EFFECTS OF NIGHTTIME NAPS ON BODY TEMPERATURE CHANGES, SLEEP PATTERNS, AND SELF-EVALUATION

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1 J. Human Ergol., 10: , 1981 EFFECTS OF NIGHTTIME NAPS ON BODY TEMPERATURE CHANGES, SLEEP PATTERNS, AND SELF-EVALUATION OF SLEEP Kazuya MATSUMOTO Department of Hygiene, Kyorin University School of Medicine, 20-6 Shinkawa, Mitaka, Tokyo 181, Japan The effects of nighttime naps on physiological functions and self-evaluation of sleep were studied by changing the time at which a nap was taken. Eight healthy male subjects stayed overnight under five different conditions, i.e., with a 2-hr nap between 22: 00-00:00 (N1), 02: 00-04: 00 (N2), 04: 00-06: 00 (N3) or 06: 00-O8 : 00 (N4) or without any nap (SD) between a previous full night's sleep and a day sleep on the day following the night. Decrease in rectal temperature during the night was more marked for conditions N2, N3 and N4, with a lesser extent of individual differences, than for SD and N I. The mean decrease was in the order of N1, N2, N3, and N4. Latency of the onset of each of the sleep stages 51, S2, S3, or S4 was significantly shorter in N2, N3, and N4 than in N 1. Latency of rapid eye movement sleep (REND sleep) shortened and the amount of S4 or RENT sleep increased significantly in the order of N1, N2, N3, and N4. The self evaluation of the sleep depth and the rapidness of sleep onset correlated highly with these sleep parameters. N3 and N4 were evaluated to have resulted in a better sleep than N1. This suggests that when the nap was inserted between midnight and early morning, the ordinary circadian rhythm was more likely to be maintained. While the sleep polygrams and subjective self evaluation indicated the restorative effect of a 2-hr nap, its timing should be chosen so as to conform with the physiological circadian rhythm. Recent studies on patterns of sleep taken at different hours of the day have shown that they vary according to the circadian rhythm. This temporal patterning of sleep stages has been shown to be maintained throughout the 2 hr in a day, including daytime sleeps. The proportions of rapid eye movement (RENT) sleep and stage 4 sleep in naps taken following a normal night of sleep are shown Received for publication April 13,

2 174 K. MATSUMOTO to change progressively as the time of sleeping becomes later and later in the day (MARON et al., 1964; WEBS et al., 1966; WEBB and AGNEW, 1967; KARACAN et al., 1970; ENDO, 1977). Morning naps closely resemble the terminal portion of nocturnal sleeps, with a large proportion of REM stage and a lesser amount of stage 4 sleep, whereas late afternoon and evening naps resemble the first one-third period of a normal night's sleep, with a limited proportion of REM sleep and a large amount of stage 4 sleep. These studies have established that REM sleep has a clear circadian rhythmicity and that stage 4 sleep depends on the length of the period of prior wakefulness. Acute phase-shift experiments have demonstrated that the time-of-day effect on sleep patterns persists for 1-2 weeks after a 120 K- 180 K reversal of the bedrest-activity cycle (WEITZMAN et al., 1970; TAUB and BERGER, 1973; MOSES et al., 1975; CARSKADON and DEMENT, 1975; WEITZMAN et al., 1974, 1980). This response of REM sleep in accordance with changes in bedrest time is a characteristic of endogenous, self sustained circadian rhythms (ASCHOFF et al., 1967: 1975). With the development of industries, shift-work systems including night duties have been spreading yearly. It has been pointed out that various types of health impairments are more frequently seen among shift workers doing night duties than among day workers (RUTENFRANZ et al., 1976, 1977; COLQUHOUN et al., 1971; Shift Work Committee, Japan Association of Industrial Health, 1979). These impairments apparently result from disturbances in the sleepwake cycle which are associated with sleep loss and cumulative fatigue. Frequent night duties seem to aggravate these disturbances. As a countermeasure to alleviate these unfavorable effects of night duties, the importance of naps during night duties has been investigated by KOGI (1962), COLQUHOUN et al., (1975) and others. Though the taking of naps during night duties is practiced in many workshops in Japan, possibly more frequently than in Europe, the physiological effects of such naps are not yet well understood (COLQUHOUN et al., 1975). In the present study, the effects of nighttime naps as against those of total sleep deprivation were studied by changing the time at which a nap was taken. The effects were compared by analyzing changes in body temperature and sleep patterns occurring during the naps, and by self-evaluation of the sleep. The Table 1. An overview of experimental * sleep at home; (*) sleep in the laboratory; the first four nights of (*) are baseline nights. 06: 00; N4, nap between 06: 00-08: 00; SD, 27-hour sleep deprivation.

3 NIGHTTIME NAPS ON BODY TEMPERATURE 175 purpose of the study was to find suitable time zones for a nighttime nap as a measure to promote the health of workers. METHODS The subjects were 8 healthy males, age 21 to 24, who were otherwise living with a normal sleep-wake cycle. To habituate them to the experimental environment, they were asked to sleep in a laboratory room for 4 consecutive nights. The experiment was then performed following the experimental schedules shown in Table 1 and Fig. 1. The subjects took a 2-hour nap during 4 different 27-hour periods of sleep deprivation which started after a full night's sleep (00: 00-07: 00) and lasted until the time of day sleep (10: 00-07: 00) on the following day. The periods in which the examinees lay in bed for taking a nap were as follows: (1) late evening nap (22: 00-00: 00), referred to as Nl, (2) midnight nap (02: 00-04: 00), N2, (3) late night nap (04: 00-06: 0), N3, and (4) early morning nap (06 : : 00), N4. Sleep was not permitted during the whole period of 27 hr in a fifth schedule referred to as SD. The subjects were divided into four groups and the five schedules were randomly allocated to each group of examinees. Each sleep including naps was taken in a sealed, semi-soundproof room. Polygrams were recorded for the whole period of sleep on an electropolygraph. These consisted of EEG, EC, E MG, ECG, and respiratory movement. The electrodes for EEC, EG, and EMC were arranged according to ECTSCAFEEN and DALES' (1960, on an electropolygraph. Rectal temperature was measured with a thermistor thermometer at 10-min intervals starting from the in-bed time. The judgment of sleep stages was made according to ECHTSCAEFEN and ALES' (1960 criteria which define stages 1-4 (referred to as S1-S4) and stage E (rapid eye movement). Immediately after waking from respective sleeps, the examinees were told to note on a fixed form their self evaluations of "deepness of sleep," "rapidness of sleep onset" and "mood on waking." During the waking period, the examinees stayed in the laboratory room. Oral temperature, flicker fusion frequency, sleepiness (CARSKADON and DEMENT, 1977) and fatigue complaints (SAITO et al.,1970; K0GI et al.,1970) were recorded at sleep schedules for four groups of subjects. N1, nap between 22: 00-00: 00; N2, nap between 02: 00-04: 00; N3, nap between 04: 00-

4 176 K. MATSUMOTO Fig. 1. The five experimental schedules with and without a nighttime nap. one-hour intervals. Rectal temperature was measured at 2-hour intervals. As a result, the examinees had about 40 min per hour of free time in which they were allowed to watch TV, listen to the radio or read a magazine. While staying in the laboratory, they were asked not to leave the laboratory room, not to sleep except during the pre-determined nap period, and to refrain from taking drugs, alcohol, coffee, and other drinks, The experiment usually lasted for about one week for each of the five schedules. When an examinee was allowed to sleep at home, he was told to sleep only during the normal sleeping hours at night. RESULTS Rectal temperatures Figure 2 shows changes in the rectal temperature of individual cases during the experimental sleep-wake cycle for each of the five nap conditions. Rectal temperature abruptly fell, soon after lying in bed for the pre-experiment full night's sleep, attaining a minimum between 04: 00-06: 00 in the morning. However it rose soon after getting up at 07: 00 after the night's sleep, maintaining a high level during the daytime. During the experimental nap between 22:00 and 10: 00, however, the rectal temperature curve varied according to different nap conditions. In the SD and N1 conditions, some cases showed a drop while others maintained a relatively high temperature. However, in the other three nap conditions, and especially in N3 and N4, rectal temperature showed a steep drop during the night and early morning hours, with no distinct individual differences in the patterns. Also in these three conditions, the temperature tended to rise abruptly early in the morning after the minimum was attained. This tendency persisted until the middle of the day sleep. These results show two distinctly different patterns in rectal temperature changes, one in the SD and N1 conditions, and the other in the N2, N3, and N4 conditions. Figure 3 shows the changes, at 10-min. Intervals, in the mean rectal temperature averaged for the eight subjects during naps in the four different nap conditions. In all these conditions, rectal temperature tended to drop gradually

5 NIGHTTIME NAPS ON BODY TEMPERATURE 177 Fig. 2. Rectal temperature curves of eight subjects during five experimental sleep schedules.

6 178 K. MATSUOTO Fig. 3. Mean rectal temperature curves for different conditions of a 120mmin nap. O, nap taken between 22:00-00:00 (N1); œ, 02: 00-04: 00 (N2);, 04: 00-06: 00 (N3);, 06:00-08:00 (N4). Fig. 4. Means and standard deviations of the latency of each sleep. from the time of lying in bed. However, despite the drop, the temperature remained relatively high in the N1 condition, and was lower in the order of N2, N3, and N4 conditions. The analysis of variance revealed that the differences among the four nap conditions were significant at the 0.01 level.

7 NIGHTTIME NAPS ON BODY TEMPERATURE 179 Table 2. Time in minutes spent for different sleep stages during a nighttime nap. * Significant difference from condition N1 (p<0.0s);p< Sleep patterns during nap When the mean total sleep time during the 120-min nap period was given as the sum of time for S1, 52, 53, 54, and REIN sleep, the value was 99.8 min for N1, min for N2, min for N3, and min for N4. The total sleep time was shorter for Ni compared with that for other three conditions. There was no significant difference in sleep latency (Fig. 4) for S l-s4 between the conditions, except for NI (Fig. 4). RE sleep latency was the longest in N1, and tended to shorten in the order of N2, N3 and N4. The difference between RE latency in Ni and that in N3 or N4 was highly significant (p<0.01). As for the sleep stages in the 120-min period (Table 2), the amount of 54 and that of RE sleep increased in the order of Nl<N2<N3<N4. The difference in the amount of 54 or RE sleep between N1 and N3 was significant at the 0.05 level, and also between NI and N4 at the level. Self evaluation of sleep state The results of recording self evaluations of the naps taken were rated on a five-point scale for deepness and on a four-point scale for "rapidness of sleep onset," "mood on waking," or "sufficiency of the preceding nap." These evaluations were made in such a way that the score was higher if the sleep was poorer. As shown in Fig. 5, the score for 'deepness of sleep' was the worst for NI. Compared with Nl, it was better for N2 (p<0.05), N4 (p<0.45) and N3 (p<0.001). "Rapidness of sleep onset" likewise had the worst score for NI. It was significantly better for N3 and N4 (p<0.001) compared with N1. The score of "mood on waking" was worse for N2 and N3 (p<0.05) than for N1. But with "sufficiency of sleep," the score of the self evaluations did not vary significantly between the four conditions. Table 3 shows correlation coefficients between the self evaluations of the nap and the sleep parameters or rectal temperature immediately after the nap. It can be seen that both the self evaluations of "deepness of sleep" and "rapidness of sleep onset" showed significant positive correlations with S4 latency, REM sleep latency, waking time and the amount of S 1 sleep. These self evaluations had

8 180 K. MATSUMOTO Fig. 5. Self evaluations of sleep state for four different nap conditions. * Significant difference from condition N1(p<0.05); ** p<0.01. significant negative correlations with the amount of S4 or REM sleep. "Deepness of sleep" had a significant positive correlation with rectal temperature immediately before and immediately after lying in bed, whereas "rapidness of sleep onset" showed such a correlation only with rectal temperature immediately before lying in bed. The self evaluations of "mood on waking" and "sufficiency of the sleep immediately preceding" showed only low correlations with the sleep

9 NIGHTTIME NAPS ON BODY TEMPERATURE 181 Table 3. Correlation coefficients computed between sleep and self-evaluations of sleep. Significant difference * p<0.05; ** p<0.01;** p<0,001, parameters, and "mood on waking" showed a significant negative correlation only with rectal temperature immediately after the nap. DISCUSSION The present study indicated that rectal temperature during the nighttime varied with the timing of a 2-hour nap. Fall in rectal temperature was more marked for the N2, N3, and N4 nap schedules than for the SD and Nl schedules. In the N2, N3, and N4 schedules, rectal temperature rose steeply in the early morning after reaching its lowest point. This rise was maintained until the first half of the day sleep period. Also in the N2, N3, and N4 conditions, individual differences in the pattern of rectal temperature change were characteristically small. The falling tendency of the rectal temperature in N2, N3 and N4 was possibly accelerated because the nap was inserted in a period coinciding with the falling stage of the circadian rhythm physiological functions. In other words, lowering of metabolism and circulatory functions resulting from the nap took place when the nocturnal hypofunction due to the circadian rhythm was developing. Thus a nap, even as short as 2 hr, could exert similar effects on different individuals, depending on the time of its insertion. The 2-hour naps were able to help reinforce the underlying circadian rhythm, making the fall and rise of the rectal temperature more explicit. The results suggest that a short nap interrupting a nighttime sleep deprivation period could have a significant effect on the workers efficiency, as in the case of a night duty condition.

10 182 K. MATSUMOTO COLQUHOUN et al. (1975) investigated what effect a 4- or 8-hour sleep would have on the efficiency of work during the immediately following 04 : : 00 period. A drop in efficiency of work between 04: 00 to 08 : 00 was definitely less when the subjects took a 4-hr nap after working from 20: 00 to 00: 00 than when they did not. The drop was still marked, compared with the decrease for control subjects who slept for 8 hr from 20: 00 to 04: 00. The difference was especially outstanding in the signal detection rate and the false report rate. Thus, a nap taken during night duty, along with sufficient sleep or repose before the night duty, could prevent to a certain degree the efficiency of work from falling. The present results showed that rectal temperature during a nighttime nap tended to fall gradually with time from the onset of sleep for all of the respective experimental conditions, and that its level was lowest for N4, followed by N3, N2, and N1 in ascending order. This would be in accord with the circadian rhythm. On the other hand, a number of studies (MARON et al.,1964; WEBB and AGNEW, 1967; KARACAN et al., 1970; AGNEW and WEBB, 1973; ENDO, 1977; ATSUMOTO, 1978a; 1980) have reported the circadian rhythm of REM sleep, i.e., along with its increase with time from midnight to early morning and decrease from noon to early evening, REIN sleep was well retained for all of the respective nap schedules used in the present study. The present results also demonstrated that the amount of REM sleep increased in the order of N1 <N2<N3 <N4. Therefore, the amount of REM sleep is considered to be in accordance with the basic circadian rhythm. WEBB and AGNEW (1971) and ENDO et al. (1977; 1975) reported that the amount of S4 sleep was dependent on the length of prior wakefulness. In the present experiment, the interval between getting up from the night's sleep and the beginning of the nap was 15 hr for N1, 19 hours for N2, 21 hours for N3, and 23 hours for N4, S4 sleep increasing in proportion to the length of prior wakefulness. Views are divergent and not yet established as to what role the different sleep stages play in relation to recovery from fatigue. However, since the amounts of S4 and REM sleep were greatest and the latency of S4 and RE stages the shortest in the N3 and N4 schedules, these two conditions might favor recovery from fatigue. Furthermore, the self evaluations of sleep showed that the examinees subjectively considered themselves to have slept deeper and earlier in N3 and N4 than in N1. The "mood on waking," however, was evaluated as the worst for N2 and N3, probably because the examinees were forced to get up at the time when the functional level of the body was at its lowest in the circadian rhythm. CARSKADON and DEMENT (1977) found that under a min sleep-wake schedule, the sleepiness score increased with the lapse of time from night to early morning and decreased again in the afternoon. Furthermore, they reported that the sleepiness score correlated positively with the amount of non-rem sleep of S3 and S4, and that it correlated significantly negatively with the amount of REM sleep. The self-evaluation concerning sleep may be strongly influenced by two factors,

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