern is the normative sleep pattern."
REFERENCES 1. Broughton R. Biorhythmic variations in consciousness and psychological functions. Can Psychol Rev 1975;16:217-39. 2. Feltin M, Broughton R. Differential effects of arousal from slow wave versus REM sleep. Psychophysiology 1968;5:231. 3. Fort A, Mills IN. Influence of sleep, lack of sleep and circadian rhythm on short psychometric tests. In: Colquhoun WP, ed. Aspects of human efficiency: diurnal rhythm and loss of sleep. London: English University Press, 1972. 4. Browne RC. The day and night performance of teleprinter switchboard operators. Occup Psychol 1949;23:16. 5. Bjerner B, Holm A, Swensson A. Diurnal variation in mental performance. Br J lnd Med 1955;21: 10310. 6. Broughton R. Sleep disorders: disorder of arousal. Science 1968;159:107~8. 7. Broughton R. Confusional sleep disorder: interrelationship with memory consolidation and retrieval in sleep. In: Boag TJ, Sampbell D, eds. A triurne concept of the brain and behavior. Toronto: University of Toronto Press, 1973. 8. Blake MUF. Time of day effects on performance in a range of tasks. Psychonomic Science 1967;9:34950. 9. Hockey GRI, Colquhoun WP. Diurnal variation in performance: a review. In: Colquhoun WP, ed. Aspects of human efficiency: diurnal rhythm and loss of sleep. London: English University Press, 1972. 10. Muscio B. Fluctuation in mental efficiency. BrJ PsychoI1920;10:327-44. 11. Wyatt S, Weston HC. A performance test under industrial conditions. Br J Psychol 1920;10:293-309. 12. Laird DA. Relative performance of college students as conditioned by time of day and day of week. J Exp Psychol 1925;8:5~3. 13. Colquhoun WP. Biological rhythms and human performance. London: Academic Press, 1971. 14. Webb WB. Sleep behavior as a biorhythm. In: Colquhoun WP, ed. Biological rhythms and human performance. London: Academic Press, 1971. 15. Hume KI, Mills IN. Rhythms of REM and slow-wave sleep in subjects living on abnormal time schedules. Waking Sleeping 1977;1:291-6. 16. Webb WB. The spontaneous termination of sleep [Abstract]. Sleep Res 1977;6:118. 17. Weitzman ED, Czeisler CA, Zimmerman IC, Ronda 1M. Timing of REM andd stage 3 and 4 during temporal isolation in man. Sleep 1980;2:391-408. 18. Gagnon P, De Koninck I. Reappearance of EEG slow waves in extended sleep. Electroencephalogr Clin Neurophysiol 1984;58: 155-7. 19. Rechtschaffen A, Kales A, eds. A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects. Washington, DC: Public Health Service. U.S. Government Printing Office, 1968. 20. De Koninck I, Koulack D. Dream content and adaptation to a stressful situation. J Abnorm Psychol 1975;84:25~0.
21. De Koninck I, Gagnon P, Lallier S. Sleep positions in the young adult and their relationship with the subjective quality of sleep. Sleep 1983;6:52-9. 22. Horne lA, Ostberg O. Individual differences in human circadian rhythms. Bioi PsychoI1977;5:17~90. 23. Hartmann E. The functions of sleep. New Haven Yale University Press, 1973. 24. Webb WB, Agnew HW. Sleep stage characteristics of long and short sleepers. Science 1970;168:146-7 25. Monroe U. Psychological and physiological differences between good and poor sleepers. J Abnorm Psychol 1967;72:255-64. Sleep, Vol. 8, No.2, 1985
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26. Broughton, RJ. Three central issues concerning ultradian rhythms. In: Schulz H, Lavie P., eds. Ultradian rhythms. Berlin: Springer-Verlag, 1985 (in press). 27. Webb WB, Agnew HW. Stage 4 sleep: influence of time course variables. Science 1971;174:1354-6. 28. Webb WB. Sleep, the gentle tyrant. Englewood Cliffs: Prentice-Hall, 1975.
Reapparition des ondes lentes pendant Ie sommeiJ prolonge avec coucher dephase Une etude tri-partite a ete entreprise afin de verifier l'hypothese de Broughton l qui veut que l'activite delta chez l'humain soit gouvernee par un rythme ultradien approximatif de 12 h. La premieere partie de l'experience etait constituee de 2 nuits d'adaptation de 8 h sui vies d'une nuit prolongee de 15 h avec coucher a minuit. Un retour significatif de sommeil lent (SL) a ete enregistre 12 h et 32 min apres la premiere apparition de SL. La deuxieme partie etait constituee d'une nuit d'adaptation de 8 h suivie d'une nuit prolongee avec coucher dephase a 0400 h. Un retour mixte de SL a ete observe avec une premiere augmentation significative it 1228 h et une seconde a 1745 h (i.e., 13 h et 32 min apres la premiere apparition de SL). Dans la troisieme partie, Ie coucher a de nouveau ete dephase a0400 h mais cette fois les sujets ont eu trois nuits d'adaptation it ce nouveau regime avant la nuit prolongee. Un seul retour significatif de SL a ete observe it 1656 h, i.e., 12 h et 24 min apres la premiere apparition sans augmentation aux alentours de midi, demontrant ainsi Ie meme phenomene retrouve dans la premiere partie. Ces resultats suggerent que Ie retour de SL entre 1200 et 1500 h est un phCnomene relativement bien ancre puisqu'il y etait encore present lorsque Ie coucher a ete soudainement dephase. II semble, cependant, que Ie SL soit relie a un rythme bimodal d'approximativement 12 h qui peut etre dephase avec I' habituation.
Sleep. Vol. 8. No.2. 1985
