Combination of bright light and caffeine as a countermeasure for impaired alertness and performance during extended sleep deprivation

Transcription

1 J. Sleep Res. (1997) 6, Combination of bright light and caffeine as a countermeasure for impaired alertness and performance during extended sleep deprivation KENNETH P. WRIGHT JR., PIETRO BADIA, BRYAN L. MYERS and STEVEN C. PLENZLER Department of Psychology, Bowling Green State University, Bowling Green, OH, USA Accepted in revised form 15 November 1996; received 15 August 1996 SUMMARY Effects of four conditions (Dim Light-Placebo, Dim Light-Caffeine, Bright Light- Placebo and Bright Light-Caffeine) on alertness, and performance were studied during the night-time hours across 45.5 h of sleep deprivation. Caffeine (200 mg) was administered at and hours and bright-light exposure (>2000 lux) was from to hours each night. The three treatment conditions, compared to the Dim Light-Placebo condition, enhanced night-time performance. Further, the combined treatment of caffeine and all-night bright light (Bright Light-Caffeine) enhanced performance to a larger degree than either the Dim Light-Caffeine or the Bright Light- Placebo condition. Beneficial effects of the treatments on performance were largest during the early morning hours (e.g. after hours) when performance in the Dim Light-Placebo group was at its worst. Notably, the Bright Light-Caffeine condition was able to overcome the circadian drop in performance for most tasks measured. Both caffeine conditions improved objective alertness on the Maintenance of Wakefulness Test. Taken together, the above results suggest that the combined treatment of bright light and caffeine provides an effective intervention for enhancing alertness and performance during sleep loss. KEYWORDS alertness, bright light, caffeine, performance, shift work, sleep deprivation INTRODUCTION 1991; Dawson and Campbell 1991; Horne et al. 1991; Daurat et al. 1993) and the ingestion of stimulants such as caffeine Work during the early morning hours is often accompanied by (Borland et al. 1986; Rogers et al. 1989; Walsh et al. 1990, reduced levels of alertness and performance. In general, between 1995; Schweitzer et al. 1992; Smith et al. 1993; Bonnet and and hours, performance and alertness are at their Arand 1994a,b; Bonnet et al. 1995; Muehlbach and Walsh lowest levels (e.g. Colquhoun 1984; Krueger 1989; Smith 1992) 1995). While these studies show that naps, bright light and resulting in poor work efficiency and in an increased risk of caffeine enhance night-time alertness and performance, most accidents (e.g. Lauber and Kayten 1988; Gold et al. 1992; still report a circadian trough in alertness and performance Dinges 1995; Horne and Reyner 1995). The degradation in between and hours. The present study focused on alertness and performance which occurs during the early whether a combination of treatments could overcome the morning hours can be attenuated by taking naps (Dinges et circadian trough in alertness and performance. al. 1987; Rogers et al. 1989; Bonnet 1991; Schweitzer et al. Since caffeine is thought to enhance alertness and 1992; Rosekind et al. 1995), exposure to bright light (e.g. performance by antagonizing the neuromodulator adenosine Campbell and Dawson 1990; French et al. 1990; Badia et al. in the central and peripheral nervous systems (Snyder 1985; Nehlig et al. 1992), and since bright light is thought to enhance night-time alertness and performance by reducing melatonin Correspondence: Kenneth P. Wright, Jr., Department of Psychology, Bowling Green State University, Bowling Green, Ohio 43403, USA. and enhancing body temperature (e.g. French et al. 1990; Badia Tel.: + (419) ; fax: + (419) ; et al. 1991), it was hypothesized that combining caffeine and kwright@bgnet.bgsu.edu bright-light treatments, thus utilizing both mechanisms, would European Sleep Research Society

2 Bright light and caffeine 27 lead to greater effects on alertness and performance than either Pharmacal Co., Chicago, IL, USA) or a placebo (200 mg sugar) treatment used alone. To test this hypothesis, the current study was administered in a double-blind manner with water and investigated the effects of caffeine and bright light, alone and food at and hours each night. in combination, on night-time alertness and performance in Both nights from to hours, subjects remained humans. These treatments were compared to each other and under either dim or bright illumination depending on the to a dim light placebo control condition across 2 consecutive treatment condition. Light sources in the bright-light room nights of sleep deprivation. consisted of a bank of four light boxes (Lighting Resources, Columbus, OH, USA; Apollo Light Systems Orem, UT, USA), METHOD placed 1.5 m in front of the subjects. In addition, three 500 watt halogen lamps, shielded from direct illumination of the Subjects eyes, illuminated the background. During the performance Forty-six, healthy male participants (aged y) who gave testing on computer tasks, two light boxes provided a minimum written informed consent were studied during 45.5 h of sleep of 2000 lux. At all other times subjects received 2500 lux. Lux deprivation. Participants showed regular sleep/wake schedules measurements were taken at eye level. Subjects wore black (self-reported bedtime between and hours and wake shirts, eye black, and the computer monitor was surrounded time between and hours) and were low to moderate by a hood to prevent glare from interfering with performance caffeine users ( mg daily). Only moderate caffeine users testing. were tested to minimize tolerance and withdrawal effects. Sleep When subjects were not performing tasks, they were schedules and caffeine intake were verified with logs for the week permitted to watch taped movies, play video games prior to study. In addition, subjects were free of medication and ( hours only), read and write, play cards, or nicotine use. Alcohol and caffeine use was prohibited for 24 h converse with a member of the research staff. prior to participation. All subjects obtained a physical exam at the Student Health Centre. Over 200 subjects were screened. Dependent measures Of these, 46 met the criterion for participation and 40 of these completed the study. Five subjects, two receiving caffeine and Several neurobehavioural measures were used to examine three receiving placebo, terminated participation after 1 night alertness and performance during the night-time hours. These of sleep deprivation and one subject was removed from analysis measures were recorded in 3 h blocks from to hours owing to violating the experimental protocol. The average age each night. and weight of subjects were 19.2 y and 75.5 kg (166.5 lbs). Subjects weight, age and morning/eveningness type (Horne Performance tasks and Osterberg 1976) were similar for each treatment condition. Subjects were paid for their participation. Performance batteries were composed of tasks from the Walter Reed Performance Assessment Battery, the United Triservice Performance Assessment Battery, and several tasks from the Experimental protocol sleep laboratories of the University of Pennsylvania and Participants arrived at the laboratory at hours Thursday Bowling Green State University (Thorne et al. 1985; Gillooly and remained until hours Sunday. Thursday evening, et al. 1990; Dinges et al. 1993, 1994). These tasks included the participants were introduced to performance tasks, given Dual Task, the Wilkinson Four Choice Reaction Time, the practice, and then slept from to hours. After Switching Task, a modified version of the Psychomotor awakening, they remained awake for the next 45.5 h. A modified Vigilance Task, Two-Column Addition, Continuous constant routine procedure was used (Minors and Waterhouse Recognition, Reaction Time task Time Uncertainty Block, 1985; Czeisler et al. 1990). During the daytime Digit Recall, Probed Forced Memory Recall, and the Thurstone ( hours), ambient illumination was maintained at word-generating task. Ζ100 lux. Two subjects were assigned to each experimental Subjects practised performance tasks from to room where they remained seated (nonrecumbent) in chairs hours Thursday night as well as from to hours throughout the study except during bathroom breaks. Ambient Friday day. Each subject practised tests until asymptotic temperature in the rooms was maintained at 25.6±1.7 C (78 performance was obtained (i.e. until there was less than a 10% ±3 F). Food was provided in aliquots every 3 h. The diet was change in performance between the last two practice trials). based on 3600 cal/day (RDA+20%; USDA 1981) to allow for Data were lost for two subjects in the Switching and modified increased energy need during sleep deprivation. Each aliquot Psychomotor Vigilance Tasks. consisted of 450 cal. Only throughput scores were analysed except for tasks not Subjects were randomly assigned to one of four conditions: having a throughput score. Throughput is derived from Dim Light-Placebo (Ζ100 lux/200 mg sugar), Bright Lightper accuracy and speed data yielding a measure of correct responses Placebo (2500 lux/200 mg sugar), Dim Light-Caffeine (Ζ100 minute. The throughput measure is thus sensitive to changes lux/200 mg caffeine), and Bright Light-Caffeine (2500 lux/ in accuracy and speed performance. Table 1 includes a list of 200 mg caffeine). Either caffeine ( Eleveine Alva-Amco all performance tasks and measures analysed. As seen, several

3 28 K. P. Wright et al. Table 1 Mean percentage of trials for which performance was better under treatment compared to placebo for each condition Bright light caffeine Dim light caffeine Bright light placebo All Trials All Trials All Trials trials after trials after trials after h h h Dual task throughput (T) Dual task control losses Modified psychomotor vigilance task reaction time Wilkinson throughout Switching task mannequin throughput (T) Switching task math throughput Probed force memory recall weak associates (T) Probed force memory recall strong associates Digit recall throughput Two-column addition throughput Continuous recognition throughput (T) Thurstone (words generated) Reaction time (time uncertainty block) throughput tasks have two measures which were analysed, each measuring adifferent component of the task. Bright Light-Caffeine condition. An analysis was conducted for the broad EEG bands of Delta ( Hz), Theta ( Hz), Alpha ( Hz), and Beta ( Hz). Measures of alertness Measures of alertness included the Maintenance of Wakefulness Data analysis Test (MWT; Mitler et al. 1982), Stanford Sleepiness Scale (SSS; Data for each alertness test and each performance measure Hoddes et al. 1973), and spectral analysis of the EEG. Latencies listed in Table 1 were analysed using to sleep on the MWT were scored using the criteria of (Caffeine Light Night) repeated measure ANOVAs. In Recthschaffen and Kales (1968). Sleep onset was defined as addition, Time-of-Night effects were examined for each night three continuous 30 s epochs of any stage of sleep. If subjects (four times a night). To compensate for possible violations of remained awake for 15 min, the test was ended and a score of repeated measure assumptions (e.g. sphericity, homogeneity of 15 was given for that test. For MWT and spectral data, variance) and to decrease Type I error rate, Huynh-Feldt EEG activity was acquired over three brain sites (Fz, Cz, Oz) Correction Factors were used in all analyses where appropriate. referenced to linked mastoids using a Grass Model 7 Polygraph. Modified Bonferroni Correction Factors were used to correct Standard mentalis EMG, and EOG activities were also recorded for multiple comparisons (Keppel 1982). to assist in sleep stage scoring. Each EEG amplifier (Grass The last practice trial for each performance task was model 7P511) was set at a sensitivity of 5 μv/mm and used a examined to test for differences among the groups prior to frequency bandwidth (filter settings) of Hz. A 60 Hz treatment onset. These data are shown in Figs 1 and 2 with notch filter was used to attenuate electrical interference noise. the last practice trial (baseline) labelled with a P on each A 50-μV sign wave signal was used for calibration. Impedances performance graph. Results showed no significant differences for the EEG electrodes were maintained at less than 10 Kohms. among the groups for the last practice trial. However, several For EEG spectrals, 1 min of eyes closed data were acquired nonsignificant trends (differences in baseline) were noted. To at a rate of 200 samples per s per channel with a 12-bit Data equate groups the latter data were transformed into change Translations 2821 A-D board. Data were assessed for changes scores by subtracting the baseline score from all subsequent in amplitude by submitting the digitized EEG data to a Fast time points. Tasks for which data were transformed are labelled Fourier Transform (FFT) in manually selected epochs (Rhythm in Table 1 with a (T). vs. 9.0; Stellate Systems, Westmount, Canada). The EEG data For descriptive purposes, the percentage of trials for which were then accumulated in power units (μv 2 /Hz) over a performance was better under treatment conditions compared frequency bandwidth of Hz, in 1 s bins. Artifacts were to Dim Light-Placebo was tabulated. Specifically, means for removed from data prior to analysis. Spectra of epochs with each performance task for each trial in the caffeine and brightlight EEG artifacts (e.g. muscle artifact, amplifier blocking) were treatment conditions were compared to the Dim Light- eliminated on the basis of visual inspection. If fewer than 30 s Placebo condition. If better performance occurred in the of artifact-free epochs were available, the spectral was omitted caffeine and bright-light treatment conditions compared to from further processing. Data were lost for one subject in the Dim Light-Placebo, a score of 1 was given to the treatment

4 Bright light and caffeine 29 Figure 1. Performance on (a) the Dual Task Control Losses, and (b) the Wilkinson Four Choice Reaction Time task during the last practice trial prior to sleep deprivation (P=17.00 h) and every 3 h nightly. =significant difference from Dim Light-Placebo (DLP); Ε=significant difference between Bright Light-Caffeine (BLC) and Bright Light-Placebo (BLP) or BLC and Dim Light-Caffeine (DLC);+=significant difference between BLP and DLC. for that trial. Otherwise, if performance was the same or lower Dim Light Placebo, performance was better in the Bright Lightin the treatment group compared to Dim Light-Placebo, a Caffeine condition for 83% of trials on Night 1 and 94% of score of 0 was assigned. The total number of 1s were then trials on Night 2. The Dim Light-Caffeine condition showed tabulated for each condition separately and divided by 52 (13 better performance for 83% of trials on Nights 1 and 79% of measures, see Table 1, multiplied by 4 performance trials for the trials on Night 2; the Bright Light-Placebo condition each night of sleep deprivation). The final data were thus a showed better performance relative to Dim Light-Placebo for percentage of the trials that overall performance was better 58% of trials on Night 1 and for 75% of trials on Night 2. under that treatment compared to Dim Light-Placebo. Table 1 provides a descriptive analysis for each performance measure separately for all trials and for trials after hours. RESULTS In addition to the above descriptive analysis, results of the Performance measures ANOVA analyses for tasks showing significant main effects or significant interaction effects are provided in Table 2. Most Performance data are summarized as the percentage of trials tasks show a main effect for either Caffeine, Light, or both. In performance was better under each treatment condition addition, a main effect for Night was observed for most tasks. compared to the Dim Light-Placebo condition. Compared to Also, a few Light Night interactions were obtained. In

5 30 K. P. Wright et al. Table 2 Analysis of variance summary table for (caffeine condition light condition night) repeated measures analysis Task Measure d.f. F F F F F F F Caffeine(C) Light(L) Night(N) C L C N L N C L N Dual task Control losses 1, Switching task Math throughput 1, (.054) (.053) 2.37 Switching task Mannequin 1, (.07) 3.50(.07) 0.01 throughput Thurstone Number of words 1, generated Wilkinson four choice Throughput 1, reaction time Modified psychomotor reaction time 1, vigilance task Note. Values in parentheses represent P-values of trends. P<.05. compared to placebo (Fig. 2a,b). Little effect of bright light was observed. However, the combined treatment of Bright Light and Caffeine showed the best performance on the Modified Psychomotor Vigilance Task on both nights of sleep deprivation (Fig. 2c). Thurstone number of words generated Although significant main effects were observed for the number of words generated (Table 2), the only significant post-hoc comparison seen was for better performance in the Bright Light- Caffeine condition than in the Dim Light-Placebo condition at hours on Night 2 (P<0.0063). general, performance was worse across each night and also tended to be worse on Night 2 compared to Night 1. Also, Dim Light-Caffeine enhanced performance for most performance tasks, whereas Bright Light-Placebo tended to enhance performance for some tasks. The Bright Light-Caffeine condition was by far the most powerful treatment for enhancing performance during the night-time hours. Notably, unlike the other treatments, the combination of Bright Light and Caffeine prevented the circadian drop in performance for most tasks. The Bright Light-Caffeine condition also showed improved performance compared to the Dim Light-Caffeine and Bright Light-Placebo conditions for several tasks. Results for specific tasks are summarized next. Dual task control losses and Wilkinson four choice reaction time Alertness Maintenance of Wakefulness Test (MWT) Figure 1 presents data for the Control Loss measure of the A significant main effect for Caffeine F (1, 36)=18.11, P<0.01 Dual Task and for the Throughput measure of the Wilkinson showed higher levels of alertness for caffeine compared to the Task. In general, night-time performance was better under Dim placebo conditions (Means and standard errors for each night Light-Caffeine and Bright Light-Placebo vs. Dim Light-Placebo are listed at bottom of Fig. 3). The effect of caffeine on the (Fig. 1a,b). However, the Bright Light-Caffeine condition MWT was larger on Night 2 compared to Night 1 as observed showed the most marked effects on performance. Performance with a significant Caffeine Night interaction F (1, 36)=9.25, was significantly better in the Bright Light-Caffeine condition P<0.01. Multiple comparisons show no significant differences compared to Dim Light-Placebo for almost all measurements among the four treatment conditions for the average latency taken on both nights of sleep deprivation. The combined to sleep on Night 1. However, there was a trend for a shorter treatment of bright light and caffeine also showed significantly latency to sleep in the Dim Light-Placebo condition compared improved performance compared to the Dim Light-Caffeine to the Bright Light-Caffeine condition on Night 1 (P=0.04; a and Bright Light-Placebo conditions for several measurement modified Bonferoni Correction Factor required a P<0.003 each night. for significance). Note that no subjects fell asleep during the allowable 15-min period in the Bright Light-Caffeine condition on Night 1. On Night 2, latency to sleep was significantly Switching task mannequin throughput, switching task math shorter in the placebo conditions compared to the caffeine throughput and the modified psychomotor vigilance task conditions (all P<0.003). Significantly shorter latencies to sleep Switching Task Math and Mannequin Throughput per- on Night 2 occurred for all conditions when compared to Night formance were significantly better under caffeine conditions 1 (all P<0.003). Time-of-Night effects were also observed. As

6 Bright light and caffeine 31 Figure 2. Performance on the (a) Switching Task Mannequin Throughput (b) Switching Task Math Throughput and (c) the modified Psychomotor Vigilance Task, during the last practice trial prior to sleep deprivation (P=17.00 h) and every 3 h nightly during 2 nights of sleep deprivation. =significant difference from DLP; Ε=significant difference between BLC and BLP or BLC and DLC;+=significant difference between BLP and DLC.

7 32 K. P. Wright et al. Table 3 Subjective sleepiness on the stanford sleepiness scale interactions for Caffeine Night F (1, 36)=23.24, P<0.01), and Light Night F (1, 36)=5.18, P<0.05) were observed. Post-hoc Dim light- Dim light- Bright light- Bright-light comparisons revealed significantly higher subjective sleepiness placebo caffeine placebo caffeine n n n on Night 1 for the placebo conditions than in the caffeine conditions (all P<0.003). Furthermore, Bright Light-Caffeine Night ± ±0.24 a,c 4.33± ±0.31 a,b,c showed lower subjective sleepiness compared to Dim Light- Night ± ± ±0.24 a,b 4.70±0.28 a,b Caffeine on Night 1 (P<0.003). No effect of Bright Light-Placebo on subjective sleepiness was observed on Night 1. However, On Note. Mean±SEM. a=significantly less sleepy than Dim Light- Placebo (P<0.003), b=significantly less sleepy than Dim Lightconditions compared to the bright light conditions (all P<0.003). Night 2 higher subjective sleepiness was observed in the dim light Caffeine (P<0.003), c=significantly less sleepy than Bright Light- Placebo (P<0.003), n=significntly less sleepy on Night 1 vs. Multiple comparisons also revealed greater sleepiness on Night Night 2 (P<0.003). 2 compared to Night 1 for all conditions (all P<0.003, except for a trend for the Bright Light-Placebo condition, P=0.024; a modified Bonferoni Correction Factor required a P<0.003 to be significant). seen in Fig. 3, caffeine under both dim and bright light Spectral Analysis of the EEG maintained longer latencies to sleep compared to Dim and Objective alertness was also measured using power spectral Bright Light Placebo conditions the last measurement on Night analysis of EEG activity. Absolute power data at brain site Cz 1 and after hours on Night 2 (all P<0.0063). for the Delta, Theta, Alpha and Beta bands were examined. The alerting effects of caffeine also carry over into the Results showed some differences in EEG activity among the morning hours as revealed by a significant main effect for caffeine and bright light conditions. However, no consistent Caffeine F (1, 36)=20.78 on the hours MWT. pattern of EEG arousal for any of the caffeine or light treatments was observed. Stanford sleepiness scale (SSS) Mean sleepiness scores on the SSS are presented in Table 3. Main DISCUSSION effects for Caffeine F (1, 36)=8.29, P<0.01, for Light F (1, 36)= As predicted, the combination of bright light and caffeine was 8.77, P<0.01), and Night F (1, 36)=175.55, P<0.01) as well as the most effective treatment for enhancing night-time alertness Figure 3. Latency to sleep on the Maintenance of Wakefulness Test during sleep deprivation every 3 h nightly and once during the morning. =significant difference from DLP; Ε=significant difference between BLC and BLP or BLC and DLC;+=significant difference between BLP and DLC. Trends noted as P-values in parentheses. Values in lower left corner are the average (SEM) latency to sleep scores for each condition on each night.

8 Bright light and caffeine 33 and performance across the sleep deprivation period. This similar, however, the timing of caffeine administration was manipulation was sufficient to overcome the circadian trough different. Bonnet et al. administered 400 mg of caffeine at in night-time performance for most tasks measured. Although hours each night or 150 or 300 mg at hours on the the efficacy of this intervention in applied settings remains to first night of sleep loss and then every 6 h there after. The be determined, the current data suggest that the combination hours time is near the peak of the melatonin curve and of bright light and caffeine may provide an effective intervention the trough of the temperature curve. In contrast, the current in cases where sleep loss and fatigue contribute to poor study first administered caffeine just prior to the onset of performance (e.g. continuous and sustained operations, shift melatonin secretion and very near the temperature peak (20.00 work). hours) each night. The differential findings suggests that the Separately, both the Dim Light-Caffeine and Bright Light- timing of caffeine administration may be an important factor in Placebo conditions effectively enhanced performance during its ability to sustain alertness and performance across extended the night-time hours compared to the Dim Light-Placebo periods of sleep loss. condition. In addition, the Dim Light-Caffeine condition A study by Penetar et al. investigated whether caffeine could showed superior performance compared to the Bright Light- enhance alertness and performance following 49 h of sleep Placebo condition for several performance measures. The Dim deprivation. A large dose of caffeine (600 mg 70 kg 1 ) Light-Caffeine condition also enhanced objective alertness administered at hours ameliorated the effects of sleep (MWT) and reduced subjective sleepiness (SSS). Differences deprivation on performance (for up to 12 h) and on alertness among the treatments on the MWT, especially on Night 1, (for up to 4.5 h) compared to placebo. Whereas, moderate doses may have been greater had a time period longer than 15 min of caffeine (150 and 300 mg 70 kg 1 ) improved performance in been used. Little effect of the Bright Light-Placebo condition some tasks but not in others. These results suggest that larger on the MWT was observed; however, some effect on the SSS doses of caffeine than used in the present study may be required occurred. Thus, caffeine doses of 200 mg twice a night appears to enhance alertness and performance for periods of sleep more effective in enhancing night-time alertness and deprivation longer than 48 h. performance than exposure to 2500 lux of bright light. Whether In the present study, little consistent effect of the caffeine and similar results would occur when comparing or combining light treatments on spectral EEG data was observed when other doses of caffeine and light remains to be determined. compared to the Dim Light-Placebo condition. Bright light As noted, others have shown exposure to bright light to has been shown to increase beta EEG activity (Badia et al. enhance performance during the night-time hours (Campbell 1991) and, in general, stimulants such as caffeine increase beta and Dawson 1990; French et al. 1990; Badia et al. 1991; and decrease theta EEG (Nehlig et al. 1992). Dawson and Campbell 1991; Horne et al. 1991; Daurat et al. 1993). However, results from one study failed to show an effect of bright light (1500 or 3000 lux) on night-time performance Mechanisms (Dollins et al. 1993). Given that the majority of studies, Correlational evidence shows rhythms in alertness and including the present one, show a positive effect of bright light performance to be related to the circadian rhythms of melatonin on performance, the negative results of the study by Dollins and temperature (e.g. Akerstedt et al. 1979; Dinges et al. 1995). et al. may be related to methodological factors as noted by the In general, when melatonin is high and temperature is low, authors in their article. performance and alertness are low. Experimental evidence Results for the Dim Light-Caffeine condition are consistent shows administration of exogenous melatonin to reduce with previous studies showing caffeine ingestion to improve temperature levels, shorten the latency to sleep, and reduce performance across the night-time hours (Borland et al. 1986; performance levels (e.g. Lieberman et al. 1984; Badia et al. Rogers et al. 1989; Schweitzer et al. 1992; Smith et al. 1993; 1993; Dawson and Encel 1993). Since exposure to bright Bonnet and Arand 1994a,b; Muehlbach and Walsh 1995). light during the night-time hours suppresses melatonin and Caffeine ingestion has also been shown to decrease both attenuates the nocturnal decrease in temperature (e.g. Lewy et subjective (SSS) and objective measures of sleepiness (MSLT al. 1980; Badia et al. 1991), improved alertness and performance and Repeated Test of Sustained Wakefulness RTSW) during bright-light exposure are thought to be related to compared to a placebo control (e.g. Walsh et al. 1990). changes in melatonin and temperature (Campbell and Dawson Two studies have reported effects of caffeine on alertness 1990; French et al. 1990; Badia et al. 1991; Horne et al. 1991; and performance during sleep deprivation periods of more Daurat et al. 1993). Whether the alertness and performance than 24h (Penetar et al. 1994; Bonnet et al. 1995). Bonnet et enhancing effects of bright light and caffeine are related to al. reported beneficial effects of caffeine on the first night of immediate or delayed effects on the circadian pacemaker can sleep deprivation, whereas on the second night both caffeine not be determined with the current experimental design. and placebo groups were relatively incapacitated. The ability However, one could hypothesize that a small phase delay may of caffeine to enhance alertness and performance across 2 have occurred since more bright-light exposure occurred on nights of sleep deprivation in the current study is contrary to the delay portion of the phase response curve (Czeisler et al. the latter results. A comparison between the studies shows that 1989). Whether caffeine has phase shifting effects remains subject characteristics and caffeine dose administered were to be determined. However, examination of melatonin and

9 34 K. P. Wright et al. temperature curves for subjects in the current study (presented ACKNOWLEDGEMENTS in Wright et al. 1997) suggests that a large phase shift (e.g. We thank the laboratory technician, L. Santiago and the Type 0 resetting) did not occur since melatonin still peaked research assistants, M. Boecker, M. Cochran, J. Ginzler, T. and temperature still troughed at night after caffeine ingestion Hawkins, N. Knippen, L. Selden, A. White. W. Wojdak, and and during the bright-light exposure. M. Woodruff. This work was supported by the Army Research The finding that bright-light exposure does not affect Institute, Contract MDA K temperature, alertness and performance during the daytime hours (when melatonin is low) provides additional support for the role of melatonin and temperature in the alerting and REFERENCES performance enhancing effects of bright light (Badia et al. 1991; Akerstedt, T., Froberg, J. E., Friberg, Y. and Wetterberg, L. 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