Homeostatic Changes in Slow Wave Sleep during Recovery Sleep ...

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For example, reanalysis of the data reported by Agnew et al. (1) shows that 90% of the. SWS (stage 3 and 4) lost during 2 consecutive nights of selective stage 4 ...
Sleep 10(6):600-605, Raven Press, Ltd., New York © 1987 Association of Professional Sleep Societies

Homeostatic Changes in Slow Wave Sleep during Recovery Sleep Following Restricted Nocturnal Sleep and Partial Slow Wave Sleep Recovery during an Afternoon Nap Andrew Tilley, Frank Donohoe, and Sharon Hensby Department of Psychology, University of Queensland, Queensland, Australia

Summary: There has been speculation and some evidence to suggest that certain fractions of sleep, notably, slow wave sleep (SWS), are under homeostatic control. In order to test this hypothesis, the sleep of eight subjects. was terminated on four different occasions after 50% of their normal baseline SWS levels had been obtained. An afternoon nap then followed during which 100%, 50%, or 25% of the SWS debt was reclaimed. A fourth condition contained no afternoon nap. The change in SWS from baseline during subsequent recovery sleep was directly related to the outstanding SWS debt, thus demonstrating that SWS is under homeostatic control. Key Words: Homeostasis-Slow wave sleep.

Loss of slow wave sleep (SWS), as a result of short-term selective or total sleep deprivation, is usually substantially reclaimed during subsequent recovery sleep (1-4). For example, reanalysis of the data reported by Agnew et al. (1) shows that 90% of the SWS (stage 3 and 4) lost during 2 consecutive nights of selective stage 4 sleep deprivation was recovered on a single recovery night. In a complementary fashion, BorMly et al. (4) have reported that 100% of the SWS lost following 1 night of sleep deprivation is reclaimed over the next 2 recovery nights. These results suggest that SWS may be under homeostatic control and that there may be a "need" for a set amount of SWS per day. Further incidental evidence supporting the notion that SWS is under homeostatic control is provided by studies showing that normal, baseline levels of SWS are usually preserved during restricted sleep regimes (5 -12) or if nocturnal sleep is replaced or augmented by naps (13,14,15). For example, the amount of SWS obtained during 7 nights of sleep restricted to 3 h/night was virtually the same as that which would have been obtained during 7 full night's sleep (5). The amount of SWS obtained in ten I-hour naps evenly spread over a 40-h period (0600 to 2200 h the next day) was exactly the same as would have been obtained in the single night's sleep which the naps replaced Address correspondence and reprint requests to Dr. A. Tilley at Department of Psychology, University of Queensland, St Lucia, Queensland 4067, Australia.

600

HOMEOSTATIC CHANGES IN SWS

601

(14). Finally, an afternoon nap has been shown to produce reductions in the amount of SWS during subsequent nocturnal sleep commensurate with the amount of SWS obtained during the nap (13, IS). The focus of the last study (1S) was on stage 4 sleep. It was reported that the reduction in nocturnal stage 4 sleep following an afternoon nap was greater than the amount of stage 4 sleep obtained in the nap. However, reanalysis of the data in terms of SWS shows that the reduction in the amount of SWS at night is equivalent to the amount obtained during an afternoon nap. The present experiment is a direct test of the hypothesized homeostasis of SWS. If SWS is under homeostatic control, then reducing the amount of SWS at night should result in homeostatic increases in SWS during subsequent sleep. Furthermore, it should be possible to supplement the recovery of the SWS debt by means of a daytime nap. For example, if SWS is restricted to SO% of baseline levels at night and 100% of the debt is repaid during a nap, the level of SWS should remain at near baseline levels during' 'recovery" sleep. If only SO% of the debt is repaid during a nap, then the level of SWS should rise 2S% above baseline levels during recovery sleep, and so on. In short, the amount of additional SWS obtained during recovery sleep should be a function of the amount of SWS debt. The present study was designed to test this hypothesis by systematically varying the amount of SWS debt repaid during an afternoon nap prior to recovery sleep. METHODS Eight healthy young women (mean age 20), all paid volunteers and students at the University of Queensland, took part in the study. Following three, nonrecording adaptation nights in the sleep laboratory, their sleep electroencephalographic (C4/Al electrode placements), electro-oculographic, and electro myographic activity was recorded for 2 consecutive nights to establish individual baseline sleep measures, which were averaged across the 2 baseline nights. Each subject returned to the sleep laboratory on four separate occasions, spaced at least one week apart. On each occasion, following 2 further adaptation nights, the subject's nocturnal sleep was continually monitored from lights out (the subject's habitual bedtime) and terminated after ~SO% of baseline SWS had been obtained. After being awakened, the subject arose, showered and dressed, and was supervised throughout the remainder of the experimental period. Most of the subjects spent their time in the psychology student's common room playing video games, studying, or chatting with other students. The restricted nocturnal sleep was then either supplemented by an afternoon nap, commencing at 1300 h, or followed by no nap at all. (The order of the four conditions was counterbalanced across subjects.) The afternoon nap was allowed to continue until ~100, ~SO, or ~2S% of the SWS debt had been repaid, at which point the subject was awakened, arose, dressed, and was supervised as before. The subject returned to bed in the early evening. Bedtime varied slightly from subject to subject and from condition to condition depending upon the total amount (pre-nap + post-nap) of wakefulness, which was fixed at around 16.S h, a normal daily amount. For example, if the subject had been woken at 0100 h after restricted sleep and had a l.S-h afternoon nap from 1300 to 1430 h~ the subject would have gone to bed at 1900 h. The subject was allowed to sleep ad libitum until awakening spontaneously the following morning.

Sleep, Vol. 10, No.6, 1987

A. TILLEY ET AL.

602

RESULTS

All sleep records were scored blind according to standard procedures (16). Entry into stage 2 sleep was taken as the point of sleep onset. Summary statistics for the various sleep measures are shown in Table 1. Table 2 shows the relationship between the amount of stage 2, SWS, REM, and total sleep time (TST) debt before recovery sleep and the change in these sleep measures from baseline during recovery sleep. TABLE 1. Sleep measures a for baseline sleep and four experimental conditions

Baseline Mean

Latency

Awake

Stage 1

Stage 2

Stage 3

16 (7)

3 (I)

8 (3)

225 (20)

22 (6)

0(0)

1 (2)

22 (12) 203

0(0)

0(0)

12 (6) 191

4 (2)

7 (2)

286 (55) 130

40 (II) -12

0(0)

3 (7)

16 (14) 209

0(0)

0(0)

10 (5) 199

2 (3)

7 (4)

308 (50) 116

44 (9) -10

0(0)

0(0)

17 (19)

0(0)

0(0)

2 (4)

100% Nap condition Restricted 14 (5) Mean Debt Nap 5 (2) Mean Debt Recovery 16 (4) Mean Debt 50% Nap condition Restricted 16 (8) Mean Debt Nap 5 (I) Mean Debt Recovery 14 (10) Mean Debt 25% Nap condition Restricted 12 (5) Mean Debt Nap 6 (2) Mean Debt Recovery Mean 11 (6) Debt No nap condition Restricted 13 (6) Mean Debt Recovery Mean 5 (2) Debt

Stage 4

SWS

REM

TST

84 (14)

106 (14)

118 (14)

457 (25)

8 (3) 14

46 (8) 38

54 (8) 52

2 (3) 116

79 (I5) 378

8 (5) 6

48 (8) -10

56 (10) -4

9 (7) 107

77 (15) 301

68 (16) 6

108 (22) -6

203 (26) 22

604 (53) 154

6 (2) 16

45 (7) 39

51 (7) 55

2 (4) 116

72 (21) 385

4 (3) 12

22 (4) 17

26 (5) 29

0(0) 116

36 (6) 349

88 (19) \3

132 (24) 3

190 (29) 44

637 (46) 169

7 (4) 15

47 (6) 37

54 (8) 52

4 (5) 114

75 (26) 382

7 (3) 201

2 (I) \3

13 (2) 24

15 (2) 37

0(0) 114

23 (4) 359

9 (6)

304 (53) 122

41 (9) -6

93 (20) 15

134 (23) 9

196 (19) 36

643 (58) 173

0(0)

4 (8)

35 (47) 190

7 (6) 15

43 (13) 41

50 (9) 56

2 (5) 116

91 (48) 366

5 (6)

7 (7)

334 (48) 81

40 (9) -3

108 (18) 17

148 (23) 14

184 (22) 50

673 (48) 150

SDs are shown in parentheses. Latency, time from lights out to sleep onset; awake, time awake following sleep onset; and TST, total sleep time. Negative numbers signify amount of debt overpaid. a Measured in minutes, rounded to nearest whole number.

Sleep, Vol. 10, No.6, 1987

603

HOMEOSTATIC CHANGES IN SWS

As shown in Table 1, the amount of SWS obtained during recovery sleep appears to be determined by the amount of SWS debt. In general, the greater the debt, the higher the amount of SWS during recovery sleep. Moreover, the change in SWS from baseline is more or less the same as the SWS debt, although there appears to be a slight overshoot in debt recovery in the 100% nap condition and a slight undershoot in the 50%, 25%, and no nap conditions. As shown in Table 2, overall, the SWS debt accounts for over 80% of the variance in the change in SWS from baseline during recovery sleep. This clearly indicates that SWS is under homeostatic control. The relationship between SWS debt and SWS change during recovery sleep is shown in Figure 1. It is possible that some of the change in SWS from baseline during recovery sleep may be due to the amount of prior TST (restricted sleep + nap), which significantly decreases across conditions from 156 min in the 100% nap condition to 91 min in the no nap condition (F328 = 5.56, P < 0.01). However, two stepwise, multiple regression analyses, with sirs debt and prior TST as predictors of SWS change, showed that prior TST added