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Extraversion and sleep: A psychophysiological study of the arousal hypothesis
Person. individ.018
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Vol. 5, NO.
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3. pp. 345-353, 1984
0191-8869/84
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EXTRAVERSION STUDY W. Psychology
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1984 Pergamon
Press Ltd
AND SLEEP: A PSYCHOPHYSIOLOGICAL OF THE AROUSAL HYPOTHESIS* BURT
Department,
THOMPSON?
and
University
of Missouri,
JOHN
H.
MUELLER
Columbia,
MO 65211, U.S.A.
(Received 22 August 1983) Summary-In order to examine Eysenck’s (1967) hypothesis concerning the relationship between extraversion and arousal, autonomic measures were taken from introverts and extraverts immediately before sleep and during 6 hr of sleep. Introverts and extraverts did not differ in heart rate (HR) or skin-potential response rate (SPR) during the pre-sleep period. A variety of sleep-related measures of arousal provided no strong support for the hypothesis that introverts are more aroused than extraverts. The only consistent autonomic difference between introverts and extraverts during sleep was a tendency for introverts to have higher SPRs in the second 3 hr of the night. The results are discussed in terms of the level of environmental stimulation present in the experimental conditions, and suggestions are made for future sleep studies of the relationship between extraversion and arousal.
INTRODUCTION
Eysenck (1967) has proposed that the behavioral differences between introverts and extraverts can be partially explained in terms of individual differences in the cortico-reticular loop which mediates cortical arousal and inhibition. According to this arousal hypothesis, extraverts have lower levels of reticular activity and are characterized by high levels of cortical inhibition and thus reduced sensitivity to environmental stimulation. Introverts, because of higher levels of reticular activity, are presumably more sensitive. In theory, this difference in arousal should manifest itself in a variety of psychophysiological measures, including the orienting response, sensory thresholds, cortical activity measured by EEG, electrodermal responses and cardiac activity. Research using the EEG as a measure of arousal has failed to reveal any strong, systematic, relationship between extraversion and arousal. However, this failure is perhaps best interpreted as neither evidence for nor against an extraversion-arousal relationship because of the many methodological problems that have plagued this research [see Gale (1981) for a review]. Studies that have employed autonomic measures of arousal have not conclusively demonstrated that introverts are more aroused than extraverts. However, these studies do indicate that some relationship exists, although it appears to be a complex one (cf. Geen, 1983). For example, Coles, Gale and Kline (1971) presented Ss with a series of tones while measuring skin resistance. Although introverts showed no greater reactivity to the tones than extraverts, and took no longer to habituate to the tones, they did show more spontaneous skin-resistance responses over the course of the whole experiment. In another study of habituation, Crider and Lunn (1971) found that introverts took longer to habituate than extraverts, a finding that is consistent with the hypothesis that introverts have higher levels of arousal than extraverts. Other studies, in which both the intensity and the frequency of tones have been varied, also indicate that the relationship between extraversion and arousal is far from simple. For example, Smith and Wigglesworth (1978) measured electrodermal responses to a 1000 Hz tone at three levels of intensity. Habituation rates for the introverts and extraverts were equal at the low and middle intensity levels, but introverts took longer to habituate to the highest intensity level, If habituation rate is inversely related to level of arousal, then these results offer some support for the arousal hypothesis. In a study in which multiple measures of arousal across different situations were used, Gange, Geen and Harkins (1979) found that introverts showed more spontaneous skin-resistance responses and higher heart rates than extraverts for all three conditions of their study. Stelmack, Bourgeois, *This paper is based on a Thesis submitted by the first author Columbia, MO. ~To whom all correspondence should be addressed. 345
to the Psychology
Department,
University
of Missouri,
346
W. BURT THOMPSON and JOHN H. MUELLER
Chian and Pickard (1979) used electrodermal, cardiac and vasomotor indices of autonomic activity to measure the orienting reaction of introverts and extraverts to visual and auditory stimulation in two independent studies. In both studies, as indicated by all three measures, introverts took longer to habituate to both types of stimuli than the extraverts. It appears, then, that introverts and extraverts differ in their responses to certain situations in ways that suggest arousal differences exist between the two groups, although the nature of these differences is not yet clear. A potentially fruitful, though relatively neglected, situation in which to investigate the extraversion-arousal relationship is the sleep state. The onset and maintenance of sleep is dependent upon a relatively low level of activity in the reticular activating system (Cohen, 1979; Hobson, 1974). Thus the sleep state provides an opportunity to investigate Eysenck’s arousal hypothesis by examining sleep-related measures of arousal, and in particular, the transition from wakefulness to sleep. Additionally, the sleep state allows continuous monitoring of spontaneous variations in several indices of autonomic nervous system activity for an extended period of time. Although many studies of the effects of personality variables (including extraversion) on sleep patterns have been reported, few have specifically addressed the extraversion-arousal question [see Taub, Hawkins and Van de Castle (1978) for a review]. Spiegel and Nuesch (1977) correlated a variety of sleep measures with scores on the Maudsley Personality Inventory (MPI) for 20 young and 20 elderly Ss. For the young Ss on the first night of sleep in the laboratory, Extraversion (E) scores were negatively correlated with wakefulness after sleep onset, wakefulness in the last third of the night, and the duration of Stage 1. These relationships did not appear on the second night, which suggests that higher E scores may be associated with faster adaptation to the sleep laboratory. This interpretation must be viewed with some caution, however, because the same pattern of correlations did not appear in the elderly Ss. Tune (1969a) correlated the self-reported sleep of 240 Ss from 20 to 70 yrs of age (mean = 51) with E scores obtained from the Heron Inventory (Heron, 1956), and found a weak relationship between E and the total amount of sleep obtained at night (r = 0.18). This effect was due to introverts over age 40 sleeping less than extraverts. Introverts and extraverts under age 40 slept the same amount (Tune, 1969b). If arousal is inversely related to total sleep length, studies of habitual long and short sleepers might be expected to find that long sleepers are more extraverted than short sleepers. This approach to the extraversion-arousal question has yielded mixed results. Hartmann, Baekeland and Zwilling (1972) found that long sleepers (who averaged more than 9 hr of sleep each night) scored higher on the Social Introversion scale of the MMPI and were judged, on the basis of an interview. to be more introverted than short sleepers (who averaged less than 6 hr of sleep). Wagner and Mooney (1975) found that long and short sleepers did not differ on either the Social Introversion scale of the MMPI or the E scale of the MPI. Costello and Smith (1963), using psychiatric patients for Ss, found that extraverts (measured with the MPI) slept much longer than introverts, a relationship that remained unchanged even on nights when the Ss were given sedatives. Such inconclusive data indicates that total sleep length is perhaps too gross a measure, and affected by too many factors. to be a useful indicator of arousal in this context. Few polygraphic sleep studies have been designed specifically to investigate the relationship between extraversion and arousal, despite the existence of testable hypotheses concerning this relationship. For example, in a discussion of REM sleep function, Cohen (1979) has suggested that extraverts may have more REM rebound than introverts following REM deprivation. This prediction is based upon the assumption that one function of REM sleep is to restore some optimal level of cortical arousal following long periods of low arousal (i.e. non-REM sleep). If. while awake, extraverts are characterized by a low, less than optimal, level of arousal that must be increased periodically through sensation-seeking activities, then this same need for arousal may appear during sleep as a strong REM motive. In a study designed to test this hypothesis. Nakazawa. Kotorii, Kotorii, Tachibana and Nakano (1975) found that extraverts did have more REM rebound than introverts. However, the validity of this finding is questionable because extraversion and neuroticism were confounded and the total sleep time on the recovery night following REM deprivation was not held constant (thus some Ss had more opportunity for REM rebound than others). Other predictions concerning sleep and arousal are even more directly related to reticular
Extraversion
and sleep
347
functioning. For example, the onset of sleep is dependent upon a low level of activity in the reticular activating system. Increases in arousal induced by evening exercise (Hobson, 1968) or caffeine (Kay, Blackburn, Buckingham and Karacan, 1976) result in delayed sleep onset. Thus, a reasonable prediction about the relationship between sleep and extraversion is that introverts, if more aroused, should take longer to fall asleep than extraverts. Bohlin (1972) measured the time to sleep onset of introverts and extraverts while they were exposed to monotonous auditory stimulation and found no difference between the groups. However, Bohlin used only a median split (rather than extreme scorers) to produce her two groups (ns = 25) and her Ss were not specifically trying to fall asleep (Bohlin, 1971). Because the study did not take place at the normal bedtime of the Ss, and the Ss were presumably trying to stay awake for the experiment, these data probably provide little information about the normal sleep-onset times of introverts and extraverts. To summarize, few sleep studies of the relationship between extraversion and arousal have been reported. Those that do exist have not typically used extreme groups (e.g. Bohlin, 1971; Spiegel and Nuesch, 1977) or are difficult to interpret because of the use of different measures of extraversion or certain experimental procedures (e.g. Bohlin’s measurement of sleep onset in Ss who were not trying to go to sleep). The present study was designed to test Eysenck’s (1967) arousal hypothesis by examining two types of arousal measures from introverts and extraverts: (a) sleep-related measures of arousal, particularly the transition from wakefulness to sleep; and (b) autonomic nervous system measures of arousal (skin-potential responses and heart rate) across several conditions (while awake and during sleep). According to the theory, Ss with higher levels of arousal (i.e. introverts) should exhibit more autonomic activity and longer sleep-onset times. both initially following lights-out and after mid-night awakenings. More highly-aroused Ss should also exhibit sleep of lower quality. as indicated by a variety of sleep measures (e.g. more stage changes, more body movement and proportionally more Stage 1 sleep).
METHOD Subjects The Ss were 9 extraverts and 9 introverts (4 men and 5 women per group), selected on the basis of their scores on the Eysenck Personality Inventory Form A (EPI; Eysenck and Eysenck, 1968). For selection purposes the EPI was administered to a total of 106 people between the ages of 18 and 24yrs. The mean Extraversion (E), Neuroticism (N) and Lie (L) scores of this total sample were 14.6, 9.4 and 3.2, respectively. All Ss whose E score was greater than 16 or less than 10, and whose L score was less than 5, were asked to participate in the present study in return for a S20 payment. Thirty-two of the 106 people tested met the criteria and 18 of these completed the study. The scores for these 18 are summarized in Table 1. Table Males Females Mean age EPI scores E N L
1. S characteristics Extraverts
Introverts
4 S 21.3
4 5 22.4
18.4 9.0 1.9
6.9 9.0 2.3
Apparatus All recordings were made on a Beckman Accutrace 100 polygraph, at a paper speed of 10 mm/set. Electrode placements and filter settings corresponded to the standardized procedures described by Rechtschaffen and Kales (1968), and consisted of: (a) right parietal left mastoid;
(C4) and right occipital
(02) electroencephalograph
(EEG)
referenced
to the
348
W. BURT THOMPSON and JOHN H. MUELLER
(b) right and left electro-oculograph (EOG) from the outer canthus referenced to the left mastoid; (c) electromyogram (EMG) from the chin. Ag/AgCl electrodes were used to record skin-potential responses (SPR), which were recorded from the medial phalanx of the middle finger of the left and right hands, referenced to a left and right abraded forearm site, respectively (Martin and Venables, 1980). The electrodes were 1 cm in diameter and attached with surgical tape. Unibase was used as the medium to produce a 0.05 M NaCl electrolyte paste for the SPR recordings. In order to obtain heart rate (HR), an EKG was taken from the left-forearm electrode. All recordings took place in a temperature-controlled sleep laboratory consisting of a soundattenuated bedroom with an adjacent room containing the recording equipment. An intercom system allowed communication between the S and experimenter at all times. Procedure On the first night, which served as an adaptation night, the S arrived at the sleep laboratory approx. 90 min before his or her usual bedtime. After signing a consent form and completing Form B of the EPI, the S dressed for bed and the recording electrodes were attached. Next, 10 min of pre-sleep recordings (EEG, EOG, EMG, EKG and SPR) were taken while the S was seated in the bedroom. The S was asked to remain quiet, relaxed, and alert during these 10 min. Following this pre-sleep period the light was turned out and the S was allowed to sleep undisturbed until his or her requested awakening time. Since it was primarily for adaptation purposes, polygraph recordings were not taken consistently during the first night of sleep. In the morning, the S filled out a brief questionnaire concerning the comfort of the sleeping arrangements and the quality of the night’s sleep. The S’s second night in the sleep laboratory took place no more than two nights after the first. The sequence of events on the second night was the same as on the first night, except that an EPI was not completed and all-night polygraph recordings were taken. In the morning the S again filled out the sleep questionnaire. Only the polygraph records from the two lo-min pre-sleep periods and the first 6 hr following lights-out on the second night were scored for analysis. The sleep records were scored according to standard procedures (Rechtschaffen and Kales, 1968). To simplify analysis, Stages 3 and 4 were collapsed into the single category of Stage delta. Our decision to analyze the first 6 hr of sleep (rather than the whole night) was based upon several considerations. First, the 6-hr recording period was more than adequate to obtain sleep-stage onsets and autonomic measures of arousal. Second, we wanted to hold constant the opportunity for the different sleep stages to occur. Since the S with the shortest sleep record slept just over 6 hr, we limited our analysis to 6 hr for all records. Assuming the average time to cycle from non-REM to REM sleep is approx. 2 hr, 6 hr of sleep would allow for three full cycles, which we judged to be sufficient to assess the effects of extraversion on sleep-stage occurrence. Additionally, the 6 hr of sleep were divided into two 3-hr halves and included in the analysis as a factor (designated ‘half’). It seemed desirable to include the first and second halves of the night as a factor in this analysis for at least two reasons. First, the effect of different levels of arousal on sleep patterns may be greatest in the second half of the night, given the motivational nature of sleep. It may be that factors such as fatigue override and mask some sleep-related indices of arousal (e.g. sleep onset following mid-night awakenings) until the immediate need for restorative sleep has been satisfied. Second, this method of analysis has already proved to be useful in analyzing For example, Spiegel and Nuesch (1977) found that the sleep of introverts and extraverts. extraversion was negatively correlated with wakefulness only in the last third of the night. Thus, an analysis by half was judged appropriate here. The dependent measures were defined and computed as follows (cf. Williams, Karacan and Hursch, 1974). HR was taken from the EKG channel by hand count from the first 15 set of every third minute of the record, and converted into beats per minute (bpm). SPR was determined by counting all responses (right hand only) with peak-to-peak amplitudes of 200 mV or greater and expressed as the number of responses per minute. The criterion for a body movement was 15 set or more of continuous movement artifact in all channels, including SPR channels (which indicate
Extraversion
and sleep
349
hand movement). Body movements were scored regardless of where they began in the record, and were converted to body movements per hour. Percent of time per stage is the proportion of the entire 6-hr sleep period spent in each stage. The mean length for each stage is the mean number of minutes of each occurrence of each stage. The number of stage changes is the number of times the sleep record changed from one stage to another. Time to sleep-stage onset is the number of minutes from lights-out to the first epoch scored as the stage in question. REM cycle is the number of minutes from the beginning of one REM period to the beginning of the next REM period. REM density is the percent of the ten 3-set periods of each REM epoch that contained rapid eye movements. From the IO-min pre-sleep periods, HR and SPR were computed. For each 3-hr half of the second night of sleep, the following measures were computed: per cent of time in Stages awake (W), 1, 2, delta and REM, the mean HR and SPR in each stage, rate of body movement (MR) in each stage, mean length of each stage and the number of stage changes. RESULTS
Pre-sleep HR and SPR The HR and SPR from each pre-sleep period were analyzed in a 2 (extraversion) x 2 (first and second night) x 2 (sex) design. For HR, there were no significant effects (Fs < 1). The mean SPR for introverts was higher than that of extraverts across the fwo pre-sleep periods (means = 1.6 and 0.99), but this difference was not significant, F(1,23) < 2. All other effects failed to reach significance as well. Sleep -onset measures The sleep-onset measures (expressed in minutes) were analyzed in a 2 (extraversion) x 2 (sex) design. There were no significant effects for any of the sleep-onset measures, Fs (1,14) < 2.1. In addition to the usual measures of sleep onset, differences were computed between the time-to-onset of Stages 2, delta and REM, and all prior sleep-stage onsets. Thus, a Stage l-Stage 2 difference was computed as a measure of how long Stage 2 occurred after the onset of Stage 1. Similarly, a Stage 2-Stage REM difference indicates how long it was after Stage 2 that the first REM period occurred. These onset differences were computed in an attempt to measure sleep onsets while controlling for variation in previous sleep-onset times. This procedure would not have been necessary if sleep-onset times for the four stages had been highly correlated, but this was not always the case. Only two of the six possible correlations between the sleep-onset times were significant: Onset 1 with Onset 2 (r = 0.95, P < 0.001) and Onset delta with Onset REM (r = 0.50, P < 0.05). The sleep-onset differences were also analyzed in the extraversion x sex design. This analysis indicated that after falling asleep (i.e. reaching Stage 2), introverts tended to reach Stage REM sooner than extraverts [means = 66.5 and 93.7, respectively: F( 1,14) = 3.84, P < 0.081, a difference in the opposite direction from that suggested by Cohen (1979). None of the other differences measures showed significant effects. In general, these sleep-onset data do not indicate that introverts and extraverts differ in initial sleep-onset times from lights-out. REM measures REM density
and REM cycle were analyzed in a 2 (extraversion) x 2 (sex) design. REM density measures showed no significant effects (Fs < 1.9). For the first REM cycle, the interaction was significant [F(1,14) = 5.75, P -C 0.051 with female introverts having a longer cycle than female extraverb (means = 91.2 and 73.1, respectively), and male introverts having a shorter cycle than male extraverts (means = 85.3 and 113.1, respectively). None of the main effects were significant. Analysis by jrst
and second half of the night
A 2 (extraversion) x 2 (sex) x 2 (half) design was used to analyze HR, SPR, MR, average stage length and percent of total sleep time for each sleep stage. The number of stage changes was also included in this analysis, but yielded no significant effects (Fs < 1.6) and so will not be discussed further. Table 2 summarizes the mean scores of extraverts and introverts on these measures for the first and second
halves
of the night.
W. BURT THOMPSON and
350 Table 2. Mean
JOHN H. MUELLER
scores and standard deviations of arousal-related dependent measures introverts for the first and second halves of the night First half
Overall HR Overall SPR Overall MR Stage length Stage changes % Stage W “/, stage I “/, stage 2 % Stage delta % Stage REM HR Stage W HR Stage I HR Stage 2 HR Stage delta HR Stage REM SPR Stage W SPR Stage 1 SPR Stage 2 SPR Stage delta SPR Stage REM MR Stage W MR Stage I MR Stage 2 MR Stage delta MR Stage REM Length Stage W Length Stage I Length Stage 2 Length Stage delta Length Stage REM
of extraverts
and
Second half
Extraverts
Introverts
Extraverts
Introverts
60.9 (10.5) I .25 (I -4) 7.7 (14.5) II.8 (3.7) 32.1 (10.9) 8.8(1l.l) 6.2 (3.5) 37.7(11.3) 32.9 (15.5) 8.7 (3.7) 67.9 (10.2) 59.9(11.1) 58.9 (9.9) 60.3 (10.3) 65.2 (9.6) 0.62 (I .O) 0.26 (0.36) 1.03(1.4) 1.96 (2.3) 0.11 (0.12) 6.5 (10.6) 2.8 (5.1) 0.19 (0.38) 0.46 (0.56) 4.1 (8.5) 6.4 (4.6) 2.7 (0.61) 16.7 (7.0) 36.2 (13.9) 14.9 (9.8)
59.7 (8.0) 0.99 (0.99) 2.6(1.0) 12.9 (3.8) 29.6 (8.6) 7.5 (6.9) 7.4 (5.6) 37.3 (4.4) 36.3 (12.9) 10.3 (6.4) 62 2 (10.5) 58.9 (8.0) 59.2 (7.9) 58.5 (8.6) 64 7 (8.9) 0 62 (0.66) 0.34 (0.56) l.O-r(l.2) 1.16(1.2) 0.48 (0.75) 7. I (8.8) I 7 (2.6) 0.96(1.1) 0.65 (1.0) 0.16 (0.46) 13.8 (13.9) 3.7 (I 5) 15.8 (4.1) 307115.8) 16.2 (7.1)
57.6 (10.2) 0.82 (0.52) 3.1 (1.1) 10.5 (2.3) 34.0 (8.8) 1.4(1.0) 6.3 (4.3) 55.1 (10.4) IO.5 (8.4) 25.0 (6.4) 71.0(15.8) 59.2 (13.6) 56.6 (10.2) 56.3(11.9) 60.3 (10.7) 0.84(1.6) 0.37 (0.53) 0.62 (0.40) 2.03 (2.2) 0.32 (0.35) 12.9(12.8) 6.2 (9.9) 0.52 (0.45) 1.3 (1.6) 1.2 (0.98) 2.6 (2.4) 2.3 (0.76) 21.9 (7.9) 16.4(19.5) 20.0 (10.3)
58.7 (7.5) 1.45(1.29) 3.0(1.3) 11.5(2.5) 31.1 (6.6) 4.6 (9.6) 6.8 (3 2) 52.2 (9 7) Il.9 (8.5) 22.7 (8.6) 66.8 (7.5) 58.1 (7.5) 57.2 (7.3) 57 4 (7.6) 62 7 (9.3) 0.57 (0.76) 0 91 (0.98) 1.60(1 7) 2.96 (2.3) 0.66 (0.67) 10.3 (22.1) 5.6 (8 2) 0.23 (0.34) 2.4 (5.7) 1.9 (5.6) 7.2 (10.9) 2.8 (1.4) 22.8 (5.6) 23.6(13.5) 18.4 (7.6)
Heart rate. For overall HR, there were no significant effects for extraversion (us < l), or for the half X extraversion interaction (Fs < 2.7). Not surprisingly, HR differed by half, as the mean HR was higher in the first half of the night [means = 60.3 and 58.2; F(1,14) = 8.48, p < 0.021. No other effects were significant. Heart rate in Stages W and 1 showed no significant effects. For Stages 2, delta and REM, there was a significant half effect, indicating higher HR in these stages during the first half of the night, Fs(1,14) = 11.34, 11.78 and 11.84, respectively (Ps < 0.01). None of the HR measures showed a significant effect for extraversion or the extraversion x half interaction. SPR, no main effects were significant. The Skin-potential response rate. For overall extraversion x half interaction approached significance [F(1,14) = 3.89, P < 0.071 as the mean SPR for extraverts was higher during the first 3 hr (means = 1.3 and 0.99) and the introvert’s SPR was higher for the second 3 hr (means = 1.5 and 0.82). The most consistent autonomic difference between introverts and extraverts was an extraversion x half interaction for SPR in Stages 1, 2 and delta. In Stage 1, the introverts and extraverts had approximately equal SPRs in the first half of the night (means = 0.34 and 0.26, respectively), but in the second 3 hr the introverts had a significantly higher SPR than the extraverts [means = 0.91 and 0.37, respectively; F( 1,14) = 4.64. P < 0.051. In Stage 2, introverts and extraverts had equal SPRs in the first half (means = 1.0) and in the second half of the night the introverts had a higher SPR than extraverts (means = 1.6 and 0.62, respectively), although this interaction only approached significance [F(1,14) = 4.37, P -c 0.061. In Stage delta, the extraverts had a higher SPR (1.96) than the introverts (1.16) in the first half, but in the second half the introverts’ SPR was higher than the extraverts’ (means = 2.96 and 2.0, respectively). This interaction was not significant [F(l,14) = 3.46, P < 0.091. In Stage REM, there was no extraversion x half interaction, and the main effect for extraversion only approached significance, with the introverts having a higher SPR (0.57) than the extraverts (0.20) [F(1.14) = 3.70, P < 0.081. In all four sleep stages (1, 2 delta and REM), the SPRs were higher in the second half of the night than in the first half, but only in Stage 1 was the difference significant, the means in the first and second halves being 0.30 and 0.64, respectively [F(1,14) = 9.85, P < 0.011. In Stage 2, the SPR means were 1.0 and 1.1 (F < 1). In Stage delta, the SPRs were 1.6 and 2.5 (F = 4.07. P < 0.07),
Extraversion
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351
and in Stage REM the means for the first and second halves were 0.29 and 0.49, respectively [F(1,14) = 3.16, P < 0.101. Percent of night per stage. For the percent of the night in Stage W, there were no effects for extraversion or for the extraversion x half interaction. Predictably, there was a tendency for Ss to spend a greater proportion of time awake during the first half of the night [means = 8.2 and 3.0; F(1,14) = 4.33, P < 0.061. For percent of Stage 1, there was no extraversion main effect (F < 1) or extraversion x half interaction (F < 1). The one significant effect for Stage 2 percent was for half, as Stage 2 not unexpectedly comprised a larger proportion of the second half of the night than the first half [means = 53.7 and 37.5; F(1,14) = 34.79, P -C 0.011. As expected, the opposite effect was found for Stage delta percent, the means being 34.6 and 11.2 in the first and second 3 hr, respectively [F( 1,14) = 34.47, P < O.Ol]. As was the case with Stages 1 and 2, there were no significant effects for extraversion or extraversion x half. For Stage REM percent, there was again a significant and predictable effect for half, with more Stage REM occurring in the second 3 hr of the night [means = 9.5 and 23.9; F(1,14) = 36.06, P < 0.011. Body-movement rate. There were no significant effects for overall MR [F(1,14) < 2.11. MR in Stage 1 showed an extraversion x sex interaction [F( 1,14) = 4.99, P < 0.051. For introverts, females had a higher MR than males (means = 6.03 and 1.7, respectively), but for extraverts males had the higher MR, 7.08 to 1.3. For MR is Stage 2, a significant extraversion x half interaction indicated that the introverts’ MR was higher than the extraverts’ in the first half of the night (means = 0.96 and 0.19, respectively), but in the second 3 hr the extraverts had a higher MR than the introverts [means = 0.52 and 0.23; F(1,14) = 5.20, P < 0.051. The main effect for extraversion was not significant for MR in Stage 2 (F = 2.3, P > 0.10). There were no significant effects of MR in Stages delta or REM. Average stage length. For overall average stage length, there was no significant effect for extraversion [F( 1,14) < 2.11. For the average length of Stage W periods, introverts had a longer mean than extraverts (10.9 to 4.8) a difference that approached significance [F(1,14) = 3.12, P < 0. lo]. The absence of an extraversion x half interaction (F < 1) indicates that this relationship remained stable across the night. A significant half main effect indicated that the average Stage W period was longer during the first half of the night (10.1) than during the second half (5.0) [F(1.14) = 5.83, P < 0.051. For the average length of Stage 1 periods, the introverts had a higher mean than the extraverts (3.2 and 2.5, respectively), but the effect was not significant [F(1,14) = 3.53, P < 0.091. Overall, the average length of Stage 2 was no different for introverts and extraverts, but there was a significant half main effect, with Stage 2 periods being longer in the second half of the night than in the first half [means = 22.3 and 16.2, respectively; F(1,14) = 10.75, P < 0.011. There was a significant half main effect for Stage delta, as these periods were longer in the first half of the night [means = 33.4 and 20.0; F(1,14) = 5.70, P < 0.051.
DISCUSSION In general, the results of the present study do not indicate that introverts and extraverts consistently differ in their levels of arousal, at least not immediately before or during sleep. However, as will be discussed shortly, this outcome may actually be consistent with a more integrative interpretation of the arousal hypothesis. During the pre-sleep awake period, introverts did not differ from extraverts on HR or SPR. There was no difference between the groups for initial sleep onset. The groups did not differ in the proportion of the night spent in the different sleep stages. Cohen’s (1979) suggestion that extraverts may have a stronger REM motive than introverts was not supported. The data in support of the arousal hypothesis are those of the tendency for introverts to have higher SPRs than extraverts in the second half of the night, and the somewhat longer average length of Stage 1 periods for introverts. There was also a tendency for introverts to remain awake longer following mid-night awakenings, indicated by the average length of Stage W periods.
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W. BURT THOMPSON and JOHN H. MUELLER
The equality of the introverts’ and extraverts’ sleep-onset times is perhaps the most damaging evidence against the arousal hypothesis. Assuming that the sleep-laboratory environment was equal in the amount of environmental stimulation it provided for introverts and extraverts, then more highly-aroused Ss should have taken longer to fall asleep. However, in light of recent research (Campbell and Hawley, 1982; Geen, 1984) which suggests that introverts have a lower level of optimal arousal than extraverts, the level of environmental stimulation must be considered when interpreting the outcome of the present study. Briefly, Campbell and Hawley (1982) observed the library environment chosen by extraverts and introverts in which to study. Extraversion was positively correlated with noise level and socializing opportunities. Geen (1984) found that introverts chose a lower level of noise to be heard during a learning task than extraverts. Introverts and extraverts showed no difference in physiological arousal when stimulated by a sound level chosen by either themselves or members of the same personality group. However, at the same intensity levels, introverts were more aroused than extraverts. Several investigators (Gale, 1981; Geen, 1984) have suggested that introverts and extraverts may show little or no difference in arousal at very low levels of stimulation. This may be due to their (theoretically) different levels of optimal stimulation, and the resulting strategies they develop to deal with deviations from that optimal level. A sleep laboratory is a relatively unstimulating environment, therefore it is not unreasonable to consider the present study as an investigation of the arousal hypothesis under extremely low levels of stimulation. If this were the case, then the failure to detect arousal differences between introverts and extraverts would not be surprising. Although this interpretation is post hoc, it leads to some specific predictions. For example, if introverts and extraverts differ in their sensitivity to environmental stimuli, then one might expect longer sleep onsets in introverts under conditions of stimulation higher than those present during this study. In other words, in a noisy environment (e.g. a college dormitory), introverts may take longer to fall asleep than extraverts, but in quiet environments their sleep onsets may be equal. In terms of habituation to the sleep laboratory, it may be that extraverts habituate to the novel environment of a sleep laboratory (e.g. the bed and electrodes) more quickly than introverts, as suggested by Spiegel and Neusch (1977). In this case, sleep-onset times and autonomic activity on the first night in the lab may be a better measure of arousal differences between introverts and extraverts. However, as discussed above, first-night sleep records were not taken during the present study, so this question remains in need of investigation. In conclusion, no consistent evidence was found in support of Eysenck’s (1967) theory relating extraversion and arousal. An interpretation of this arousal hypothesis that simply stresses the absolute arousal differences between introverts and extraverts may overlook important interactions between an individual and the environment. A more integrative hypothesis, one that predicts arousal levels on the basis of an interaction between individual differences in the optimal level of stimulation and the amount of stimulation in the environment, may provide a more complete framework for interpreting the relevant data. Several tests of this arousal hypothesis that involve sleep are still in need of investigation. One involves the measurement of sleep onset (initially and following mid-night awakenings) in environments with different levels of stimulation. If introverts prefer lower levels of stimulation than extraverts (and thus, are presumably more highly aroused at moderate levels of stimulation), then their sleep onset should be delayed under moderate to high levels of stimulation. A second test would involve measurement of the rate of habituation to the sleep laboratory, as indicated by the usual measures of sleep quality (e.g. delayed sleep onset, increased proportions of Stage 1 sleep), and autonomic measures. More highly-aroused Ss would be expected to take longer to habituate to the sleep laboratory over the course of several nights. Nebraska Wesleyan University (Lincoln, Nebr.) for Acknowledgements-We wish to thank the Psychology Department, the use of their sleep laboratory. This project would not have been possible without the friendship and technical expertise of Marty Klein. Helpful comments on an earlier draft were made by Russell Geen and Dave McDonald.
REFERENCES Bohlin G. (1971) Monotonous Neurophysiol. 31, 593401. Bohlin G. (1972) Susceptibility Person. 6, 248-254
stimulation,
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to sleep during
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a habituation
habituation procedure
of the orienting as related
reaction.
to individual
Electroenceph.
difference.
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Extraversion
and sleep
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Campbell J. B. and Hawley C. W. (1982) Study habits and Eysenck’s theory of extraversion-introversion. J. Res. Person. 16, 139-146. Cohen D. B. (1979) Sleep and Dreaming: Origins, Nature and Function. Pergamon Press, New York. Coles G., Gale A. and Kline P. (1971) Personality and habituation of the orienting reaction: tonic and response measures of electrodermal activity. Psychophysiology 8; 54-63. Costello C. G. and Smith C. M. (1963) The relationshin between personalitv. _. sleeoL and the effects of sedatives. Br. J. Psychiar. 109, 568-571. ~ ’ Crider A. and Lunn R. (1971) Electrodermal lability as a personality dimension. J. exp. Res. Person. 5, 145-150. Eysenck H. J. (1967) The Biological Basis of Personality. Thomas, Springfield, III. Eysenck H. J. and Eysenck S. B. G. (1968) Manual for the Eysenck Personality Inventory. EdITS, San Diego, Calif. Gale A. (1981) EEG studies of extraversion-introversion: what’s the next step? In Dimensions of Persona&y (Edited by Lynn R.). Pergamon Press, New York. Gange J. J., Geen R. G. and Harkins S. G. (1979) Autonomic differences between extraverts and introverts during vigilance. Psychophysiology 16, 392-397. Geen R. (1983) The psychophysiology of extraversion-introversion. In Social Psychophysiology (Edited by Cacioppo J. T. and Petty R. E.). Guilford Press, New York. Geen R. (1984) Preferred stimulation levels in introverts and extraverts: effects on arousal and performance. J. Person. sot. Psychol. In press. Hartmann E., Baekeland F. and Zwilling G. R. (1972) Psychological differences between long and short sleepers, Archs gen. Psychiat. 26, 463468. Heron A. (1956) A two-part personality measure for use as a research criterion. Br. J. Psycho/. 47, 143-251. Hobson J. A. (1968) Sleep after exercise. Science 162, 1503-1505. Hobson J. A. (1974) The cellular basis of sleep cycle control. Adu. Sleep Res. 1, 217-250. Kay D. C., Blackburn A. B., Buckingham J. A. and Karacan I. (1976) Human pharmacology of sleep. In Pharmacology of Sleep (Edited by Williams R. L. and Karacan I.). Wiley, New York. Martin I. and Venables T. (1980) Techniques in Psychophysiology. Wiley, New York. Nakazawa Y., Kotorii M., Kotorii T., Tachibana H. and Nakano T. (1975) Individual differences in compensatory rebound of REM sleep, with particular reference to their relationship to personality and behavioral characteristics. J. net-u. men/. Dis. 161, 18-25. Rechtschaffen A. and Kales A. (1968) A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. NIH Publication No. 204. Smith B. D. and Wigglesworth M. J. (1978) Extraversion and neuroticism in orienting reflex dishabituation. J. Res. Person. 12, 284296. Smith B. D., Rypma C. B. and Wilson R. J. (1981) Dishabituation and spontaneous recovery of the electrodermal orienting response: effects of extraversion, impulsivity, sociability and caffeine. J. Res. Person. 15, 233-240. Spiegel R. and Nuesch E. (1977) Correlations between personality characteristics and sleep variables in healthy subjects. In Sleep 1976: Memory, Environment, Epilepsy, Sleep Sraging (Edited by Koella W. P. and Levin P.). Karger, New York. Stelmack R. M., Bourgeois R., Chian J. Y. C. and Pickard C. (1979) Extraversion and the OR habituation rate to visual stimuli. J. Res. Person. 13, 49958. Taub J. M., Hawkins D. R. and Van de Castle R. L. (1978) Personality characteristics associated with sustained variations in the adult human sleep/wakefulness rhythm. Walking and Sleeping 2, 7-15. Tune G. S. (1969a) The influence of age and temperment on the adult human sleep-wakefulness pattern. Br. J. Psychol. 60, 431-141. Tune G. S. (1969b) Sleep and wakefulness in 509 normal adults. Br. J. Psychol. 42, 7480. Wagner M. K. and Mooney D. K. (1975) Personality characteristics of long and short sleepers. J. clin. Psycho/. 31,434-436. Williams R. L., Karacan I. and Hursch C. J. (1974) Elecrroencephalography (EEG) of H uman Sleep: Clinical Applications. Wiley, New York.
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Britain.
3. pp. 345-353, 1984
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All rights reserved
EXTRAVERSION STUDY W. Psychology
Copyright
0
$3.00+0.00
1984 Pergamon
Press Ltd
AND SLEEP: A PSYCHOPHYSIOLOGICAL OF THE AROUSAL HYPOTHESIS* BURT
Department,
THOMPSON?
and
University
of Missouri,
JOHN
H.
MUELLER
Columbia,
MO 65211, U.S.A.
(Received 22 August 1983) Summary-In order to examine Eysenck’s (1967) hypothesis concerning the relationship between extraversion and arousal, autonomic measures were taken from introverts and extraverts immediately before sleep and during 6 hr of sleep. Introverts and extraverts did not differ in heart rate (HR) or skin-potential response rate (SPR) during the pre-sleep period. A variety of sleep-related measures of arousal provided no strong support for the hypothesis that introverts are more aroused than extraverts. The only consistent autonomic difference between introverts and extraverts during sleep was a tendency for introverts to have higher SPRs in the second 3 hr of the night. The results are discussed in terms of the level of environmental stimulation present in the experimental conditions, and suggestions are made for future sleep studies of the relationship between extraversion and arousal.
INTRODUCTION
Eysenck (1967) has proposed that the behavioral differences between introverts and extraverts can be partially explained in terms of individual differences in the cortico-reticular loop which mediates cortical arousal and inhibition. According to this arousal hypothesis, extraverts have lower levels of reticular activity and are characterized by high levels of cortical inhibition and thus reduced sensitivity to environmental stimulation. Introverts, because of higher levels of reticular activity, are presumably more sensitive. In theory, this difference in arousal should manifest itself in a variety of psychophysiological measures, including the orienting response, sensory thresholds, cortical activity measured by EEG, electrodermal responses and cardiac activity. Research using the EEG as a measure of arousal has failed to reveal any strong, systematic, relationship between extraversion and arousal. However, this failure is perhaps best interpreted as neither evidence for nor against an extraversion-arousal relationship because of the many methodological problems that have plagued this research [see Gale (1981) for a review]. Studies that have employed autonomic measures of arousal have not conclusively demonstrated that introverts are more aroused than extraverts. However, these studies do indicate that some relationship exists, although it appears to be a complex one (cf. Geen, 1983). For example, Coles, Gale and Kline (1971) presented Ss with a series of tones while measuring skin resistance. Although introverts showed no greater reactivity to the tones than extraverts, and took no longer to habituate to the tones, they did show more spontaneous skin-resistance responses over the course of the whole experiment. In another study of habituation, Crider and Lunn (1971) found that introverts took longer to habituate than extraverts, a finding that is consistent with the hypothesis that introverts have higher levels of arousal than extraverts. Other studies, in which both the intensity and the frequency of tones have been varied, also indicate that the relationship between extraversion and arousal is far from simple. For example, Smith and Wigglesworth (1978) measured electrodermal responses to a 1000 Hz tone at three levels of intensity. Habituation rates for the introverts and extraverts were equal at the low and middle intensity levels, but introverts took longer to habituate to the highest intensity level, If habituation rate is inversely related to level of arousal, then these results offer some support for the arousal hypothesis. In a study in which multiple measures of arousal across different situations were used, Gange, Geen and Harkins (1979) found that introverts showed more spontaneous skin-resistance responses and higher heart rates than extraverts for all three conditions of their study. Stelmack, Bourgeois, *This paper is based on a Thesis submitted by the first author Columbia, MO. ~To whom all correspondence should be addressed. 345
to the Psychology
Department,
University
of Missouri,
346
W. BURT THOMPSON and JOHN H. MUELLER
Chian and Pickard (1979) used electrodermal, cardiac and vasomotor indices of autonomic activity to measure the orienting reaction of introverts and extraverts to visual and auditory stimulation in two independent studies. In both studies, as indicated by all three measures, introverts took longer to habituate to both types of stimuli than the extraverts. It appears, then, that introverts and extraverts differ in their responses to certain situations in ways that suggest arousal differences exist between the two groups, although the nature of these differences is not yet clear. A potentially fruitful, though relatively neglected, situation in which to investigate the extraversion-arousal relationship is the sleep state. The onset and maintenance of sleep is dependent upon a relatively low level of activity in the reticular activating system (Cohen, 1979; Hobson, 1974). Thus the sleep state provides an opportunity to investigate Eysenck’s arousal hypothesis by examining sleep-related measures of arousal, and in particular, the transition from wakefulness to sleep. Additionally, the sleep state allows continuous monitoring of spontaneous variations in several indices of autonomic nervous system activity for an extended period of time. Although many studies of the effects of personality variables (including extraversion) on sleep patterns have been reported, few have specifically addressed the extraversion-arousal question [see Taub, Hawkins and Van de Castle (1978) for a review]. Spiegel and Nuesch (1977) correlated a variety of sleep measures with scores on the Maudsley Personality Inventory (MPI) for 20 young and 20 elderly Ss. For the young Ss on the first night of sleep in the laboratory, Extraversion (E) scores were negatively correlated with wakefulness after sleep onset, wakefulness in the last third of the night, and the duration of Stage 1. These relationships did not appear on the second night, which suggests that higher E scores may be associated with faster adaptation to the sleep laboratory. This interpretation must be viewed with some caution, however, because the same pattern of correlations did not appear in the elderly Ss. Tune (1969a) correlated the self-reported sleep of 240 Ss from 20 to 70 yrs of age (mean = 51) with E scores obtained from the Heron Inventory (Heron, 1956), and found a weak relationship between E and the total amount of sleep obtained at night (r = 0.18). This effect was due to introverts over age 40 sleeping less than extraverts. Introverts and extraverts under age 40 slept the same amount (Tune, 1969b). If arousal is inversely related to total sleep length, studies of habitual long and short sleepers might be expected to find that long sleepers are more extraverted than short sleepers. This approach to the extraversion-arousal question has yielded mixed results. Hartmann, Baekeland and Zwilling (1972) found that long sleepers (who averaged more than 9 hr of sleep each night) scored higher on the Social Introversion scale of the MMPI and were judged, on the basis of an interview. to be more introverted than short sleepers (who averaged less than 6 hr of sleep). Wagner and Mooney (1975) found that long and short sleepers did not differ on either the Social Introversion scale of the MMPI or the E scale of the MPI. Costello and Smith (1963), using psychiatric patients for Ss, found that extraverts (measured with the MPI) slept much longer than introverts, a relationship that remained unchanged even on nights when the Ss were given sedatives. Such inconclusive data indicates that total sleep length is perhaps too gross a measure, and affected by too many factors. to be a useful indicator of arousal in this context. Few polygraphic sleep studies have been designed specifically to investigate the relationship between extraversion and arousal, despite the existence of testable hypotheses concerning this relationship. For example, in a discussion of REM sleep function, Cohen (1979) has suggested that extraverts may have more REM rebound than introverts following REM deprivation. This prediction is based upon the assumption that one function of REM sleep is to restore some optimal level of cortical arousal following long periods of low arousal (i.e. non-REM sleep). If. while awake, extraverts are characterized by a low, less than optimal, level of arousal that must be increased periodically through sensation-seeking activities, then this same need for arousal may appear during sleep as a strong REM motive. In a study designed to test this hypothesis. Nakazawa. Kotorii, Kotorii, Tachibana and Nakano (1975) found that extraverts did have more REM rebound than introverts. However, the validity of this finding is questionable because extraversion and neuroticism were confounded and the total sleep time on the recovery night following REM deprivation was not held constant (thus some Ss had more opportunity for REM rebound than others). Other predictions concerning sleep and arousal are even more directly related to reticular
Extraversion
and sleep
347
functioning. For example, the onset of sleep is dependent upon a low level of activity in the reticular activating system. Increases in arousal induced by evening exercise (Hobson, 1968) or caffeine (Kay, Blackburn, Buckingham and Karacan, 1976) result in delayed sleep onset. Thus, a reasonable prediction about the relationship between sleep and extraversion is that introverts, if more aroused, should take longer to fall asleep than extraverts. Bohlin (1972) measured the time to sleep onset of introverts and extraverts while they were exposed to monotonous auditory stimulation and found no difference between the groups. However, Bohlin used only a median split (rather than extreme scorers) to produce her two groups (ns = 25) and her Ss were not specifically trying to fall asleep (Bohlin, 1971). Because the study did not take place at the normal bedtime of the Ss, and the Ss were presumably trying to stay awake for the experiment, these data probably provide little information about the normal sleep-onset times of introverts and extraverts. To summarize, few sleep studies of the relationship between extraversion and arousal have been reported. Those that do exist have not typically used extreme groups (e.g. Bohlin, 1971; Spiegel and Nuesch, 1977) or are difficult to interpret because of the use of different measures of extraversion or certain experimental procedures (e.g. Bohlin’s measurement of sleep onset in Ss who were not trying to go to sleep). The present study was designed to test Eysenck’s (1967) arousal hypothesis by examining two types of arousal measures from introverts and extraverts: (a) sleep-related measures of arousal, particularly the transition from wakefulness to sleep; and (b) autonomic nervous system measures of arousal (skin-potential responses and heart rate) across several conditions (while awake and during sleep). According to the theory, Ss with higher levels of arousal (i.e. introverts) should exhibit more autonomic activity and longer sleep-onset times. both initially following lights-out and after mid-night awakenings. More highly-aroused Ss should also exhibit sleep of lower quality. as indicated by a variety of sleep measures (e.g. more stage changes, more body movement and proportionally more Stage 1 sleep).
METHOD Subjects The Ss were 9 extraverts and 9 introverts (4 men and 5 women per group), selected on the basis of their scores on the Eysenck Personality Inventory Form A (EPI; Eysenck and Eysenck, 1968). For selection purposes the EPI was administered to a total of 106 people between the ages of 18 and 24yrs. The mean Extraversion (E), Neuroticism (N) and Lie (L) scores of this total sample were 14.6, 9.4 and 3.2, respectively. All Ss whose E score was greater than 16 or less than 10, and whose L score was less than 5, were asked to participate in the present study in return for a S20 payment. Thirty-two of the 106 people tested met the criteria and 18 of these completed the study. The scores for these 18 are summarized in Table 1. Table Males Females Mean age EPI scores E N L
1. S characteristics Extraverts
Introverts
4 S 21.3
4 5 22.4
18.4 9.0 1.9
6.9 9.0 2.3
Apparatus All recordings were made on a Beckman Accutrace 100 polygraph, at a paper speed of 10 mm/set. Electrode placements and filter settings corresponded to the standardized procedures described by Rechtschaffen and Kales (1968), and consisted of: (a) right parietal left mastoid;
(C4) and right occipital
(02) electroencephalograph
(EEG)
referenced
to the
348
W. BURT THOMPSON and JOHN H. MUELLER
(b) right and left electro-oculograph (EOG) from the outer canthus referenced to the left mastoid; (c) electromyogram (EMG) from the chin. Ag/AgCl electrodes were used to record skin-potential responses (SPR), which were recorded from the medial phalanx of the middle finger of the left and right hands, referenced to a left and right abraded forearm site, respectively (Martin and Venables, 1980). The electrodes were 1 cm in diameter and attached with surgical tape. Unibase was used as the medium to produce a 0.05 M NaCl electrolyte paste for the SPR recordings. In order to obtain heart rate (HR), an EKG was taken from the left-forearm electrode. All recordings took place in a temperature-controlled sleep laboratory consisting of a soundattenuated bedroom with an adjacent room containing the recording equipment. An intercom system allowed communication between the S and experimenter at all times. Procedure On the first night, which served as an adaptation night, the S arrived at the sleep laboratory approx. 90 min before his or her usual bedtime. After signing a consent form and completing Form B of the EPI, the S dressed for bed and the recording electrodes were attached. Next, 10 min of pre-sleep recordings (EEG, EOG, EMG, EKG and SPR) were taken while the S was seated in the bedroom. The S was asked to remain quiet, relaxed, and alert during these 10 min. Following this pre-sleep period the light was turned out and the S was allowed to sleep undisturbed until his or her requested awakening time. Since it was primarily for adaptation purposes, polygraph recordings were not taken consistently during the first night of sleep. In the morning, the S filled out a brief questionnaire concerning the comfort of the sleeping arrangements and the quality of the night’s sleep. The S’s second night in the sleep laboratory took place no more than two nights after the first. The sequence of events on the second night was the same as on the first night, except that an EPI was not completed and all-night polygraph recordings were taken. In the morning the S again filled out the sleep questionnaire. Only the polygraph records from the two lo-min pre-sleep periods and the first 6 hr following lights-out on the second night were scored for analysis. The sleep records were scored according to standard procedures (Rechtschaffen and Kales, 1968). To simplify analysis, Stages 3 and 4 were collapsed into the single category of Stage delta. Our decision to analyze the first 6 hr of sleep (rather than the whole night) was based upon several considerations. First, the 6-hr recording period was more than adequate to obtain sleep-stage onsets and autonomic measures of arousal. Second, we wanted to hold constant the opportunity for the different sleep stages to occur. Since the S with the shortest sleep record slept just over 6 hr, we limited our analysis to 6 hr for all records. Assuming the average time to cycle from non-REM to REM sleep is approx. 2 hr, 6 hr of sleep would allow for three full cycles, which we judged to be sufficient to assess the effects of extraversion on sleep-stage occurrence. Additionally, the 6 hr of sleep were divided into two 3-hr halves and included in the analysis as a factor (designated ‘half’). It seemed desirable to include the first and second halves of the night as a factor in this analysis for at least two reasons. First, the effect of different levels of arousal on sleep patterns may be greatest in the second half of the night, given the motivational nature of sleep. It may be that factors such as fatigue override and mask some sleep-related indices of arousal (e.g. sleep onset following mid-night awakenings) until the immediate need for restorative sleep has been satisfied. Second, this method of analysis has already proved to be useful in analyzing For example, Spiegel and Nuesch (1977) found that the sleep of introverts and extraverts. extraversion was negatively correlated with wakefulness only in the last third of the night. Thus, an analysis by half was judged appropriate here. The dependent measures were defined and computed as follows (cf. Williams, Karacan and Hursch, 1974). HR was taken from the EKG channel by hand count from the first 15 set of every third minute of the record, and converted into beats per minute (bpm). SPR was determined by counting all responses (right hand only) with peak-to-peak amplitudes of 200 mV or greater and expressed as the number of responses per minute. The criterion for a body movement was 15 set or more of continuous movement artifact in all channels, including SPR channels (which indicate
Extraversion
and sleep
349
hand movement). Body movements were scored regardless of where they began in the record, and were converted to body movements per hour. Percent of time per stage is the proportion of the entire 6-hr sleep period spent in each stage. The mean length for each stage is the mean number of minutes of each occurrence of each stage. The number of stage changes is the number of times the sleep record changed from one stage to another. Time to sleep-stage onset is the number of minutes from lights-out to the first epoch scored as the stage in question. REM cycle is the number of minutes from the beginning of one REM period to the beginning of the next REM period. REM density is the percent of the ten 3-set periods of each REM epoch that contained rapid eye movements. From the IO-min pre-sleep periods, HR and SPR were computed. For each 3-hr half of the second night of sleep, the following measures were computed: per cent of time in Stages awake (W), 1, 2, delta and REM, the mean HR and SPR in each stage, rate of body movement (MR) in each stage, mean length of each stage and the number of stage changes. RESULTS
Pre-sleep HR and SPR The HR and SPR from each pre-sleep period were analyzed in a 2 (extraversion) x 2 (first and second night) x 2 (sex) design. For HR, there were no significant effects (Fs < 1). The mean SPR for introverts was higher than that of extraverts across the fwo pre-sleep periods (means = 1.6 and 0.99), but this difference was not significant, F(1,23) < 2. All other effects failed to reach significance as well. Sleep -onset measures The sleep-onset measures (expressed in minutes) were analyzed in a 2 (extraversion) x 2 (sex) design. There were no significant effects for any of the sleep-onset measures, Fs (1,14) < 2.1. In addition to the usual measures of sleep onset, differences were computed between the time-to-onset of Stages 2, delta and REM, and all prior sleep-stage onsets. Thus, a Stage l-Stage 2 difference was computed as a measure of how long Stage 2 occurred after the onset of Stage 1. Similarly, a Stage 2-Stage REM difference indicates how long it was after Stage 2 that the first REM period occurred. These onset differences were computed in an attempt to measure sleep onsets while controlling for variation in previous sleep-onset times. This procedure would not have been necessary if sleep-onset times for the four stages had been highly correlated, but this was not always the case. Only two of the six possible correlations between the sleep-onset times were significant: Onset 1 with Onset 2 (r = 0.95, P < 0.001) and Onset delta with Onset REM (r = 0.50, P < 0.05). The sleep-onset differences were also analyzed in the extraversion x sex design. This analysis indicated that after falling asleep (i.e. reaching Stage 2), introverts tended to reach Stage REM sooner than extraverts [means = 66.5 and 93.7, respectively: F( 1,14) = 3.84, P < 0.081, a difference in the opposite direction from that suggested by Cohen (1979). None of the other differences measures showed significant effects. In general, these sleep-onset data do not indicate that introverts and extraverts differ in initial sleep-onset times from lights-out. REM measures REM density
and REM cycle were analyzed in a 2 (extraversion) x 2 (sex) design. REM density measures showed no significant effects (Fs < 1.9). For the first REM cycle, the interaction was significant [F(1,14) = 5.75, P -C 0.051 with female introverts having a longer cycle than female extraverb (means = 91.2 and 73.1, respectively), and male introverts having a shorter cycle than male extraverts (means = 85.3 and 113.1, respectively). None of the main effects were significant. Analysis by jrst
and second half of the night
A 2 (extraversion) x 2 (sex) x 2 (half) design was used to analyze HR, SPR, MR, average stage length and percent of total sleep time for each sleep stage. The number of stage changes was also included in this analysis, but yielded no significant effects (Fs < 1.6) and so will not be discussed further. Table 2 summarizes the mean scores of extraverts and introverts on these measures for the first and second
halves
of the night.
W. BURT THOMPSON and
350 Table 2. Mean
JOHN H. MUELLER
scores and standard deviations of arousal-related dependent measures introverts for the first and second halves of the night First half
Overall HR Overall SPR Overall MR Stage length Stage changes % Stage W “/, stage I “/, stage 2 % Stage delta % Stage REM HR Stage W HR Stage I HR Stage 2 HR Stage delta HR Stage REM SPR Stage W SPR Stage 1 SPR Stage 2 SPR Stage delta SPR Stage REM MR Stage W MR Stage I MR Stage 2 MR Stage delta MR Stage REM Length Stage W Length Stage I Length Stage 2 Length Stage delta Length Stage REM
of extraverts
and
Second half
Extraverts
Introverts
Extraverts
Introverts
60.9 (10.5) I .25 (I -4) 7.7 (14.5) II.8 (3.7) 32.1 (10.9) 8.8(1l.l) 6.2 (3.5) 37.7(11.3) 32.9 (15.5) 8.7 (3.7) 67.9 (10.2) 59.9(11.1) 58.9 (9.9) 60.3 (10.3) 65.2 (9.6) 0.62 (I .O) 0.26 (0.36) 1.03(1.4) 1.96 (2.3) 0.11 (0.12) 6.5 (10.6) 2.8 (5.1) 0.19 (0.38) 0.46 (0.56) 4.1 (8.5) 6.4 (4.6) 2.7 (0.61) 16.7 (7.0) 36.2 (13.9) 14.9 (9.8)
59.7 (8.0) 0.99 (0.99) 2.6(1.0) 12.9 (3.8) 29.6 (8.6) 7.5 (6.9) 7.4 (5.6) 37.3 (4.4) 36.3 (12.9) 10.3 (6.4) 62 2 (10.5) 58.9 (8.0) 59.2 (7.9) 58.5 (8.6) 64 7 (8.9) 0 62 (0.66) 0.34 (0.56) l.O-r(l.2) 1.16(1.2) 0.48 (0.75) 7. I (8.8) I 7 (2.6) 0.96(1.1) 0.65 (1.0) 0.16 (0.46) 13.8 (13.9) 3.7 (I 5) 15.8 (4.1) 307115.8) 16.2 (7.1)
57.6 (10.2) 0.82 (0.52) 3.1 (1.1) 10.5 (2.3) 34.0 (8.8) 1.4(1.0) 6.3 (4.3) 55.1 (10.4) IO.5 (8.4) 25.0 (6.4) 71.0(15.8) 59.2 (13.6) 56.6 (10.2) 56.3(11.9) 60.3 (10.7) 0.84(1.6) 0.37 (0.53) 0.62 (0.40) 2.03 (2.2) 0.32 (0.35) 12.9(12.8) 6.2 (9.9) 0.52 (0.45) 1.3 (1.6) 1.2 (0.98) 2.6 (2.4) 2.3 (0.76) 21.9 (7.9) 16.4(19.5) 20.0 (10.3)
58.7 (7.5) 1.45(1.29) 3.0(1.3) 11.5(2.5) 31.1 (6.6) 4.6 (9.6) 6.8 (3 2) 52.2 (9 7) Il.9 (8.5) 22.7 (8.6) 66.8 (7.5) 58.1 (7.5) 57.2 (7.3) 57 4 (7.6) 62 7 (9.3) 0.57 (0.76) 0 91 (0.98) 1.60(1 7) 2.96 (2.3) 0.66 (0.67) 10.3 (22.1) 5.6 (8 2) 0.23 (0.34) 2.4 (5.7) 1.9 (5.6) 7.2 (10.9) 2.8 (1.4) 22.8 (5.6) 23.6(13.5) 18.4 (7.6)
Heart rate. For overall HR, there were no significant effects for extraversion (us < l), or for the half X extraversion interaction (Fs < 2.7). Not surprisingly, HR differed by half, as the mean HR was higher in the first half of the night [means = 60.3 and 58.2; F(1,14) = 8.48, p < 0.021. No other effects were significant. Heart rate in Stages W and 1 showed no significant effects. For Stages 2, delta and REM, there was a significant half effect, indicating higher HR in these stages during the first half of the night, Fs(1,14) = 11.34, 11.78 and 11.84, respectively (Ps < 0.01). None of the HR measures showed a significant effect for extraversion or the extraversion x half interaction. SPR, no main effects were significant. The Skin-potential response rate. For overall extraversion x half interaction approached significance [F(1,14) = 3.89, P < 0.071 as the mean SPR for extraverts was higher during the first 3 hr (means = 1.3 and 0.99) and the introvert’s SPR was higher for the second 3 hr (means = 1.5 and 0.82). The most consistent autonomic difference between introverts and extraverts was an extraversion x half interaction for SPR in Stages 1, 2 and delta. In Stage 1, the introverts and extraverts had approximately equal SPRs in the first half of the night (means = 0.34 and 0.26, respectively), but in the second 3 hr the introverts had a significantly higher SPR than the extraverts [means = 0.91 and 0.37, respectively; F( 1,14) = 4.64. P < 0.051. In Stage 2, introverts and extraverts had equal SPRs in the first half (means = 1.0) and in the second half of the night the introverts had a higher SPR than extraverts (means = 1.6 and 0.62, respectively), although this interaction only approached significance [F(1,14) = 4.37, P -c 0.061. In Stage delta, the extraverts had a higher SPR (1.96) than the introverts (1.16) in the first half, but in the second half the introverts’ SPR was higher than the extraverts’ (means = 2.96 and 2.0, respectively). This interaction was not significant [F(l,14) = 3.46, P < 0.091. In Stage REM, there was no extraversion x half interaction, and the main effect for extraversion only approached significance, with the introverts having a higher SPR (0.57) than the extraverts (0.20) [F(1.14) = 3.70, P < 0.081. In all four sleep stages (1, 2 delta and REM), the SPRs were higher in the second half of the night than in the first half, but only in Stage 1 was the difference significant, the means in the first and second halves being 0.30 and 0.64, respectively [F(1,14) = 9.85, P < 0.011. In Stage 2, the SPR means were 1.0 and 1.1 (F < 1). In Stage delta, the SPRs were 1.6 and 2.5 (F = 4.07. P < 0.07),
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and in Stage REM the means for the first and second halves were 0.29 and 0.49, respectively [F(1,14) = 3.16, P < 0.101. Percent of night per stage. For the percent of the night in Stage W, there were no effects for extraversion or for the extraversion x half interaction. Predictably, there was a tendency for Ss to spend a greater proportion of time awake during the first half of the night [means = 8.2 and 3.0; F(1,14) = 4.33, P < 0.061. For percent of Stage 1, there was no extraversion main effect (F < 1) or extraversion x half interaction (F < 1). The one significant effect for Stage 2 percent was for half, as Stage 2 not unexpectedly comprised a larger proportion of the second half of the night than the first half [means = 53.7 and 37.5; F(1,14) = 34.79, P -C 0.011. As expected, the opposite effect was found for Stage delta percent, the means being 34.6 and 11.2 in the first and second 3 hr, respectively [F( 1,14) = 34.47, P < O.Ol]. As was the case with Stages 1 and 2, there were no significant effects for extraversion or extraversion x half. For Stage REM percent, there was again a significant and predictable effect for half, with more Stage REM occurring in the second 3 hr of the night [means = 9.5 and 23.9; F(1,14) = 36.06, P < 0.011. Body-movement rate. There were no significant effects for overall MR [F(1,14) < 2.11. MR in Stage 1 showed an extraversion x sex interaction [F( 1,14) = 4.99, P < 0.051. For introverts, females had a higher MR than males (means = 6.03 and 1.7, respectively), but for extraverts males had the higher MR, 7.08 to 1.3. For MR is Stage 2, a significant extraversion x half interaction indicated that the introverts’ MR was higher than the extraverts’ in the first half of the night (means = 0.96 and 0.19, respectively), but in the second 3 hr the extraverts had a higher MR than the introverts [means = 0.52 and 0.23; F(1,14) = 5.20, P < 0.051. The main effect for extraversion was not significant for MR in Stage 2 (F = 2.3, P > 0.10). There were no significant effects of MR in Stages delta or REM. Average stage length. For overall average stage length, there was no significant effect for extraversion [F( 1,14) < 2.11. For the average length of Stage W periods, introverts had a longer mean than extraverts (10.9 to 4.8) a difference that approached significance [F(1,14) = 3.12, P < 0. lo]. The absence of an extraversion x half interaction (F < 1) indicates that this relationship remained stable across the night. A significant half main effect indicated that the average Stage W period was longer during the first half of the night (10.1) than during the second half (5.0) [F(1.14) = 5.83, P < 0.051. For the average length of Stage 1 periods, the introverts had a higher mean than the extraverts (3.2 and 2.5, respectively), but the effect was not significant [F(1,14) = 3.53, P < 0.091. Overall, the average length of Stage 2 was no different for introverts and extraverts, but there was a significant half main effect, with Stage 2 periods being longer in the second half of the night than in the first half [means = 22.3 and 16.2, respectively; F(1,14) = 10.75, P < 0.011. There was a significant half main effect for Stage delta, as these periods were longer in the first half of the night [means = 33.4 and 20.0; F(1,14) = 5.70, P < 0.051.
DISCUSSION In general, the results of the present study do not indicate that introverts and extraverts consistently differ in their levels of arousal, at least not immediately before or during sleep. However, as will be discussed shortly, this outcome may actually be consistent with a more integrative interpretation of the arousal hypothesis. During the pre-sleep awake period, introverts did not differ from extraverts on HR or SPR. There was no difference between the groups for initial sleep onset. The groups did not differ in the proportion of the night spent in the different sleep stages. Cohen’s (1979) suggestion that extraverts may have a stronger REM motive than introverts was not supported. The data in support of the arousal hypothesis are those of the tendency for introverts to have higher SPRs than extraverts in the second half of the night, and the somewhat longer average length of Stage 1 periods for introverts. There was also a tendency for introverts to remain awake longer following mid-night awakenings, indicated by the average length of Stage W periods.
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W. BURT THOMPSON and JOHN H. MUELLER
The equality of the introverts’ and extraverts’ sleep-onset times is perhaps the most damaging evidence against the arousal hypothesis. Assuming that the sleep-laboratory environment was equal in the amount of environmental stimulation it provided for introverts and extraverts, then more highly-aroused Ss should have taken longer to fall asleep. However, in light of recent research (Campbell and Hawley, 1982; Geen, 1984) which suggests that introverts have a lower level of optimal arousal than extraverts, the level of environmental stimulation must be considered when interpreting the outcome of the present study. Briefly, Campbell and Hawley (1982) observed the library environment chosen by extraverts and introverts in which to study. Extraversion was positively correlated with noise level and socializing opportunities. Geen (1984) found that introverts chose a lower level of noise to be heard during a learning task than extraverts. Introverts and extraverts showed no difference in physiological arousal when stimulated by a sound level chosen by either themselves or members of the same personality group. However, at the same intensity levels, introverts were more aroused than extraverts. Several investigators (Gale, 1981; Geen, 1984) have suggested that introverts and extraverts may show little or no difference in arousal at very low levels of stimulation. This may be due to their (theoretically) different levels of optimal stimulation, and the resulting strategies they develop to deal with deviations from that optimal level. A sleep laboratory is a relatively unstimulating environment, therefore it is not unreasonable to consider the present study as an investigation of the arousal hypothesis under extremely low levels of stimulation. If this were the case, then the failure to detect arousal differences between introverts and extraverts would not be surprising. Although this interpretation is post hoc, it leads to some specific predictions. For example, if introverts and extraverts differ in their sensitivity to environmental stimuli, then one might expect longer sleep onsets in introverts under conditions of stimulation higher than those present during this study. In other words, in a noisy environment (e.g. a college dormitory), introverts may take longer to fall asleep than extraverts, but in quiet environments their sleep onsets may be equal. In terms of habituation to the sleep laboratory, it may be that extraverts habituate to the novel environment of a sleep laboratory (e.g. the bed and electrodes) more quickly than introverts, as suggested by Spiegel and Neusch (1977). In this case, sleep-onset times and autonomic activity on the first night in the lab may be a better measure of arousal differences between introverts and extraverts. However, as discussed above, first-night sleep records were not taken during the present study, so this question remains in need of investigation. In conclusion, no consistent evidence was found in support of Eysenck’s (1967) theory relating extraversion and arousal. An interpretation of this arousal hypothesis that simply stresses the absolute arousal differences between introverts and extraverts may overlook important interactions between an individual and the environment. A more integrative hypothesis, one that predicts arousal levels on the basis of an interaction between individual differences in the optimal level of stimulation and the amount of stimulation in the environment, may provide a more complete framework for interpreting the relevant data. Several tests of this arousal hypothesis that involve sleep are still in need of investigation. One involves the measurement of sleep onset (initially and following mid-night awakenings) in environments with different levels of stimulation. If introverts prefer lower levels of stimulation than extraverts (and thus, are presumably more highly aroused at moderate levels of stimulation), then their sleep onset should be delayed under moderate to high levels of stimulation. A second test would involve measurement of the rate of habituation to the sleep laboratory, as indicated by the usual measures of sleep quality (e.g. delayed sleep onset, increased proportions of Stage 1 sleep), and autonomic measures. More highly-aroused Ss would be expected to take longer to habituate to the sleep laboratory over the course of several nights. Nebraska Wesleyan University (Lincoln, Nebr.) for Acknowledgements-We wish to thank the Psychology Department, the use of their sleep laboratory. This project would not have been possible without the friendship and technical expertise of Marty Klein. Helpful comments on an earlier draft were made by Russell Geen and Dave McDonald.
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