Stage at Awakening, Sleep Inertia and Performance

Transcription

1 Sleep Research Online 5(3): 89-97, Printed in the USA. All rights reserved X 2003 WebSciences Stage at Awakening, Sleep Inertia and Performance Corrado Cavallero and Francesco Versace Department of Psychology, University of Trieste, Italy Our purpose was to verify if the differential effect of final stage at awakening varies with the amount of sleep reduction and if this effect goes beyond the boundaries of sleep inertia. Seven university students were paid for their participation. The design included two conditions, REM (three consecutive nights) and NREM (three consecutive nights). Sleep length was progressively curtailed to 6 hrs, 4.5 hrs and 3 hrs for the first, second and third night, respectively. In the REM condition, participants were awakened during REM sleep, while in the NREM condition, participants were awakened during Stage 2. Days following experimental nights were devoted to the assessment of performance. Test sessions were scheduled upon awakening and then every three hours thereafter. Performance was measured by means of a Simple Reaction Time task and a Four Choice reaction time task. Results show that sleep inertia after NREM awakenings is more pronounced than after REM awakenings. Sleep curtailment enhances this differential effect and prolongs it beyond sleep inertia boundaries. Final stage at awakening not only exerts a differential influence on performance within the sleep inertia phase, but also impairs performance after NREM awakenings following inertia dissipation, especially when sleep is curtailed. CURRENT CLAIM: Sleep inertia following Stage 2 awakenings is more pronounced than after REM awakenings; sleep curtailment enhances this differential effect and prolongs it beyond sleep inertia boundaries. When waking from sleep, people usually experience a period of confusion in which performance, compared to pre-sleep levels, is impaired. This phenomenon, which is most dramatic and evident when awakening is abrupt, is known as sleep inertia and has been observed by a great number of researchers (Kleitman, 1963; Lubin et al., 1976; Naitoh, 1981; Dinges et al., 1981, 1985, 1987; Bonnet, 1983, 1993; Balkin and Badia, 1988; Dinges, 1989, 1990; Pivik, 1991; Naitoh et al., 1993; Mullington and Broughton, 1994; Bonnet and Arand, 1995). Sleep inertia reflects the graduality of the awakening process, just as increasing sleepiness characterizes the transitional state of sleep onset. The question of whether sleep inertia and sleepiness are of the same nature is still debated. Balking and Badia (1988) failed to find any conclusive evidence suggesting that sleep inertia is qualitatively different from typical sleepiness. Tassi and Muzet (2000), in their recent review of the literature, hypothesize that performance decrements observed in sleep inertia are due to lowered levels of arousal, while sleepiness is a state of hypo-vigilance and furthermore, that both states share the slowing down of reaction times but only sleepiness involves a reduction in performance accuracy. Regardless of its nature, sleep inertia is typically modest and short-lived following awakening from a night with a normal amount of sleep (Dinges, 1990). Sleep deprivation/reduction tends to increase its intensity (Dinges et al., 1985; Balkin and Badia, 1988) and duration (Naitoh, 1981; Dinges et al., 1987), even if results on duration have been inconsistent. For example, Naitoh (1981) found that after 50 hrs of sleep deprivation, sleep inertia lasted for several hours, while Dinges et al. (1987) showed that, even after 56 hrs of sleep deprivation, sleep inertia never lasted more than 30 minutes. This discrepancy, as pointed out by Muzet et al. (1995), is probably attributable to differences in the experimental designs and tasks used in the various studies. However, even if duration estimates vary from 1 minute (Webb and Agnew, 1964) to several hours (Naitoh, 1981), the various authors tend to agree on a maximum of 3-4 hrs. Sleep stage at awakening is an important factor in amplifying sleep inertia: waking up from slow wave sleep (SWS) is worse than waking up from REM sleep (Dinges et al., 1985). Less is known about the relationship between other NREM stages and performance, even if Scott (1969) made an attempt at studying performance as a function of the sleep stage from which participants are aroused. The author states, Preliminary results... indicate... progressively greater impairment in relation to Stage 1-REM, 2 and 3+4, respectively. Thirty years later, Jewett et al. (1999) analyzed the time course of inertia dissipation following REM and Stage 2 awakenings in participants who slept an average of 8 hrs per night and found that sleep inertia effects dissipate in about 3 hrs in both conditions but failed to find any final sleep stage effect on performance. This failure could be due to the amount of sleep preceding the final awakening; indeed, if curtailing sleep time enhances sleep inertia, it is reasonable to presume that partially depriving participants of sleep can also amplify the differential effect of sleep stage at awakening on sleep inertia. However, this line of reasoning can only be followed at a hypothetical level, as we were not able to find a single study that specifically investigated final stage effects in partially sleep-deprived participants. Correspondence: Corrado Cavallero, Ph.D., University of Trieste, Department of Psychology, Via S. Anastasio 12, Trieste, Italy, Tel: , Fax: , cavaller@univ.trieste.it.

2 90 CAVALLERO AND VERSACE Another much debated question refers to the duration of both sleep inertia and differential final stage effects. On the one hand, there is no general agreement on how long sleep inertia lasts: duration estimates vary from 1 minute (Webb and Agnew, 1964) to several hours (Naitoh, 1981), even if the various authors seem to agree on a maximum duration of 3-4 hrs. On the other hand, the only study that investigated the time course of sleep inertia following awakening from REM and NREM sleep (Jewett et al., 1999) failed to find a differential effect, even upon awakening. Moreover, nearly nothing is known about what happens beyond the agreed end point of the time course of sleep inertia. In particular, does sleep stage at awakening exert any differential effect on performance during the period that goes from hypothesized sleep inertia dissipation to successive sleep onset? In order to answer some of these questions, one has to devise an experimental design which allows for a) the comparison of the same participant s performance following awakenings alternatively in REM and NREM conditions; b) the shortening of a comparable amount of sleep granted to each participant; and c) the keeping constant of together with the amount of sleep granted to each participant the circadian phase in which participants are awakened. Keeping these premises in mind, the only viable solution seems to be comparison between awakenings in REM and Stage 2. In fact, a comparison between REM and SWS awakenings, a situation in which the most pronounced effects, if any, would be expected, would not allow for simultaneous control of sleep amount and circadian phase. The present study, which was designed following the above mentioned criteria, allows us to compare performance following awakenings in REM and NREM-Stage 2 in two different sleep reduction conditions (Low and High) in order to verify a) if the differential effect of final stage at awakening varies according to the amount of sleep reduction; and b) if this effect goes beyond what is commonly defined as the boundary of sleep inertia effects. In particular we proposed three main hypotheses: A) The average daily level of performance is better following awakening in REM than in NREM-Stage 2; B) The difference between REM and NREM awakenings in the average daily level of performance increases (or becomes significant) in the High Sleep Reduction Condition; C) In the High Sleep Reduction Condition, the difference in performance following REM and Stage 2 awakenings persists during the period that goes from hypothesized sleep inertia dissipation to successive sleep onset. types. Participants were told that they would be submitted to a reduced sleep schedule and their performance would be tested during the day, at fixed intervals. They were asked to maintain their habitual physical activity and study routines, to reduce caffeine consumption, and to not sleep during the experimental days. Experimental Design The entire experimental period was divided into three different conditions: Adaptation (one night), REM (three consecutive nights), and NREM (three consecutive nights). These took place in three different weeks according to the following schedule: Adaptation (1st week), REM and NREM in the following two weeks in counterbalanced order. During the Adaptation night, each participant slept following his/her own habitual sleep schedule in order to facilitate adaptation to the lab situation and determine sleep duration and architecture. In the REM condition, each participant underwent progressive sleep reduction and was awakened during the fourth REM phase (1st day REM Baseline), during the third REM phase (2nd day Low Sleep Reduction), and during the second REM phase (3rd day High Sleep Reduction). Participants were awakened ten minutes after the appearance of the first clear REM burst. In the NREM condition, participants were always awakened in Stage 2, in the effort to obtain comparable amounts of sleep reduction 1. In both REM and NREM conditions, participants were allowed to sleep for about six hours (roughly corresponding to four sleep cycles) during the first night (Baseline), for about four and a half hours (three sleep cycles) the second night (Low Sleep Reduction), and for about three hours (two sleep cycles) the third night (High Sleep Reduction) (Figure 1). During the wake period following each night, performance levels were assessed in test sessions that took place at fixed intervals. The first test session started immediately after awakening and then every three hours according the following schedule: 8 a.m., 11 a.m., 2 p.m., 5 p.m., 8 p.m., and 11 p.m., with the exception of Day 3 in both REM and NREM R E M Baseline Low Sleep Reduction A High Sleep Reduction A 5 Figure 1 Experimental Design Experimental Design A METHODS Baseline A Participants Eight normal sleepers (four males and four females, aged 20 to 26), all university students, were studied in pairs in the sleep laboratory. On the basis of the results of the Morningness/Eveningness Questionnaire (MEQ, Horne and Ostberg, 1976), all participants were classified as intermediate N R E M Low Sleep Reduction A High Sleep Reduction A NREM Sleep Wake A Awakening REM Sleep Testing 1 When the NREM condition was run before the REM condition, the awakening time was determined with reference to the sleep architecture of the adaptation night.

3 STAGE AT AWAKENING, SLEEP INERTIA AND PERFORMANCE 91 conditions, where an additional 5 a.m. test session took place (Figure 1). During the time intervals between test sessions, participants were allowed to perform activities of their choice (work, study, etc.) but were not allowed to sleep. In order to avoid the masking effects of a learning curve, all participants had a practice session before the beginning of the adaptation night and before the beginning of the first experimental night in each condition. Procedure Adaptation At 9 p.m., participants arrived at the lab where they received instructions regarding the following day s schedule and then practiced with the test battery for about one and a half hours, performing each task 10 times. At 11 p.m. (mean time), participants were prepared for the night. Two unipolar EEG (F3-A1, F4-A1) and two EOG (LE-A2, RE-A2) channels were recorded during the night. At 12:40 a.m. (mean time) participants went to bed and lights were turned off. Sleep onset was assessed by the appearance of the first clearly identifiable K-complex and/or sleep spindle. The following morning, one hour after being awakened, participants underwent another practice session (10 rounds of each performance task) and then they left the sleep laboratory. Experimental Conditions At 10 p.m., participants arrived at the laboratory. They practiced with the test battery for a while (completing three sessions of each performance task) and were then prepared for the night following the above-mentioned procedure. At 12:20 a.m. (mean time), participants went to bed and lights were turned off. After being awakened, participants were tested following the same schedule for both experimental conditions, as illustrated in Figure 1. Materials Each test session lasted about 10 minutes and consisted of three parts: a) measurement of timpanic temperature (by means of a Thermoscan Plus thermometer, Thermoscan Inc.); b) assessment of subjective vigilance levels (Stanford Sleepiness Scale (SSS) [Hoddes et al., 1973], and Global Vigor/Affect Scale (GVAS) [Monk, 1989]); and c) assessment of performance levels by means of two tests (described below) presented in randomized order. The seven statements of the SSS appeared on the computer screen, and the participant made his/her choice by pressing the corresponding key on the numerical keyboard of the computer. The GVAS was administered in a paper and pencil version. The two performance tests had been previously created with the Micro Experimental Laboratory (MEL) software (Schneider, 1988, 1990), and they ran on 486 IBM compatible computers. The first was a Simple Reaction Time Task (SRTT). In this test, participants are asked to press the keyboard space bar immediately after the presentation of a red circle on the computer screen, presented 60 times at randomized intervals (Inter-Trial Interval (ITI) ranging from 600 to 2000 msecs). The entire task takes about two minutes. The second was a Four-Choice Reaction Time Task (4CRTT). In this test, the target (a red circle) is presented in one of four different positions arranged in a square on the computer screen. Participants are asked to press the key on the numeric keypad corresponding to the position of the target as fast as possible. The total number of trials is 40, ten for each possible target position, presented with a 1000 msecs ITI for a total duration of about two minutes. The dependent variables were reaction times for the SRTT and reaction times plus error rates for the 4CRTT. RESULTS Physiological Data Sleep recordings were independently scored according to Rechtschaffen and Kales s criteria by two experienced scorers (the two authors), who reached an interscorer reliability of Discrepancies were then reconciled and all analyses were performed on the reconciled version. No discrepancies were found regarding sleep stage at awakening. As a result of this analysis, one participant had to be excluded from the original sample, because her awakenings did not conform to standards 2. Further analyses were therefore carried out on the data from the remaining seven participants (four who had followed the REM- NREM schedule and three the NREM-REM schedule). To verify the comparability of the intensity of sleep reduction in the REM and NREM conditions, total sleep time (TST) was used as dependent variable in a two-way (Condition by Day) ANOVA. Not surprisingly, we found a significant main effect for Day (F 2,12 =186.20; p<0.0001; average TST for Day 1 (Baseline) was ±10.50 minutes, Day 2 (Low Sleep Reduction) was ±17.62, Day 3 (High Sleep Reduction) was ±12.21 minutes), while the interaction Condition by Day was not significant (Table 1). Since the above analysis showed that the two conditions were comparable regarding TST for the three sleep reduction levels, we proceeded analyzing sleep architecture for the two conditions within each day. Three ANOVAs were performed (one for each day) using Condition (REM and NREM) and Sleep Stage (Stage 1, Stage 2, SWS, REM, Movement and Wake Time after Sleep Onset) as factors. The interaction Condition by Stage was not significant in any of the three ANOVAs, thus indicating no main differences in sleep architecture between the REM and NREM conditions. Mean percentages of each sleep stage for the three days in REM and NREM are shown in Table 1. As far as sleep efficiency (time asleep/time in bed) is concerned, we found no difference between the two conditions or among the three days (Table 1). Temperature data for both REM and NREM Conditions were subjected separately to Cosinor Analysis to verify if experimental condition affected circadian rhythmicity. Table 2 presents the results derived from the analyses, where it can be noted that circadian rhythmicity is maintained in both conditions with no major differences between the two. 2 One awakening, initially classified as NREM, was indeed a REM one; thus, the NREM condition was incomplete.

4 92 CAVALLERO AND VERSACE Table 1 Sleep Measures for Baseline, Low and High Sleep Reduction in REM and NREM Conditions TST St1 St2 SWS REM W M SE SR AC Mins % % % % % % % Baseline REM (16.76) (1.14) (4.89) (8.40) (5.97) (2.50) (0.55) (2.76) NREM (18.71) (0.72) (3.25) (4.33) (5.62) (1.47) (2.16) (5.88) Low Sleep Reduction REM (31.98) (0.87) (8.62) (3.67) (5.67) (0.49) (1.57) (5.94) NREM (6.38) (0.71) (4.42) (8.70) (6.83) (0.23) (5.64) (7.71) High Sleep Reduction REM (14.41) (0.88) (8.50) (12.30) (4.58) (0.23) (1.89) (2.92) NREM (14.41) (1.04) (4.83) (8.40) (6.62) (0.85) (2.83) (3.61) NOTE: SR=Sleep Reduction; AC=Awakening Condition; TST=Total Sleep Time; St1=Stage 1; St2=Stage 2; SWS=Slow Wave Sleep; REM=Rapid Eye Movement Sleep; W=Wake Time; M=Movement Time; SE=Sleep Efficiency. Table 2 Cosinor Summary of Temperature Circadian Rhythm in REM and NREM Conditions Condition N PR Mesor(SD) Amplitude Acrophrase (h min) Mean (95% CI) Mean (95% CI) REM (0.32) 0.29 (0.17 to 0.42) (14.21 to 17.50) NREM (0.28) 0.27 (0.12 to 0.42) (15.09 to 18.53) NOTE: PR=Percent Rhythm measure of the strength of the circadian rhythm; Mesor=Rhythm-determined average; Amplitude=measure of one-half the extent of rhythmic change in a cycle; Acrophase=peak time with reference to local midnight. Subjective Data For each condition (REM and NREM) and for each scale SSS and GVA (separately for Vigor and Affect subscales) mean values were calculated for test sessions between the hours of 11 a.m. and 11 p.m. in the Baseline condition. All values from the Low and High Sleep Reduction conditions were then converted into deviations from these means in order to eliminate the effects of interindividual differences. Mean deviation scores were then calculated for four different periods: Awakening; Morning (averaging the 8 a.m. and 11 a.m. sessions for Low Sleep Reduction and the 5 a.m., 8 a.m., and 11 a.m. sessions for the High Sleep Reduction conditions); Afternoon (averaging the 2 p.m. and 5 p.m. sessions); and Evening (averaging the 8 p.m. and 11 p.m. sessions). Deviation scores for each scale (SSS and GVA separately for the Vigor and Affect subscales) were used as dependent variables in a three-way ANOVA with Awakening Condition (REM and NREM), Sleep Reduction (Low and High) and Period (Awakening, Morning, Afternoon, and Evening) as factors. Stanford Sleepiness Scale. We found significant main effects of a) Sleep Reduction (Low=0.83±0.41; High=1.59±0.41; =46.59; p=0.0005); b) Period (Awakening=2.39±0.87, Morning=0.83±0.34,Afternoon=0.62±0.40, Evening=1.00±0.46; =20.01; p<0.0001). Post-hoc comparisons for Period showed that the mean deviation at Awakening was significantly different from those at Morning, Afternoon and Evening (Tukey s HSD, p<0.05), while no significant differences among these last three periods could be detected. No interaction reached significance. Global Vigor Affect. Regarding the VIGOR subscale, we found significant main effects of a) Sleep Reduction (Low= ; High = ; F 1,5 =12.48; p=0.0167); b) Period (Awakening= , Morning= , Afternoon= -8.84, Evening= ; F 3,15 =6.48; p=0.005). Post-hoc comparisons for Period showed that the mean deviation at Awakening is significantly different from those at Morning, Afternoon and Evening (Tukey s HSD, p<0.05), while no significant differences among these last three periods could be detected. Again, no interaction reached significance. As far as the AFFECT subscale is concerned, no significant differences were found for main or interaction effects. Performance Data Reaction Time For each condition (REM and NREM) and for each task (simple and four-choice reaction time), mean reaction times were calculated for test sessions between the hours of 11 a.m. and 11 p.m. in the Baseline Condition. All reaction times from the low and high sleep reduction conditions were then converted into deviations from these means in order to eliminate the effects of interindividual differences. Mean deviation scores were then calculated for four different

5 STAGE AT AWAKENING, SLEEP INERTIA AND PERFORMANCE 93 periods: Awakening; Morning (averaging the 8 a.m. and 11 a.m. sessions for Low Sleep Reduction and the 5 a.m., 8 a.m., and 11 a.m. sessions for the High Sleep Reduction conditions); Afternoon (averaging the 2 p.m. and 5 p.m. sessions); and Evening (averaging the 8 p.m. and 11 p.m. sessions). Simple Reaction Time Task (SRTT). Deviation scores for SRTT were used as the dependent variable in a three way ANOVA with Condition (REM and NREM), Sleep Reduction (Low and High) and Period (Awakening, Morning, Afternoon, and Evening) as factors. We found a significant main effect of Awakening Condition (REM=15.14; NREM=24.10; =6.77; p=0.0405) (Hypothesis A), Sleep Reduction (Low=12.91; High=26.33; =7.34; p=0.0352), and Period (Awakening=50.23, Morning=13.42, Afternoon=6.24, Evening=8.58; =15.76; p<0.0001). Significant interactions were Awakening Condition by Period ( =4.07; p=0.0226) and Sleep Reduction by Period ( =3.48; p=0.0377). Post-hoc comparisons for Period showed that the mean deviation at Awakening is marginally significantly different from that at Morning (Tukey s HSD, p<0.10), and significantly different from those at Afternoon and Evening (Tukey s HSD, p<0.05), while no significant differences among these last three periods could be detected. Post-hoc comparisons for the Condition by Period interaction showed that the difference between the REM and NREM conditions is marginally significant upon awakening (Tukey s HSD, p<0.10), while no significant difference was found for the Morning, Afternoon, or Evening (Table 3). Table 3 SIMPLE REACTION TIME TASK Mean Deviation Scores for Awakening Condition (REM/NREM), Sleep Reduction (LOW/HIGH) and Period (Awakening, Morning, Afternoon, Evening). Main Effects and Interactions. AC =6.77; p= (15.35) (21.90) Main Effects SR =7.34; p= LOW HIGH (13.39) (24.09) P =15.76; p< Aw Mo Af Ev (36.92) (12.59) (15.16) (12.78) AC by SR =4.44; p= LOW (11.38) (17.27) HIGH (20.75) (27.63) First Order Interactions AC by PR =4.07; p= Aw (25.61) (49.22) Mo (12.55) (14.54) Af (10.42) (21.06) Ev (16.21) (10.69) SR by P =3.48; p= LOW HIGH Aw (26.83) (50.05) Mo (7.32) (19.28) Af (13.73) (17.99) Ev (10.70) (16.07) Second Order Interaction AC by SR by P =2.31; p= LOW HIGH Aw (19.81) (36.74) (35.61) (65.76) Mo (13.81) (6.92) (15.34) (24.32) Af (10.31) (47.33) (15.16) (21.06) Ev (10.61) (12.91) (22.41) (11.93)

6 94 CAVALLERO AND VERSACE Post-hoc comparisons for the Sleep Reduction by Period interaction showed that the difference between the Low and High Reduction conditions is marginally significant only at awakening (Tukey s HSD, p<0.10), while no significant difference was found for the Morning, Afternoon, or Evening (Table 3). Four Choice Reaction Time Task (4CRTT). Deviation scores for the Four-Choice Reaction Time Task were used as the dependent variable in a three-way ANOVA with Condition (REM and NREM), Sleep Reduction (Low and High) and Period (Awakening, Morning, Afternoon, and Evening) as factors. We found a marginally significant main effect of Awakening Condition (REM=11.33; NREM=22.38; =4.79; p=0.0713) (Hypothesis A), and significant main effects of Sleep Reduction (Low=13.02; High=20.69; =17.21; p=0.0060) and Period (Awakening=49.59, Morning=10.33, Afternoon=3.63, Evening=3.88; =45.37; p<0.0001). The only significant interaction was Sleep Reduction by Period ( =3.96; p=0.0247). Post-hoc comparisons for Period showed that the mean deviation at Awakening is significantly different from those at Morning, Afternoon and Evening (Tukey s HSD, p<0.01), while no significant differences among these last three periods could be detected (Table 4). Post-hoc comparisons for the Sleep Reduction by Period interaction showed that the difference between the Low and High Reduction conditions is marginally significant only at Awakening Table 4 FOUR CHOICE REACTION TIME TASK Mean Deviation Scores for Awakening Condition (REM/NREM), Sleep Reduction (LOW/HIGH) andperiod (Awakening, Morning, Afternoon, Evening). Main Effects and Interactions. AC =4.79; p= (12.01) (20.31) Main Effects SR =17.21; p= LOW HIGH (14.28) (16.60) P =45.37; p< Aw Mo Af Ev (23.82) (14.12) (16.33) (11.26) AC by SR =3.16; p= LOW (10.42) (20.09) HIGH (13.93) (21.16) First Order Interactions AC by PR =1.07; ns Aw (17.55) (37.53) Mo (14.14) (16.26) Af (10.94) (23.61) Ev (14.30) (9.87) SR by P =3.96; p= LOW HIGH Aw (21.15) (28.76) Mo (12.46) (17.06) Af (19.61) (15.50) Ev (11.78) (11.82) Second Order Interaction AC by SR by P =0.70; ns LOW HIGH Aw (22.41) (40.78) (25.56) (35.58) Mo (15.15) (12.75) (14.59) (24.97) Af (16.06) (26.27) (8.33) (25.55) Ev (12.80) (11.81) (16.68) (9.55)

7 STAGE AT AWAKENING, SLEEP INERTIA AND PERFORMANCE 95 (Tukey s HSD, p<0.10), while no significant difference was found for the Morning, Afternoon, or Evening (Table 4). At this point, we know that when sleep time is artificially curtailed, a) waking from NREM generally has a worsening effect on successive performance than waking from REM does (Hypothesis A); b) four and a half hours of sleep produces significantly fewer negative effects than three hours of sleep; c) the negative effect of sleep reduction is at its maximum immediately after awakening and then decreases progressively; and d) greater sleep reduction produces (disregarding final stage at awakening) the worst performance levels upon awakening. In order to verify the hypothesis that the difference between REM and NREM awakenings in the average daily level of performance increases (or becomes significant) in the High Sleep Reduction Condition (Hypothesis B), planned comparisons for the interaction Awakening Condition x Sleep Reduction were conducted with the following results: Low Sleep Reduction Condition no significant differences were observed between the REM and NREM conditions in the mean deviation for SRTT (REM=10.20, NREM=15.63; =1.49; p=0.2679), and the 4CRTT (REM=8.86, NREM=17.17; =2.30; p=0.1797) (Tables 3 and 4); High Sleep Reduction condition both SRTT and 4CRTT revealed significant differences between the REM and NREM conditions in the mean deviation (SRTT- REM=20.09, NREM=32.56; =16.42; p= CRTT-REM=13.80, NREM=27.59; =7.37; p=0.0349) (Tables 3, 4 and Figure 2). In general, progressive sleep reduction increases the REM- NREM difference; for both tasks (SRTT and 4CRTT) the difference is not significant, though it does go in the expected direction when Sleep Reduction is Low and becomes significant when reduction is High. In order to verify the hypothesis that in the High Sleep Reduction Condition performance differences following REM and Stage 2 awakenings persist during the period that goes from hypothesized sleep inertia dissipation to successive sleep onset (Hypothesis C), planned comparisons were carried out with the following results: Simple Reaction Time Task (SRTT) Low Sleep Reduction Condition: no significant differences between the REM and NREM conditions were observed for any period considered. High Sleep Reduction Condition: we found a significant difference between REM and NREM not only at Awakening (REM=45.90, NREM=80.39, =7.15; p=0.0368), but also in the Morning (REM=16.66, NREM=29.58; =7.01; p=0.0381) and in the Afternoon (REM=5.38, NREM = 12.24; =6.31; p=0.0457). (Table 3 and Figure 3); Four Choice Reaction Time Task (4CRTT) Low Sleep Reduction Condition: no significant differences between the REM and NREM conditions were observed for any period considered. High Sleep Reduction Condition: we found a marginally significant difference between the REM and NREM conditions at Awakening (REM=51.23, NREM=69.17, =4.25; p=0.0850) and a significant one in the Morning (REM=3.56, NREM=25.55, =6.65; p=0.0418), (Table 4 and Figure 4). These results show that when sleep is reduced to about two sleep cycles, the REM/NREM difference is evident, not only at awakening, as expected, but also during the morning and afternoon. Error Rate Error rates (number of errors/total number of trials) for the 4CRTT were extremely low for all levels of Awakening Condition, Sleep Reduction and Period, ranging from a DEVIATION SCORE (ms) DEVIATION SCORE (ms) DEVIATION SCORE (ms) Mean Deviation Score (ms) and Standard Error of the Awakening x Sleep Reduction Interaction for Simple Reaction Time (SRT) and Four-Choice Reaction Time (4CRT) Tasks Figure 2 *** ** LOW HIGH LOW HIGH SLEEP REDUCTION SLEEP REDUCTION SRT 4CRT Figure 3 Mean Deviation Score (ms) and Standard Error for Simple Reaction Time (SRT) Task for Each Period in REM and NREM Conditions Following Low and High Sleep Reduction AW MO AF EV AW MO AF EV LOW HIGH SLEEP REDUCTION Figure 4 Mean Deviation Score (ms) and Standard Error for Four-Choice Reaction Time (4CRT) Task for Each Period in REM and NREM Conditions Following Low and High Sleep Reduction AW MO AF EV AW MO AF EV LOW HIGH SLEEP REDUCTION ** * ** ** REM NREM REM NREM REM NREM

8 96 CAVALLERO AND VERSACE minimum of 0 to maximum of (i.e., from 0 to 1 error out of 40 trials). The scarcity of the data did not allow any kind of reasonable statistical analysis. Such a low error rate would indicate that performance accuracy in this task was not at all influenced by the experimental manipulation. DISCUSSION Our experimental design made it possible to compare the effects of sleep stage at awakening following different amounts of sleep without any confounding effect due to either circadian rhythm or sleep architecture. The lack of differences between the two Awakening Conditions (REM and Stage 2) in all the physiological parameters considered, confirmed the soundness of the initial idea of limiting our comparison to Stage 2 and REM awakenings. The lack of a REM/NREM difference in subjective ratings is not surprising; the resolution power of these measures is in fact too low to reveal anything different from the obvious relationship between the amount of sleep participants receive and their sensations of tiredness. Amethodological issue that should be stressed here, instead, is the high resolution power of the performance tasks we used; both are very short in duration but both show significant decreases after experimental sleep reduction, even when the total sleep time difference between the Low and High Sleep Reduction Conditions was under two hours. Our tasks were short in duration, but highly demanding for the participant, because Inter-Trial Intervals ranged from 600 to 2000 msecs and therefore required a high rate of response by the participant. This result confirms the idea proposed by various authors (e.g., Dinges and Kribbs, 1991; Gillberg and Akersdtedt, 1998) that in order to detect sleep loss deficits, it is not necessary to use very long tasks because, as Gillberg and Akersdtedt (1998) stated, there is no safe duration for a monotonous task, since sleep deprivation effects on performance can be observed even during the first trials of the task. Another interesting aspect of our results is the apparently different sensitivity of the two tasks to sleep stage at awakening effects; the SRTT shows a significant difference between the Awakening Conditions while the 4CRTT does not. One might speculate that the relatively higher complexity of the 4CRTT might partially counteract the adverse effects of being awakened in NREM, but considering that the p value (0.08) associated with the REM/NREM differences at awakening is two-tailed, one should opt for a more cautious interpretation and attribute the failure to reach significance to a lack of statistical power. Our results replicate the finding that sleep reduction can amplify sleep inertia effects upon awakening (as testified by the marginally significant difference between the Low and High Sleep Reduction Conditions for simple reaction times and choice reaction times). This result, however, cannot be unambiguously attributed to the amount of sleep reduction; we cannot, in fact, rule out the possibility of a circadian confound since the awakening scores were obtained at different times for each sleep reduction condition. More interestingly, the performance results obtained here tend to confirm the previously-hypothesized (Scott, 1969) differential effect exerted by sleep stage at awakening on subsequent performance. This difference increases with the increase of sleep loss 3. In fact, in the High Sleep Reduction Condition, the difference between the REM and NREM conditions in the post-awakening test session reaches (for simple reaction times) and approaches significance (for four-choice reaction times). Moreover, when sleep reduction is high, the differential effect of sleep stage at awakening continues until the morning test session for the 4CRTT and until the afternoon test session for the SRTT. This difference cannot be interpreted as a consequence of the sleep inertia effect since there is considerable agreement among sleep researchers that sleep inertia maximum duration is 3-4 hours. For this reason, we think that our results, provided they are replicated in a larger sample, should lead to a reconsideration of the elements which determine a participant s alertness level throughout the day. Folkard and Akerstedt (1991), for example, proposed a threeprocess model of alertness regulation, according to which alertness can be predicted by three parameters: Process S (sleep pressure, which is a function of the time since awakening); Process C (representing sleepiness due to circadian influences); and Process W (which refers to sleep inertia). Should our results be replicated, the addition of a fourth component to the model would be in order. We think that this fourth component, final stage at awakening, not only exerts a differential influence on performance within the sleep inertia phase, but also impairs performance after NREM awakenings following inertia dissipation, especially where alertness levels following sleep deprivation/reduction are concerned. 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