POLYPHASIC SLEEP / WAKE PATTERNS AND THEIR SIGNIFICANCE TO VIGILANCE (*>
Jiirgen Z u l l e y and J o s e f B a i l e r
Max Planck I n s t i t u t e f o r P s y c h i a t r y , Munich, F.R.G.
SUMMARY
Under c o n d i t i o n s i n which no r e s t r a i n t s on s l e e p and minimized a l t e r n a t i v e s t o s l e e p a r e g i v e n , t h e sleep/wake p a t t e r n e x h i b i t s a p o l y p h a s i c d i s t r i b u t i o n • T h i s r e s u l t was found i n a study i n which 12 s u b j e c t s were p o l y g r a p h i c a l l y recorded i n two s e s s i o n s , each l a s t i n g 32 hours. During one s e s s i o n , t h e s u b j e c t s had c o n s t a n t b e d r e s t f o r t h e e n t i r e p e r i o d , w h i l e t h e o t h e r s e s s i o n s t a r t e d w i t h 8 hours o f s l e e p d e p r i v a t i o n , f o l l o w e d by constant bedrest. During t h e day, s u b j e c t s s l e p t a t average i n t e r v a l s o f 4 hours. A f t e r s l e e p d e p r i v a t i o n , t h e recovery s l e e p covered t h e time i n t e r v a l o f t h e f i r s t two nap episodes o f t h e c o n d i t i o n without s l e e p d e p r i v a - t i o n . Compared t o b a s e l i n e a l e r t n e s s , mood and performance a r e decreased d u r i n g these two c o n d i t i o n s . T h i s r e s u l t suggests t h e presence o f an u n d e r l y i n g p o l y p h a s i c placement o f s l e e p and wake- f u l n e s s . Under t h e g i v e n c o n d i t i o n , t h e r e a l i z a t i o n o f such a sleep/wake p a t t e r n leads t o o v e r s l e e p i n g , which r e s u l t s i n a de- t e r i o r a t i o n i n t h e p s y c h o l o g i c a l v a r i a b l e s .
I n t r o d u c t i o n
I n numerous s t u d i e s (e.g. Wever, 1979) i t has been concluded t h a t t h e human sleep/wake system i s governed by a c i r c a d i a n c l o c k mechanism. Such an u n d e r l y i n g r e g u l a t i o n has a l s o been assumed i n s t u d i e s which focused on t h e i n f l u e n c e o f t h e sleep/wake p a t t e r n on performance (Taub & Berger, 1973).
Other s t u d i e s i n t h i s c o n t e x t have analyzed t h e p s y c h o l o g i c a l consequences o f r e s t r i c t e d s l e e p (Mullaney e t a l . , 1983), t h e r e c u p e r a t i v e f u n c t i o n o f a nap a f t e r s l e e p d e p r i v a t i o n ( N a i t o h , 1981) and t h e i n f l u e n c e o f a d d i t i o n a l s l e e p g i v e n a s scheduled naps (Dinges e t a l . , 1981; Godbout & M o n t p l a i s i r , 1986).
(*) Dedicated t o P r o f .Dr. D. v. Zerssen on t h e o c c a s i o n o f h i s 60th b i r t h d a y
The i n i t i a l experiments o n l y c o n s i d e r e d c i r c a d i a n a s p e c t s o f s l e e p and wakefulness* T h e r e f o r e , i n t e r f e r i n g i n f l u e n c e s o f t h e c i r c a d i a n p a t t e r n were excluded. A d d i t i o n a l s l e e p episodes (naps) a r e such i n f l u e n c e s . By g i v i n g t h e experimental i n s t r u c t i o n " P l e a s e a v o i d napping", such s l e e p episodes were suppressed (Wever, 1979; p. 5 2 ) . I f s u b j e c t s s l e p t i n s p i t e o f t h e i n s t r u c t i o n s , t h e s e s l e e p episodes were not c o n s i d e r e d as s l e e p i n subse- quent d a t a a n a l y s i s .
R e c e n t l y i t has been shown t h a t t h e assumption o f a monophasic c i r c a d i a n sleep/wake m o d u l a t i o n has t o be q u e s t i o n e d (Campbell & Z u l l e y , 1985). S i n g l y by removing t h e i n s t r u c t i o n "Please a v o i d napping", t h e frequency o f napping i n c r e a s e s remarkably (Webb & Agnew, 1974). N e v e r t h e l e s s , experimental i n - s t r u c t i o n s a r e not t h e o n l y f a c t o r which can i n f l u e n c e t h e sequence o f s l e e p i n g and waking. Self-imposed b e h a v i o r o f t h e s u b j e c t s a l s o seems t o be e f f e c t i v e . S u b j e c t s who g e t t i r e d but do not want t o go t o s l e e p can choose an a l t e r n a t i v e b e h a v i o r t o s l e e p ( i . e . d r i n k i n g a cup o f c o f f e e o r d o i n g e x e r c i s e ) .
These a s p e c t s l e d t o experiments where t h e occurrence o f spontaneous s l e e p was f a v o r e d . I n t h e s e experiments no i n s t r u c t i o n s were g i v e n as t o when o r when not t o s l e e p . Furthermore, t h e s u b j e c t s had minimized behavio-
r a l a l t e r n a t i v e s t o s l e e p . I n such a " d i s e n t r a i n e d " environment a c l e a r bimodal d i s t r i b u t i o n o f s l e e p and wakefulness c o u l d be shown w i t h i n t h e c i r c a d i a n day (Cairpbell & Z u l l e y , 1985). T h i s supports t h e assumption t h a t t h e human sleep/wake system i s c h a r a c t e r i z e d not o n l y by a s t r o n g p r o p e n s i t y t o o b t a i n a s l e e p episode once p e r c i r c a d i a n day, but a l s o by a prominent tendency f o r a s h o r t e r s l e e p episode t o o c c u r halfway between s u c c e s s i v e
" n i g h t s l e e p " e p i s o d e s . T h i s phase p o s i t i o n would correspond t o t h e e a r l y a f t e r n o o n , a t i m e when t h e v a s t m a j o r i t y o f napping o c c u r s i n d a i l y l i f e
(Dinges, 1983; Ogunremi, 1978; Okudeira, 1983; Webb, 1978).
T h i s f i n d i n g has been i n t e r p r e t e d by assuming t h a t a decrease i n a p u t a - t i v e s l e e p t h r e s h o l d r e v e a l s a second, l e s s r o b u s t phase p o s i t i o n f o r s l e e p . I n r e a l l i f e such a decrease i n t h e s l e e p t h r e s h o l d may be due t o l i f e - s t y l e
( s t u d e n t s , g e r i a t r i c p o p u l a t i o n ) . I n r e s e a r c h such a decrease may be caused by experimental c o n d i t i o n s (no i n s t r u c t i o n s about when o r when not t o s l e e p ; minimized b e h a v i o r a l a l t e r n a t i v e s t o s l e e p ) .
A r e d u c t i o n o f motor a c t i v i t y caused by c o n s t a n t b e d r e s t would a d d i t i - o n a l l y decrease such a p u t a t i v e s l e e p t h r e s h o l d . I n such a study i t was shown t h a t napping o c c u r s more f r e q u e n t l y , l e a d i n g t o t h e assumption o f a
p o l y p h a s i c d i s t r i b u t i o n o f s l e e p and wakefulness i n t h e c i r c a d i a n day (Camp- b e l l , 1984). N e v e r t h e l e s s , t h e l i n k between t h e sleep/wake p a t t e r n and raood, a l e r t n e s s and performance has not y e t been examined under such c o n d i t i o n s . T h e r e f o r e , t h e q u e s t i o n a r i s e s t o what e x t e n t a spontaneous p o l y p h a s i c e x p r e s s i o n o f s l e e p and wakefulness would i n f l u e n c e t h e course o f nood, a l e r t n e s s and performance.
Methods
Twelve h e a l t h y v o l u n t e e r s (7 females, 5 males; ages 17-46) p a r t i c i p a t e d i n t h i s study* Each s u b j e c t underwent two s e s s i o n s i n t h e s l e e p l a b o r a t o r y , each l a s t i n g 32 hours, s t a r t i n g a t 23:00* One s e s s i o n began w i t h constant b e d r e s t ( c o n d i t i o n S ) . The o t h e r s e s s i o n comprised o f an 8-hour p e r i o d o f t o t a l s l e e p d e p r i v a t i o n f o l l o w e d by constant bedrest f o r t h e remaining 24 hours ( c o n d i t i o n TSD). I n constant bedrest t h e s u b j e c t s had t o c o n s t a n t l y l i e i n bed, where they c o u l d t a k e t h e i r snacks a s they wished.
The c o n t r o l o f t h e dim i l l u m i n a t i o n i n t h e room was a t t h e d i s c r e t i o n o f t h e s u b j e c t , who had no knowlege o f t h e t i m e o f day. Subjects had no p o s s i - b i l i t y t o read, w r i t e , l i s t e n t o music, o r e x e r c i s e . I t was p o s s i b l e t o cxxtitunicate w i t h s u b j e c t s (e.g. r e g a r d i n g experimental i n s t r u c t i o n s ) by an
intercom system.
The f o l l o w i n g measurements were recorded c o n t i n u o u s l y : EBG, BOG, EMG, r e c t a l temperature and w r i s t a c t i v i t y .
S u b j e c t s were asked t o e s t i m a t e t h e i r mood and a l e r t n e s s h o u r l y on sepa- r a t e 100 itm v i s u a l analogue s c a l e s ( a l e r t n e s s : 0=drowsy, 100=alert; mood:
0=negative, 1 0 0 = p o s i t i v e ) . A t t h e same time a performance t e s t (Test d2:
c o n c e n t r a t i o n speed t e s t , Brickenkanp 1975) was c a r r i e d o u t .
The sequence o f t h e two c o n d i t i o n s (S; TSD) was randomized. Before each s e s s i o n , each s u b j e c t underwent b a s e l i n e measurements i n which he e s t i m a t e d h i s mood and a l e r t n e s s and c a r r i e d o u t t h e performance t e s t s h o u r l y d u r i n g h i s normal d a i l y l i f e .
The c r i t e r i o n used here t o d e f i n e a s l e e p episode was: a sequence o f s u c c e s s i v e u n i n t e r r u p t e d 6-minute i n t e r v a l s o f p o l y g r a p h i c a l l y recorded s l e e p i f t h e s e were n o t i n t e r r u p t e d by sequences o f wakefulness l a s t i n g l o n g e r t h a n 30 min.
A major s l e e p episode (MSE) was d e f i n e d a s any sleep episode i n i t i a t e d i n t h e n i g h t phase (20:00 - 7:00). A nap was d e f i n e d a s any s l e e p episode i n i t i a t e d between 7:00 and 20:00.
R e s u l t s O v e r a l l sleep/wake p a t t e r n
O o n d i t i o n S
I n t h i s s e s s i o n , t h e s u b j e c t s s l e p t an average o f 18.8 4 1.8 hours ( 5 9 % ) . H i e d i s t r i b u t i o n o f s l e e p over t h e day and t h e n i g h t s can be seen i n T a b l e 1. I t i s shown t h a t t h e r e l a t i v e amount o f s l e e p t a k e n d u r i n g t h e day was about h a l f o f t h e amount t a k e n d u r i n g t h e n i g h t s . Furthermore, compared t o t h e f i r s t n i g h t , t h e amount o f s l e e p was s l i g h t l y reduced i n t h e second n i g h t .
On average, t h e f i r s t major s l e e p episode (MSE) s t a r t e d a t 23:54 ± 24 min w h i l e t h e second MSE was i n i t i a t e d a t 23:16 ± 88 min. Sleep o n s e t l a t e n c y i n t h e f i r s t MSE had a mean o f 28.0 £ 16 min. The d u r a t i o n o f t h e s l e e p e p i - sodes throughout t h e e n t i r e experiment showed a c l e a r c i r c a d i a n v a r i a t i o n w i t h l o n g s l e e p episodes i n i t i a t e d d u r i n g t h e f i r s t n i g h t , s h o r t s l e e p episodes i n i t i a t e d d u r i n g t h e day and a g a i n l o n g s l e e p episodes i n i t i a t e d i n t h e second n i g h t (Table 2 ) . The waking episodes ( o n l y those l o n g e r t h e n 30 min) were l o n g e r when i n i t i a t e d d u r i n g t h e day t h a n d u r i n g t h e n i g h t (Table 2 ) .
The frequency o f napping ( s l e e p episodes i n i t i a t e d between 7:00 and 20:00) p e r s u b j e c t showed a c l e a r peak w i t h 3 naps (6 s u b j e c t s ) , w h i l e 3 s u b j e c t s t o o k 2 naps, 2 s u b j e c t s t o o k 5 naps, and one s u b j e c t t o o k 6 naps.
With r e s p e c t t o t h e placement o f t h e roost f r e q u e n t 3-nap s t r u c t u r e , i t was p o s s i b l e t o c l e a r l y d i f f e r e n t i a t e t h e i r t i m e o f o c c u r r e n c e . A l l f i r s t naps
(nap 1) were taken between 7:00 and 12:00 w i t h a mean a t 9:30. A l l second naps (nap 2) were t a k e n between 11:30 and 16:30 w i t h a mean a t 13:45. A l l t h i r d naps (nap 3) were t a k e n between 16:00 and 20:00 w i t h a mean a t 18:15.
The placement o f t h e s e naps i s used i n t h e f o l l o w i n g f o r t h e d i f f e r e n t i a t i o n o f a l l naps. The upper p a r t o f F i g u r e 1 shows a summation histogram o f t h e number o f s u b j e c t s a s l e e p w i t h a d i f f e r e n t i a t i o n i n t o d i f f e r e n t groups o f naps. The t h r e e groups o f naps can be seen t o be between two MSEs. The phase p o s i t i o n s o f t h e nap groups and t h e amount o f s l e e p w i t h i n t h e s e groups can be seen i n T a b l e 3. The placement o f t h e o v e r a l l groups o f naps i s n e a r l y i d e n t i c a l t o t h e placement o f naps by t h o s e s u b j e c t s showing t h e 3-nap s t r u c t u r e .
O o n d i t i o n TSD
One s u b j e c t was e x c l u d e d from t h e f o l l o w i n g d a t a a n a l y s i s because he s l e p t c o n t i n u o u s l y f o r 24 hours w i t h t h e e x c e p t i o n o f one i n t e r r u p t i o n
( i n t e r v e n i n g wakefulness longer t h a n 30 minutes). I n t h i s s e s s i o n , t h e o t h e r s u b j e c t s s l e p t an average o f 17.5 t 2.3 hours ( 7 3 % ) . The s l e e p was d i s t r i - buted over t h e day and t h e n i g h t w i t h more a b s o l u t e amount o f s l e e p d u r i n g t h e day t h a n d u r i n g t h e n i g h t (Table 4 ) .
The onset o f t h e f i r s t s l e e p episode a f t e r TSD (recovery sleep) was a t 7:17 t 14 min, w h i l e t h e MSE began a t 23:27 ± 75 min i n t h e f o l l o w i n g n i g h t . S l e e p onset l a t e n c y i n t h e recovery s l e e p was 6.0 min ± 5.4 min. The dura- t i o n o f t h e s l e e p episodes showed long s l e e p episodes a f t e r TSD (recovery s l e e p ) , s h o r t e r episodes d u r i n g t h e r e s t o f t h e day and long s l e e p episodes a g a i n i n t h e n i g h t phase (Table 5 ) .
time of day
F i g u r e 1
Sutmation histogram o f t h e number o f s u b j e c t s a s l e e p over t h e e n t i r e e x p e r i - mental episode i n l o c a l t i m e . Upper p a r t : c o n d i t i o n S; lower p a r t : c o n d i t i o n TSD. S l e e p episodes a r e d i f f e r e n t i a t e d w i t h r e s p e c t t o a 3-nap s t r u c t u r e
(see t e x t > .
T a b l e 1* Amount o f s l e e p (mean t SD) i n each phase i n a b s o l u t e (hours) and r e l a t i v e v a l u e s (percentage o f each phase) under c o n d i t i o n S.
Phase Time Span Mean amount of sleep hours %
1st night 23:15 - 07:00 6.6 • 0.5 85 Day 07:01 - 23:00 6.2 t 1.5 39 2nd night 23:01 - O7:00 6.0 1 0.8 75
T a b l e 2» D u r a t i o n and frequency o f s l e e p and waking episodes (mean
± SD) i n i t i a t e d i n t h e r e s p e c t i v e phase under c o n d i t i o n S.
Phase Time span Duration of
Sleep episodes Wake episodes
hours n hours n
Night 23:00 - 07:00 8.1 t 1.5 12 0 Day 07:00 - 20:00 1.5 ± 1.0 41 2.3 ± 1.6 53 Night 20:00 - 07:00 7.3 ± 1.7 17 1.1 ± 0.4 5
Table 3. Mean p o s i t i o n o f each nap phase and mean amount o f s l e e p i n each nap phase (hours: mean t SD; %: mean) under c o n d i t i o n S.
Mean phase p o s i t i o n
Hours %
Nap 1 10:00 1.9 ± 0.6 41
Nap 2 14:00 1.9 ± 0.7 30
Nap 3 18:00 1.3 ± 1.0 38
T a b l e 4. Amount o f s l e e p i n each phase i n a b s o l u t e (mean t SD) and r e l a t i v e v a l u e s (mean p e r c e n t o f each phase) under c o n d i t i o n TSD.
Phase Time span Amount o f s l e e p
hours %
Day 07:00 - 23:00 10.6 ± 1.7 66
N i g h t 23:00 - 07:00 6.9 t 0.8 86
T a b l e 5. D u r a t i o n and frequency o f s l e e p and waking episodes (mean ± SD) i n i t i a t e d i n t h e r e s p e c t i v e phase under c o n d i t i o n TSD.
1
Phase Time span D u r a t i o n o f
S l e e p episodes n Wake episodes n
Morning 07:00 - 08:00 6.7 ± 2.0 11 0
Day 09:00 - 20:00 2.0 ± 1.4 19 1.8 ± 1.7 27
N i g h t 20:00 - 07:00 6.0 • 3.3 13 2.6 ± 2.7 8
W i t h r e s p e c t t o t h e placement o f s l e e p . F i g u r e 1 (lower p a r t ) shows t h a t t h e f i r s t s l e e p episode a f t e r TSD (recovery s l e e p ) c o v e r s t h e t i m e o f nap 1 and nap 2, w h i l e nap 3 can a g a i n be c l e a r l y d i s t i n g u i s h e d , o c c u r r i n g a t about t h e same t i m e as under c o n d i t i o n S (mean 17:45; range 14:30 - 21:00).
The r e l a t i v e amount o f s l e e p i n t h i s t i m e span i s s i m i l a r t o t h e amount i n t h e r e s p e c t i v e t i m e span under c o n d i t i o n S ( i . e . 40%). Under b o t h c o n d i - t i o n s , t h e MSE i n t h e second n i g h t does not d i f f e r w i t h r e s p e c t t o placement and d u r a t i o n (Table 5 ) .
A l e r t n e s s and mood
For t h e s e s c a l e s nonparametric s t a t i s t i c s were necessary because t h e d i s t r i b u t i o n showed s i g n i f i c a n t d e v i a t i o n s from t h e normal d i s t r i b u t i o n . Under t h e b a s e l i n e c o n d i t i o n t h e s u b j e c t s h o u r l y e s t i m a t e d t h e i r s u b j e c t i v e a l e r t n e s s w i t h a median o f 67 and mood w i t h a median o f 74 on a v i s u a l analogue s c a l e . Comparing t h e c o n d i t i o n s , t h e v a l u e s decreased under c o n d i - t i o n S f o r b o t h v a r i a b l e s w i t h a median o f 55 f o r a l e r t n e s s and 62 f o r mood.
On average, t h e s u b j e c t s f e l t more drowsy and l e s s c o m f o r t a b l e under t h e b e d r e s t c o n d i t i o n .
W i t h r e g a r d t o t h e d i u r n a l v a r i a t i o n a f t e r t h e r e s p e c t i v e s l e e p episodes under c o n d i t i o n S, a l e r t n e s s shows a steady i n c r e a s e a f t e r each succeeding nap. Mood, a f t e r an i n i t i a l i n c r e a s e , decreases a f t e r t h e f i r s t nap.
A f t e r s l e e p d e p r i v a t i o n , a l e r t n e s s and mood show t h e same mean v a l u e s d u r i n g b e d r e s t as under c o n d i t i o n S w i t h a median o f 54 f o r a l e r t n e s s and 63 f o r mood.
The d i u r n a l v a r i a t i o n a f t e r TSD ( f o l l o w i n g t h e recovery s l e e p ) i s e q u i v a - l e n t t o t h a t o f c o n d i t i o n S, w i t h a steady i n c r e a s e i n a l e r t n e s s and a decrease i n mood a f t e r t h e i n i t i a l i n c r e a s e . I t can be summarized t h a t TSD
i n i t i a l l y caused a decrease i n a l e r t n e s s and mood, which was t h e n compen- s a t e d a f t e r t h e recovery s l e e p e p i s o d e .
Performance
Under b o t h experimental c o n d i t i o n s s u b j e c t s showed a decrease i n t h e i r performance t e s t compared t o t h e b a s e l i n e c o n d i t i o n . As can be seen i n F i g u r e 2, t h e h i g h e s t v a l u e s were observed under t h e b a s e l i n e c o n d i t i o n , shewing a steady i n c r e a s e throughout t h e day. C o n d i t i o n S showed s i g n i f i - c a n t l y lower v a l u e s w i t h a t r o u g h i n t h e a f t e r n o o n . S l e e p d e p r i v a t i o n caused a d r a m a t i c decrease i n performance, which recovered throughout t h e b e d r e s t episode ( c o n d i t i o n TSD), r e a c h i n g t h e same v a l u e as under c o n d i t i o n S.
A more d e t a i l e d a n a l y s i s shows t h a t under c o n d i t i o n S, t h e f i r s t nap had
a b e n e f i c i a l e f f e c t , showing t h e h i g h e s t v a l u e f o l l o w e d by a steady decrease a f t e r each suoceding nap. Under c o n d i t i o n TSD, a f u r t h e r decrease i n p e r f o r - mance became obvious. T h i s s l e e p d e p r i v a t i o n e f f e c t i s compensated d u r i n g t h e course o f t h e bedrest c o n d i t i o n .
D i s c u s s i o n
Under c o n d i t i o n s a l l o w i n g spontaneous e x p r e s s i o n o f t h e sequence o f s l e e p i n g and waking, a p o l y p h a s i c d i s t r i b u t i o n o f t h e occurrence o f s l e e p becomes obvious. Under these c o n d i t i o n s , t h e placement o f s l e e p shows p r e - f e r r e d phase p o s i t i o n s throughout t h e day besides major s l e e p episodes i n t h e n i g h t phase. The p e r i o d i c occurrence o f day s l e e p a t i n t e r v a l s o f about 4 hours has a l s o been found i n e a r l i e r s t u d i e s . Nakagawa (1980) r e p o r t e d a sleep/waking c y c l e l e n g t h o f approximately 4 hours f o r s u b j e c t s c o n f i n e d t o
Change in performance
high Median of performance in 4 and 6 hour blocks
520-
500-
CD
c
tX)
E o 480-
<_
o>
C L
460-
440<
'"'low 0-3 4-7 7-12 13-18 19-0 time of day
F i g u r e 2
Performance measurements i n t h e t h r e e c o n d i t i o n s and d u r i n g s l e e p d e p r i v a - t i o n . V a l u e s (median) a r e averaged over 4 and 6-hour b l o c k s .
bed f o r 10-12 hours d u r i n g t h e day. I n experiments w i t h o u t t i m e cues, when s u b j e c t s showed i n t e r n a l d e s y n c h r o n i z a t i o n o f rhythms, s e v e r a l peaks i n t h e d i s t r i b u t i o n o f s u b j e c t i v e n i g h t s l e e p c o u l d be observed d u r i n g t h e c i r c a - d i a n day ( Z u l l e y and Wever, 1982). These peaks had a mean i n t e r v a l o f 5.74 hours. I n a study where s u b j e c t s were recorded d u r i n g 60 hours o f e n f o r c e d b e d r e s t , t h e median day phase sleep/waking c y c l e l e n g t h was 4.65 hours
( C a n p b e l l , 1984).
These r e s u l t s support t h e assumption t h a t t h e huran sleep/wake system i s more a c c u r a t e l y d e s c r i b e d by a p o l y p h a s i c d i s t r i b u t i o n w i t h a 4-hour p e r i o d . The a c t u a l frequency o f napping seems t o depend on t h e degree t o which t h e c o n d i t i o n s a l l o w such napping and on t h e presence o f b e h a v i o r a l o p t i o n s t o s l e e p . T h i s a s p e c t can be d e s c r i b e d by a p u t a t i v e s l e e p t h r e s h o l d which i s decreased under t h e above mentioned experiments. T h i s s l e e p t h r e s h o l d has been r a i s e d e x p e r i m e n t a l l y by a l l o w i n g more b e h a v i o r a l o p t i o n s t o s l e e p (Caiqpbell & Z u l l e y , 1985). I n t h i s study, s u b j e c t s showed a c l e a r l y bimodal d i s t r i b u t i o n o f s l e e p and wakefulness. The nap peak o c c u r r e d a t about t h e same t i m e as nap 2 i n t h e p r e s e n t experiment. I n o t h e r words, nap 1 and nap 3, t h e two a d j a c e n t phase p o s i t i o n s f o r s l e e p , were masked by t h e decrease o f s l e e p p r o p e n s i t y w h i l e nap 2 s t i l l remained. A p o s s i b l e i n t e r p r e t a t i o n o f t h i s i s t h a t , by r a i s i n g t h e s l e e p t h r e s h o l d , t h e two l e s s r o b u s t phase p o s i t i o n s f o r sleep were d i m i n i s h e d , w h i l e t h e more r o b u s t phase p o s i t i o n
(nap 2) c a n s t i l l be seen, p l a c e d halfway between t h e two major s l e e p e p i s o d e s . Such a bimodal d i s t r i b u t i o n can a l s o be seen i n experiments i n which no i n s t r u c t i o n s r e g a r d i n g s l e e p were g i v e n o r when s u b j e c t s napped i n s p i t e o f t h e i n s t r u c t i o n s ( Z u l l e y & Campbell, 1985). I f , i n a d d i t i o n , t h e p r o p e n s i t y f o r s l e e p i s s t i l l decreased by experimental i n s t r u c t i o n s , t h e sleep/wake p a t t e r n becomes monophasic. T h i s r e s u l t can be seen i n e x p e r i - ments where napping i s e f f e c t i v e l y suppressed by i n s t r u c t i o n s (Wever, 1979).
The r e s u l t t h a t some s u b j e c t s show a monophasic s l e e p placement, even when napping i s a l l o w e d , c o u l d be e x p l a i n e d by t h e assumption t h a t t h e s l e e p t h r e s h o l d shows s t r o n g i n d i v i d u a l d i f f e r e n c e s . T h i s r e s u l t s i n t h e phenome- non t h a t , a t a g i v e n s l e e p t h r e s h o l d (when napping i s a l l o w e d ) , some s u b j e c t s f e e l u n a b l e t o t a k e a nap ( Z u l l e y & C a n p b e l l ; i n p r e p a r a t i o n ) .
T h i s v a r i a t i o n i n t h e degree o f s l e e p p r o p e n s i t y can a l s o be seen i n d a i l y l i f e . I n newborns, M e i e r - K o l l (1979) showed " t h a t t h e u l t r a d i a n 4-hour rhythm e s t a b l i s h e d a l r e a d y a t b i r t h does not d i s a p p e a r d u r i n g t h e f i r s t month o f l i f e " .
I t can be assumed t h a t , i n newborns, t h e s l e e p t h r e s h o l d i s decreased and t h a t , w i t h a g i n g , t h e t h r e s h o l d i n c r e a s e s . T h i s assumption i s supported by t h e f i n d i n g t h a t , i n c h i l d r e n , t h e p o l y p h a s i c p a t t e r n becomes b i p h a s i c , which i n our western c u l t u r e i s r e p l a c e d by a ironophasic p a t t e r n i n a d u l t s . T h i s h y p o t h e s i s i s a l s o supported by s t u d i e s which show t h a t , i n o t h e r c u l t u r e s w i t h l e s s s t r u c t u r e d s o c i o c u l t u r a l c o n d i t i o n s , a b i p h a s i c p a t t e r n s t i l l remains (Soldatos e t a l . , 1983). I n t h e g e r i a t r i c p o p u l a t i o n t h e b i p h a s i c p a t t e r n reappears, assuming a decrease o f t h e s l e e p t h r e s h o l d w i t h age. T h i s p a t t e r n can become p o l y p h a s i c as has been shown i n demented pa- t i e n t s ( S p i e g e l e t a l . , 1985). I n a l l these cases, t h e d i f f e r e n t p r e f e r r e d phase p o s i t i o n s f o r s l e e p correspond t o those p o s i t i o n s found i n t h e present experiment.
Thus, i t can be concluded t h a t t h e human sleep/wake system i s b a s i c a l l y expressed by p o l y p h a s i c d i s t r i b u t i o n o f s l e e p and wakefulness w i t h a p e r i o d o f about 4 hours. Yet, i t depends on t h e environmental c o n d i t i o n s and on s e l f - i m p o s e d b e h a v i o r a l c o n t r o l s t o which e x t e n t t h e separate peaks o f s l e e p p r o p e n s i t y become obvious. By d e c r e a s i n g t h e s l e e p t h r e s h o l d , a ironophasic c i r c a d i a n p a t t e r n changes i n t o a b i p h a s i c (about 12 hours) p a t t e r n , and a f u r t h e r decrease r e v e a l s a p o l y p h a s i c (about 4 hours) p a t t e r n .
I n our d a i l y l i f e , we a r e a b l e t o f u n c t i o n w e l l w i t h o u t napping. T h i s does not d i m i n i s h t h e r e l e v a n c e o f naps i n understanding t h e nature and f u n c t i o n s o f s l e e p . I t might be assumed t h a t v a r i a t i o n s i n t h e s l e e p t h r e s - h o l d p a r t i a l l y have t h e f u n c t i o n o f adapting t o environmental and i n d i v i d u a l needs. T h i s can c l e a r l y be seen i n c h i l d h o o d and o l d age. I n t h i s sense, t h e p r e s e n t study f o r c e d t h e s u b j e c t s t o s l e e p more than they a c t u a l l y needed.
The consequence was a decrease i n a l e r t n e s s and mood as w e l l as i n p e r f o r - mance. Such e f f e c t s can a l s o be seen i n cases where s u b j e c t s o v e r s l e e p (Taub e t a l . , 1971). I n c o n d i t i o n S i t i s obvious t h a t enforced bedrest a f t e r a normal n i g h t s l e e p causes o v e r s l e e p i n g . I n c o n d i t i o n TSD i t can be assumed t h a t , a f t e r t h e f i r s t l o n g s l e e p episode, t h e s u b j e c t s had recovered from s l e e p d e p r i v a t i o n and t h a t t h e subsequent s l e e p a g a i n caused o v e r s l e e p i n g . V i g i l a n c e i s n e g a t i v e l y i n f l u e n c e d by o v e r s l e e p i n g (Taub e t a l . , 1971). T h i s was a l s o seen i n t h e present study. I n d a i l y l i f e such o v e r s l e e p i n g i s l i m i t e d by s o c i a l and o c c u p a t i o n a l p r e s s u r e , keeping not o n l y an e f f e c t i v e t i m e schedule but a l s o more e f f i c i e n t v i g i l a n c e .
I f , however, t h e normal amount o r sequence o f s l e e p i n g and waking cannot be c o n t i n u e d , a s i g n i f i c a n t l y b e n e f i c i a l e f f e c t o f napping has been found
(Dinges e t a l . , 1986). By assuming a p o l y p h a s i c o r g a n i z a t i o n o f s l e e p and wakefulness, i t c a n be h y p o t h e s i z e d t h a t a s c h e d u l i n g o f s l e e p and w a k e f u l - ness w i t h r e g a r d t o t h e u n d e r l y i n g s l e e p p r o p e n s i t y may be b e n e f i c i a l under c i r c u m s t a n c e s which do not a l l o w a normal amount o f s l e e p .
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