On four experimental nights, 16 subjects had their sleep interrupted for 60 minutes by forced awakening at the time when 40 minutes of nonrapid eye movement ...
Sleep. 15(3);217-225
© 1992 American Sleep Disorders Association and Sleep Research Society
Isolated Sleep Paralysis Elicited by Sleep Interruption *T. Takeuchi, tAo Miyasita, *Y. Sasaki, tM. Inugami and :j:K. Fukuda
*Department of Psychology, Waseda University, Tokyo, Japan; tDepartment of Psychology, Tokyo Metropolitan Institute for Neurosciences, Tokyo, Japan; and tDepartment of Educational Psychology, Fukushima University, Fukushima, Japan Summary: We elicited isolated sleep paralysis (ISP) from normal subjects by a nocturnal sleep interruption schedule. On four experimental nights, 16 subjects had their sleep interrupted for 60 minutes by forced awakening at the time when 40 minutes of nonrapid eye movement (NREM) sleep had elapsed from the termination of rapid eye movement (REM) sleep in the first or third sleep cycle. This schedule produced a sleep onset REM period (SOREMP) after the interruption at a high rate of 71.9%. We succeeded in eliciting six episodes oflSP in the sleep interruptions performed (9.4%). All episodes ofISP except one occurred from SOREMP, indicating a close correlation between ISP and SOREMP. We recorded verbal reports about ISP experiences and recorded the polysomnogram (PSG) during ISP. All of the subjects with ISP experienced inability to move and were simultaneously aware of lying in the laboratory. All but one reported auditorylvisual hallucinations and unpleasant emotions. PSG recordings during ISP were characterized by a REM/W stage dissociated state, i.e. abundant alpha electroencephalographs and persistence of muscle atonia shown by the tonic electromyogram. Judging from the PSG recordings, ISP differs from other dissociated states such as lucid dreaming, nocturnal panic attacks and REM sleep behavior disorders. We compare some of the sleep variables between ISP and non-ISP nights. We also discuss the similarities and differences between ISP and sleep paralysis in narcolepsy. Key Words: Isolated sleep paralysis-Hypnagogic hallucinations-Sleep onset REM period (SOREMP)-Sleep interruption-REM/W dissociation-Narcolepsy.
Narcoleptic patients often experience sleep paralysis at sleep onset with or without hypnagogic hallucinations. These symptoms are part of the main tetrad of narcolepsy, i.e. cataplexy, sleep attacks, sleep paralysis and hypnagogic hallucinations. During sleep paralysis, patients are unable to move and speak. Many studies have also reported the occurrences of sleep paralysis despite the absence of other symptoms of narcolepsy such as sleep attacks and cataplexy (18), and this is called isolated sleep paralysis (ISP) (9). Several studies using questionnaires have indicated that the prevalence of ISP in the normal population ranges from 5% to 16% (10-12). Recently, far higher prevalence rates were reported, with 39% of normal black Americans (7) and 40% ofJapanese students (13) stating that they experienced ISP. In narcolepsy, Hishikawa et al. (14,15) have used Accepted for publication December 1991. Address correspondence and reprint requests to Tomoka Takeuchi, % Department of Psychology, Tokyo Metropolitan Institute for Neurosciences, 2-6 Musashidai, Fuchu-city, Tokyo 183, Japan.
polygraphic recordings to show that sleep paralysis occurs only at the sleep onset rapid eye movement (REM) period (SOREMP). However, only Fukuda (16) has attempted to record ISP in normal individuals during naps after sleep deprivation. Thus, no study has yet recorded laboratory-documented ISP in normal individuals and confirmed whether or not it occurs during SOREMP. Even in normal individuals, SOREMP often occurs under certain conditions, such as acute reversal of the sleeIrwake cycle (17), a mUltiphasic sleeIrwake schedule (18) or sleep interruption (19-22). Miyasita et al. (21) reported that their sleep interruption method elicited SOREMP on about 30% ofthe interrupted nights in normal subjects. Thus, the sleep interruption method appears to be a reliable tool for eliciting SOREMP. In this study, our goal was to elicit ISP and examine polygraphic recordings during the ISP. The findings in narcoleptic patients led us to expect that ISP would occur in normal subjects at SOREMP. Thus, we adopted a sleep interruption method that was thought likely to elicit SOREMP in normal individuals.
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T. TAKEUCHI ET AL.
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Sleep Onset
Awake
Baseline Night
't*
1-------- 7.5 brs of Siup Episode - - - - - - - - - - - 1 Experimental Night -Condition E2nd
Cycle
Experimental Night -Condition l-
4th
Cycle
1---------- 7.5 brs of SlaapEpisode - - - - - - - - - - - - . 1
o
~
FIG. 1. Experimental design.
•
NREM RE M Task tUmin
Questionna ires 4 Dream Reports
for seven consecutive nights. The first and second nights were adaptation nights, and the four experimental nights Subjects began following the third baseline night. Figure 1 shows the experimental design. Experimental nights were inWe administered a questionnaire on life habits interrupted for about 60 minutes by forced awakening cluding ISP experiences to 1,314 college students. A after 40 minutes of nonrapid eye movement (NREM) total of 564 (43%) had experienced ISP once or more. sleep had elapsed from the termination of REM sleep From them, we selected 16 subjects (8 men and 8 in the first (early condition: two nights) or the third women, aged 18-21 years) using the following screen(late condition: two nights) sleep cycle. The order of ing criteria: First, they had to have experienced ISP at the two conditions was counterbalanced between the least twice. Second, they had to have experienced neisubjects. ther cataplexy nor sleep attacks. This criterion was used At the time of sleep interruption, one of the experto eliminate narcoleptic patients. In the questionnaire, imenters awakened the subject by calling the indiviwe used the Japanese word "kanashibari", which is name through an earphone. During sleep interdual's known to 98.4% of Japanese students (13) and is idenruption, subjects were given an auditory vigilance task tical in meaning with the term ISP. We defined ISP in for 40 minutes at the desk inside the bedroom. After the questionnaire as the inability to move and speak the task, they were allowed to return to sleep. After while lying fully conscious or in a dreamlike state, with the second sleep onset, subjects were awakened again or without visual and/or auditory hallucinations. when 5 minutes of REM sleep had elapsed (Fig. 1). Immediately after being awakened, the subjects were asked about their state of consciousness just prior to Procedure the .experimenter's call (waking, sleeping, dreaming, Each subject was assigned to a darkened, sound- havmg ISP, etc.). If the answer was dreaming or having attenuated bedroom in the laboratory where they slept ISP, subjects were administered questionnaires, which METHODS
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ELICITED ISOLATED SLEEP PARALYSIS
seen on 17 nights (26.6%), whereas a very short REM latency (5.49 ± 5.10 minutes; SOREMP) was seen on the other 46 nights (71.9%). With regard to the early condition, only one individual on one night (TI-2) did not reach REM sleep, as he woke up voluntarily to report ISP before the onset of the REM period. There were 18 SOREMPs (58.1%) and 13 non-SOREMPs (41.9%) recorded during the early condition, whereas 28 SOREMPs (87.5%) andfournon-SOREMPs (12.5%) occurred in the late condition. ISP was elicited once in 4 subjects and twice in one subject: a total of six cases (6/64 sleep interruptions = 9.4%). These ISP cases occurred only when subjects Recording returned to sleep after an interruption, and they were The sleep period including the interruption period not reported when the subjects were awakened for sleep was monitored polygraphically with an electroenceph- interruption ·or in the mornings. Except for the abovealogram (EEG) (C 3 and 0 1 or C4 and O 2 referred to A2 mentioned case (TI-2), ISP was experienced when SOor AI), horizontal electrooculogram (EOG) and men- REMP occurred. talis electromyogram (EMG). All sleep records were scored every 30 seconds according to the standard cri- Sleep parameters before ISP teria (23). Sleep onset was defined as the first appearance of continuous stage 1, as REM sleep sometimes The sleep parameters from the second lights-out to occurred prior to the first epoch of stage 2 following the experimenter's call for the six cases showing ISP sleep interruption. We defined REM latency as the time and the results of HLA typing are given in Table 1. from sleep onset to the first epoch of REM. We defined SOREMP% represents the proportion ofSOREMP on the REM period with a latency ofless than 25 minutes the four (or three) sleep-interrupted nights. There were as SOREMP, according to the study of Miyasita et al. no significant differences in the mean SOREMP% be(21), which showed a bimodal distribution of the REM tween the 5 subjects who experienced ISP and the 11 latencies after interruption with two clusters separated subjects who did not (ISP = 80.0 ± 29.15% vs. nonby 25 minutes. We also recorded various other phys- ISP = 70.5 ± 25.71%; t = 0.617, df= 14, NS). iological measurements such as the heart rate, respiThe mean second sleep latency was not significantly ration, finger plethysmogram, rectal temperature and different among the 6 nights with ISP and the 58 nights skin potential activity. Blood samples were examined without ISP (ISP = 8.3 ± 9.03 minutes vs. non-ISP = to type human leukocyte antigens (HLA) for four of 13.7 ± 22.98 minutes; Welch's t = 1.151, df= 13.42, the 5 subjects who showed ISP. NS). However, after excluding one ISP night with an extremely long sleep latency (NA), the mean latency of the other five ISP nights was significantly shorter RESULTS than that of the 58 nights without ISP (ISP = 4.7 ± Following 64 sleep interruptions, the usual REM 2.71 minutes vs. non-ISP = 13.7 ± 22.98 minutes; latency (47.29 ± 13.00 minutes; non-SOREMP) was Welch's t = 2.778, df= 56.12, p < 0.008). The mean
contained a checklist of details of their dreams or ISP and rating scales for the impression left by their experience. Upon completion of the questionnaires, the subjects were asked to report on their experiences through an intercom. We recorded their verbal reports with a tape recorder for later transcription. Subsequently, they completed inventories such as a sleepiness scale and a mood adjective checklist (MACL). Following this procedure, the subjects were allowed to return to sleep again and were not awakened until they reached 7.5 hours of net sleep.
TABLE 1. Sleep parameters for each case with isolated sleep paralysis (minutes)
Case
Sex
Condition
F F M M M F
Early Early Late Early Late Early
Rate of SOREMP (SOREMP%)
Second sleep latency
REM latency
Time from REM onseta
114 (25) 11.5 6.0 5.5 MH 4.0 4/4 (100) 0.0 8.0 TI-I 3/3' (100) 5.5 9.0 4.0 (4.0)b., TI-2 2.5 3/3' (100) -' NA 3/4 (75) 26.0 8.0 5.0 AK 2.5 2.5 2.0 4/4 (100) Mean (80.0) 8.3 5.2 5.3 SD (29.15) 8.24 4.08 1.94 a The time elapsed from REM onset to awakening (TI-2: from sleep onset). b Only case AK was not typed for HLA-DR2. Of the four subjects tested, two were DR2 positive. c Subject TI did not exhibit REM sleep in case TI-2. KK
Stage when being called REM REM W W REM W
HLA-DR2b
+ +
Not examined
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T. TAKEUCHI ET AL.
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TABLE 2. Salient features of the subjective experiences during ISP
a
Case
Subjective state of consciousness before being called (judgement by PSG)
KK MH TI-l TI-2 NA AK
Drowsy Drowsy Waking Waking Sleeping Waking
(REM) (REM) (W) (W) (REM) (W)
ISP when calleda End End Middle Middle Middle End
Hallucinations
Anxiety and/or terror
Dreaming before/after ISP
+
Tactile
Auditory
Visual
+ + + +
+
+
+ +
+
+ + + +
+
+
+ + +
End means that ISP had terminated when the subject was calkd by the experimenter. Middle means that ISP was continuing.
second REM latency was not different among the :5 SOREMP nights with ISP and the 41 SOREMP nights without ISP (ISP = 5.2 ± 4.56 minutes vs. non-ISP == 5.5 ± 5.22 minutes; t = 0.133, df = 44, NS). At the time when individuals were awakened by the experimenter, the stage as judged by polysomnography (PSG) was either REM or W. W refers to the stage when the subject awakened voluntarily during or after ISP before the experimenter's call. Features of ISP The features of the documented ISP based on the subject's verbal reports and their answers to the ques.tionnaires are summarized in Table 2. All the verbal reports about the state of consciousness before the experimenter's call coincided with our judgement by PSG (shown in parentheses). All cases reported both an inability to move and an awareness of lying on the bed in the laboratory. All but one experienced at least one hallucinatory modality. Five of the six ISP resulted in feelings of anxiety or terror. In four cases, dreams were noted in addition to ISP: TI-l, NA and AK experienced ISP immediately after dreaming, whereas for KK it occurred just before dreaming. Individual verbal reports and the corresponding PSGs
CaseKK ISP occurred on the sixth night. After 4.5 minute:s of stage I, stage 2 sleep continued for 6 minutes until body movement occurred. The tonic EMG reached its lowest level during stage 2 sleep. Soon after the body movement, REMs appeared, and the EEG showed a mixed pattern of alpha waves and low-voltage slow waves. According to the standard criteria (23), this period was judged as stage REM. In addition to atonia and REMs, eye blinks with artifacts in the EMG/EOG channels were often observed. This state continued for 6 minutes, until KK was called by the experimenter. Judging from her report, we estimated that ISP 0wI'>~./v"'..M...,JIf
R-EOG L-R-EOG
1100 1100
llIm Over
C-O-EEG L-EOG
FIG. 2. Sleep diagram and polygraphic recording during isolated sleep paralysis (MH). The sleep diagram represents the period from the second lights-out (...) to lights-on and the experimenter's call (4). The upper and lower recordings are continuous. The horizontal bar with an asterisk indicates the periods during which PSG recordings are shown [(I): upper (2): lower]. C, central; 0, occipital; L, left; R, right; EMG, mentalis EMG; H.R., heart ratlt. Suppression of the tonic EMG preceded SOREMP. The REM period, accompanied by abundant alpha EEG, REMs and slow eye movements (SEMs), continues for 1.5 minutes. Interestingly, REMs seem to attenuate the alpha activity (upper and lower PSGs). The EMG continues at its lowest level throughout the period. This stage is terminated by a body movement, which corresponded to the subject's report that she turned over in bed (lower PSG).
He reported that he dreamed prior to ISP. The scene of the dream, in which he was pressed against the wall of a gymnasium by someone, changed into the laboratory bedroom. He woke up and opened his eyes,
which was identified by PSG. At that time he felt that his body was still under pressure. He tried to move his hands but could not. He heard strange sounds like a cassette tape winding quickly. After a while, he could Sleep, Vol. 15, No.3, 1992
T. TAKEUCHI ET AL.
222 TI
o
2
7 min
--
c;;
Eye Open
Eye Open
Take Receiver
Iso
,
L-R-EOG
----~~
~-~~-----------_~~-'~T-~~
______________
~~--v~~v-~.
EMG
FIG. 3. Sleep diagram and polygraphic recording during isolated sleep paralysis (TI-2). This chart shows eye openings. After the first time the eyes opened, abundant alpha waves continued for 30 seconds until the second eye opening, and then the tonic EMG and artifacts showed voluntary awakening when the subject took up the intercom receiver to report ISP (extreme right). (See legend of Fig. 2.)
pick up the intercom receiver voluntarily to report his ISP.
firmed by the television monitor. Figure 3 corresponds to the two eye openings. In this case, distinguishable REMs were not observed during ISP.
Case TI-2 TI-2 slept soon after lights-out with 3 minutes of sleep latency. He said that he slept soundly for a while without dreaming. He heard a strange sound and felt paralysis of his arms and legs. During the paralysis, he opened his eyes twice and saw light coming through an apparatus that projects infrared rays for the television monitor. He felt that 30 seconds had passed from the first time his eyes opened, and he then was able to pick up the intercom and report his paralysis to us. Suppression ofthe tonic EMG preceded sleep onset. As shown in the sleep diagram in Fig. 3, following 1.5 minutes of stage 1 sleep, alpha waves appeared for about 30 seconds, and this state was judged to be stage W. Subsequently, the EEG showed a low-voltage mixedfrequency pattern with the suppression of tonic EMG. This state continued for about 2 minutes and was judged to be stage 1 because of the lack of prominent REMs. Then, eye opening appeared twice, which was conSleep, Vol. 15, No.3, 1992
CaseNA NA reported ISP on the fourth night. He was called by the experimenter just after having paralysis. After lights-out, he did not sleep for 26 minutes. The tonic EMG reached its lowest level after 5.5 minutes of stage 1 sleep had elapsed. Three minutes after that, REM sleep appeared and continued for 4.5 minutes until the experimenter's call. The last 30 seconds of this REM period showed abundant alpha waves immediately after a burst of REMs. His report indicated that ISP occurred directly following a dream. He felt ISP coming on and his coverlet rising up. When he tried to open his eyes, the room lights were turned on, and an experimenter called him.
CaseAK AK experienced ISP on the seventh night. The tonic EMG reached its lowest level before sleep onset. Stage
223
ELICITED ISOLATED SLEEP PARALYSIS AI
5 I
~
=' .;;;
...-
-
;;;
:l
....
..j:,...
9
10
11 mil
• I
Eye Open
~
O-EEG
R-EOG 1100
L-R-EOG
1100
EMG
H. -12SOJ1V
1sec:.
FIG. 4. Sleep diagram and polygraphic recording during isolated sleep paralysis (AK). This chart shows an alpha-wave train and its blocking by REMs (extreme left), which is followed by an intermittent alpha burst lasting for about 30 seconds. Then, eye opening with artifacts appears (extreme right), and the EEG changes into a continuous alpha pattern. (See legend of Fig. 2.)
1 sleep continued for 2.5 minutes, with sporadic short bursts of alpha waves, and then REMs appeared. Figure 4 shows the chart at the time when 1.5 minutes had passed from the first appearance of REMs. We called the subject 4 minutes after the eyes opened as shown in the sleep diagram of Fig. 4. Her ISP developed after a dream. In the dream, she saw the "god of thunder" fly above her, and then her feet were paralyzed. She was frightened at her paralysis and woke up. She saw the strange face of a man on the wall of the bedroom. This waking corresponded to the eye opening in Fig. 4, which we also identified on the television monitor. We heard her breathing laboriously through the speaker. She also said that she could not breath well during and after the awareness ofISP. After waking, she tried to call the experimenter but could not do so. She made an effort to counter the paralysis by attempting to move her legs. Then the paralysis in her legs disappeared and subsequently that of the rest of her body. }.
tory or by PSG. None of them had ever experienced cataplexy and/or sleep attacks. In narcolepsy, SOREMP often appears in the first cycle of a night (14, 24), whereas it is rarely seen in normal individuals. In this study, none of the subjects showed SOREMP in the first cycle throughout the experiment. We cannot entirely exclude the possibility that our subjects may later develop the symptoms of narcolepsy. Thus, longitudinal observation is needed on the subsequent course of their sleep paralysis. Nevertheless, none of them were diagnosed as narcoleptics at the time when we conducted this study. Our results showed that the interruption schedule applied in this study is a reliable method for eliciting SOREMP. We adopted the NREM-40-minute schedule of Miyasita et al. (22). They reported that the occurrence of SOREMP (SOREMP%) increased as the preawakening NREM duration became longer, with 75% SOREMP being observed at a preawakening NREM duration of 60 minutes. Thus, the timing of awakening in the NREM-REM cycle is very crucial DISCUSSION for producing SOREMP. We selected the NREM-40It should be noted that none of our subjects evi- minute condition to avoid the risk of REM sleep ocdenced clinical signs of narcolepsy either in their his- curring within the 60 minutes of NREM sleep, esSleep. Vol. 15. No.3. 1992
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T. TAKEUCHI ET AL.
pecially in a late condition. We achieved a 73% incidence of SOREMP, which was as high as we ha.d expected. Of the six cases ofISP, five occurred from SOREMP. On the other hand, no ISP occurred on the non-SOREMP night. Though TI-2 did not reach full-stage REM, the de synchronized low-voltage EEG and atonia before his voluntary waking were suggestive of a REM stage. Furthermore, his ISP occurred shortly after returning to sleep, a time similar to all the other cases ofISP (Table 1). Therefore, it seems likely that ifmore time had elapsed without voluntary awakening, REMs would have appeared in this case also. Overall, these results imply a close relationship between SOREMP and ISP. We cannot exclude the possibility that ISP occurs during NREM sleep because we did not evaluate NREM awakenings. However, we did not find any signs ofISP occurring in NREM sleep by PSG and the subjects' verbal reports. Thus, we suppose that most ISP occurs under conditions that induce SOREMP, such as disruption of the circadian sleep-wake rhythm (17, 18) or interruption of the REM-NREM cycle (19-22). Among the five cases of ISP at SOREMP, two (KK and NA) differed somewhat from the others in the features ofISP. Their attacks seemed to be weaker, and they had difficulty distinguishing their paralysis from a dream. The REM latencies were much longer in these two cases than in the other three (Table 1), where ISP was severe and clearly different from dreaming. For example, MH showed 0 minutes of REM latency and reported the most severe paralysis in her life. These results imply that distinct ISP may occur in the setting ofSOREMP with a very short REM latency. Hishikawa et a1. (14, 15) also found that narcoleptic patients experience sleep paralysis only at the time of SOREMP with a very short REM latency. These findings suggest that there is a close correlation between the severity ofISP and SOREMP latency. The verbal reports of our subjects corresponded with those of narcoleptics as reported by Hishikawa et a1. (15). All but one (NA) reported auditory/visual hallucinations and unpleasant emotions. All subjects were fully conscious of lying in the laboratory and of an inability to move during ISP. These major elements of ISP reported by our normal subjects seem to be the same as those found in narcolepsy. It is likely that, both in normal individuals and narcoleptic patients, sleep paralysis with/without hypnagogic hallucinations has common psychological associations, which evoke unpleasant emotions or fear. As mentioned above, ISP occurred at SOREMP with a very short REM latency and showed abundant alpha EEG with persistent atonia as well as the paralysis reported in narcoleptics by Hishikawa et a1. (15). They Sleep, Vol. 15, No.3, 1992
found a significant increase in alpha activity during SOREMP with sleep paralysis compared to that without sleep paralysis. Thus, ISP in the normal population apparently has a common physiological background to sleep paralysis in narcolepsy. In the severity of the paralysis, however, ISP differs from sleep paralysis in narcolepsy. All of our subjects could terminate ISP either voluntarily or when their name was called. On the other hand, Hishikawa et a1. (15) reported that some narcoleptic patients could not terminate their paralysis even when called or when shaken physically. Further, some of their patients could not sit up in bed without support by the experimenter, whereas there was no such case among our subjects. Thus, suppression of tonic muscle atonia during ISP appears to be weaker than that during sleep paralysis in narcolepsy. With regard to quantitative aspects of the severity of paralysis and the difficulty of recovery, the paralysis in narcolepsy appears to be more pronounced than that seen in ISP in normal individuals. Figures 2-4 indicate that ISP is characterized by abundant alpha-wave activity and atonia. These figures demonstrate state dissociation, which means the simultaneous appearance of the elements of two states, i.e. Wand REM (25). There were two types of state dissociation. One was the intrusion of an alpha EEG (i.e. an element of W) into REM sleep (Figs. 2 and 4). This is interpreted as a REM stage with a high level of arousaL The other type was the persistence of REM atonia into W (Fig. 3). During ISP, short muscle twitches or eye opening, which were suggestive of arousal response, did not disturb the subjects' atonia. The typical sequence of both types of ISP was as follows. Alpha waves intruded into REM sleep, an arousal response occurred, atonia persisted despite the waking EEG pattern and ISP was ended with a major body movement or by the experimenter's call. Thus, we conclude that ISP occurs in the transition between stages REM andW. Interesting PSG features were found in cases MH (Fig. 2) and AK (Fig. 4), where the alpha train was intermittently blocked by REMs, as though the subjects were looking at something. In particular, the alpha blocking in AK may have represented her visual hallucination in a manner analogous to a visual image blocking the alpha wave activity in the waking state. The documented ISP occurred during the dissociation of the REM/W stage. Judging from the PSG data, ISP differs from other dissociated states of these stages, such as lucid dreaming or REM sleep behavior disorders (RBD). Lucid dreaming occurs in unequivocal REM sleep (26), whereas ISP occurred during ambiguous REM sleep or in the transition between REM and W. Alpha-wave activity during lucid dreaming (27)
:t,:
ELICITED ISOLATED SLEEP PARALYSIS was not as high as during ISP, which showed alpha bursts or trains. Schenck et al. (28) polygraphically indicated that RBD was associated with intermittent tonic EMG augmentation in conjunction with REMs. On the other hand, ISP showed a suppressed EMG accompanied by REMs. Furthermore, ISP also appears to differ from nocturnal panic attacks, although both involve frightening experiences. Panic attacks typically occur at the transition between stages 2 and 3 (29) and thus represent NREM/W dissociation, whereas ISP occurred at the transition between REM and W. Thus, ISP is polygraphically and physiologically different from lucid dreaming, RBD and panic attacks. Finally, our results showed a close relationship between SOREMP and ISP. It may be that the occurrence ofSOREMP is a necessary condition for inducing ISP but is not sufficient by itself. There may be other factors promoting the occurrence of ISP in addition to SOREMP. Fukuda et al. (13) used a questionnaire survey and found that the conditions preceding ISP in daily life are physical tiredness and psychological stress. The sleep interruption itself and performing the vigilance task while sleepy seemed to be a physically and psychologically stressful situation for some of the subjects. This situation may thus have increased the probability ofISP. In addition to the exogenous pre-ISP situation, we could also consider that some endogenous factors may affect the occurrence ofISP. HLA-DR2 is strongly associated with narcolepsy (30). However, the association between HLA-DR2 and ISP was uncertain in our small study: two subjects were DR2 positive and two were negative. Thus, further investigations are needed to examine the role of endogenous factors and individual differences as well as that of the pre-ISP psychophysiological factors. Acknowledgements:
Part of this research was supported
by a grant from the Ministry of Education and Culture, Japan (no. 01510093).
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