Adaptation of performance during a week of simulated night work

ERGONOMICS, 5FEBRUARY, 2004, VOL. 47, NO. 2, 154 165 Adaptation of performance during a week of simulated night work NICOLE LAMOND*, JILL DORRIAN, HELEH J. BURGESS, ALEX L. HOLMES, GREGORY D. ROACH, KIRSTY MCCULLOCH, ADAM FLETCHER and DREW DAWSON The Centre for Sleep Research, The Queen Elizabeth Hospital, South Australia Keywords: Night work; Performance deficits; Adaptation; Alcohol intoxication. This study aimed to provide a comparative index of the performance impairment associated with the fatigue levels frequently experienced in workplaces that require night work. To do this, we equated fatigue-related impairment with the impairment resulting from varying levels of alcohol intoxication. Fifteen young individuals participated in two counterbalanced conditions which required them to (1) work seven consecutive 8-h night shifts, and (2) consume an alcoholic beverage at hourly intervals until their blood alcohol concentration (BAC) reached 0.10%. In each condition, performance was measured at hourly intervals using a 10-min psychomotor vigilance task (PVT). Analysis indicated that as BAC increased, performance impairment significantly increased. Similarly, response times significantly increased during the first six simulated night-shifts, and lapse frequency significantly increased during the first two shifts. Equating the two conditions indicated that the first simulated night shift was associated with the greatest degree of performance impairment. In general, the impairment at the end of this shift was greater than that observed at a BAC of 0.10%. During the second and third simulated night shifts, the performance impairment was less than on the first night, but greater than that observed at a BAC of 0.05%. For the final four nights, the performance decrements generally did not exceed those observed at a BAC of 0.05%. This suggests that during a week of consecutive night shifts, adaptation of performance occurs. 1. Introduction Recent studies have provided evidence to suggest that fatigue and alcohol intoxication have qualitatively and quantitatively similar effects on performance (Dawson and Reid 1997, Lamond and Dawson 1999, Williamson and Feyer 2000). More importantly, they suggest that following 20 to 25 h of wakefulness in the early hours of the morning, the performance decrements are equivalent to those observed at a BAC of 0.10%. That is, moderate levels of fatigue produce performance decrements equivalent to or greater than those observed at levels of alcohol intoxication deemed unacceptable when driving, working and/or operating dangerous equipment. *Author for correspondence at The Centre for Sleep Research, 5th Floor CDRC Building, The Queen Elizabeth Hospital, Woodville Road, Woodville SA 5011, Australia. e-mail: nicole.lamond@unisa.edu.au Ergonomics ISSN 0014-0139 print/issn 1366-5847 online # 2004 Taylor & Francis Ltd http://www.tandf.co.uk/journals DOI: 10.1080/00140130310001617930

Adaptation of performance during simulated night work 155 Since approximately 50% of shiftworkers typically spend at least 24 h awake on the first night shift in a roster (Knauth et al. 1980, Tepas et al. 1981), these findings have important implications for industries requiring shiftwork. If generalized to an applied setting, they suggest that on the first night a shift worker would show a performance decrement similar to or greater than is acceptable for alcohol intoxication. However, shiftworkers are usually rostered on for several consecutive nights (Tepas 1985, Knauth 1995). Thus, while the findings of previous studies equating the effects of alcohol and one night awake provide a useful comparative index, they have limited generalizability in the real world except for the first shift in a roster or single night shifts. This is particularly the case given that shiftwork related fatigue is not caused solely by the effects of acute sleep deprivation. Rather, it is more often the result of cumulative partial sleep deprivation over a series of nights. Due to extended period of prior wakefulness, shift workers working nights will frequently experience acute sleep deprivation at the commencement of the first nightshift (Knauth et al. 1980, Akerstedt 1998). In addition, the subsequent desynchronization of the sleep and circadian timing systems reduces the quality and duration of daytime sleep periods. Previous research has shown that workers on nightshift will typically sleep between 2 and 4 h less during the day than at night (Foret and Lantin 1972, Foret and Benoit 1978, Tilley et al. 1982, Torsval et al. 1989, A kerstedt et al. 1991). Therefore, the initial acute sleep deprivation from night one is often further exacerbated by reduced daytime sleep quality and quantity over a sequence of nightshifts. As a result, the circadian performance deficits typically observed on night shift may be exacerbated by the cumulating sleep debt that commonly occurs during the course of a shift week (Tilley et al. 1982). Indeed, a large number of both laboratory and field investigations report cumulative sleep loss as a primary cause of the increased accident and lowered productivity over a week of night shifts (Dahlgren 1981, Tilley et al. 1982, Monk and Wagner 1989). When a series of night-shifts are worked, the potential for circadian adaptation is also a consideration that needs to be taken into account. Several studies have demonstrated functionally significant circadian adjustment following acute shifts of the sleep-wake schedule (Czeisler et al. 1990, Dawson and Campbell 1991, Minors and Waterhouse 1993, Harma et al. 1994, Dawson et al. 1995). Available evidence suggests that adaptation of performance efficiency over successive night shifts may occur in conjunction with circadian resynchronization (e.g. Colquhoun et al. 1968). However, adjustment to night work is relatively slow, and even after a considerable number of successive night shifts, adaptation is usually only partial (Colquhoun et al. 1968, Folkard et al. 1978, Knauth et al. 1978, Sack et al. 1992). Consequently, while performance may improve in conjunction with circadian adaptation, it is unlikely to return to levels observed in day workers. To provide policy makers with an ecologically valid comparison, it is necessary to compare the effects of fatigue and alcohol across a sequence of nightshifts. As maximum fatigue levels in shiftworkers are most commonly reported with shift patterns that require long sequences of nightshift (Folkard 1981, Tilley et al. 1982), the current study systematically compared the performance impairment that occurred during seven consecutive nights of simulated shift work with that caused by alcohol intoxication. The primary aim was to extend the findings of previous studies that have equated the effects of a single night of sleep loss with the effects of

156 N. Lamond et al. alcohol, and in doing so, provide a comparative index of the fatigue levels frequently experienced in workplaces that require night work. 2. Method 2.1. Participants Fifteen (eight females, seven males) healthy individuals, aged 18 to 27 years, with a mean body mass index of 22.3 (+ 2.3 kg m 2 ), participated in the current study. The participants were non-smokers who did not regularly consume large caffeine (5 350 mg/day) or alcohol (46 drinks/week) doses, and participated in a moderate amount of exercise (410 h/week). Those recruited had no current health problems and were not taking any medication other than an oral contraceptive (all females). Participants reported no history of sleep problems and were not habitual nappers, nor had they undertaken shift work or transmeridian travel in the past month. 2.2. Procedure Each participant attended two experimental conditions: (1) a simulated night shift condition, and (2) an alcohol intoxication condition. The experimental conditions were administered in a randomized and counterbalanced fashion with at least 7 days between sessions. Participants attended the conditions in groups of three or four. In both conditions the room temperature was set at 258C. During the sleep periods, light levels in the bedrooms were extremely low (light off, no windows, door to corridor shut with corridor light also off). During the testing periods, all inside lights were turned on and the windows were completely covered by heavy curtains such that levels varied from 35 300 lux depending on where each participant sat. Throughout the study, participants were permitted to consult timepieces but were not permitted to set alarms. Participants were not allowed to nap during their free time or the testing periods, and were required to abstain from caffeine and other stimulants for the entire study period. Exercise and showers were not permitted during the simulated night shifts. 2.2.1. Simulated night shift condition: Figure 1 presents a schematic representation of the protocol. For the night shift condition, participants were required to attend the laboratory for nine consecutive nights. These included an adaptation and a baseline night sleep, directly followed by seven consecutive night shifts and the subsequent daytime sleep periods. During each sleep period, polysomnographic, body temperature, respiratory and cardiac data were collected (these measures will be reported elsewhere). In addition, participants wore wrist actigraphs and were required to complete sleep wake diaries for the 7 days preceding, and during the study. On both the adaptation and baseline night, participants arrived at the laboratory at 1700 h, and were assigned to their individual bedroom. Using pre-study sleep diaries and wrist actigraph data, mean bedtimes for the previous week were determined and then assigned as that participant s bedtime for these adaptation and baseline nights. Participants were instructed to sleep until they naturally woke. After they awoke, participants were free to leave the laboratory and follow their normal daily routines until they returned at 1700 h. Following the baseline night, participants began the seven nights of simulated shiftwork. Participants were instructed to arrive at the laboratory by 1900 h each night, or earlier if they required dinner (served at 1800 h). From 2000 h onwards,

Adaptation of performance during simulated night work 157 ADAPTATION freetimeinlab sleep period BASELINE freetimeinlab sleep period SHIFT 1 freetimeinlab performance testing SHIFT 2 sleep period freetimeinlab performance testing SHIFT 3 sleep period freetimeinlab performance testing SHIFT 4sleep period freetimeinlab performance testing SHIFT 5 sleep period freetimeinlab performance testing SHIFT 6 sleep period freetimeinlab performance testing SHIFT 7 sleep period freetimeinlab performance testing sleep period 20 minutes outside ALCOHOL DAY performance testing freetimeinlab sleep period 8:00 10:00 12:00 14:00 16:00 18:00 20:00 22:00 0:00 2:00 4:00 6:00 Figure 1. Schematic representation of the simulated night shifts and alcohol condition. participants were confined to the living quarters where they were allowed to read, watch TV, study, listen to music or play games. During each shift (starting at 2300 h and ending at 0700 h) participants completed performance testing at hourly intervals. In the intervening periods, participants had free time. They were permitted, but not required, to snack every 2 h following the testing session of that hour. However, they were not permitted to exercise, shower, sleep or leave the living quarters during the testing period. Careful monitoring by the researchers ensured wakefulness during the night. Following the final testing session (at 0700 h), participants were taken outside for 20 min each morning to simulate the natural light exposure a night shiftworker would receive when driving home from a night shift. After this, each participant had breakfast and went to bed at approximately 0800 h. Again, all participants were instructed to sleep until they naturally awoke. If they slept beyond 1900 h, participants were woken by the researcher (this occurred once for two of the participants). Upon awakening, participants were free to shower and leave the laboratory until 1900 h. Throughout the study participants were explicitly and repeatedly told that once out of bed, they had to stay awake until the next scheduled sleep period and were not allowed to take naps. Wrist actigraphs were used to confirm this. 2.2.2. Alcohol intoxication condition: Participants arrived at the sleep laboratory at 1900 h on the night prior to the alcohol condition. They retired at approximately 2300 h and were woken at 0800 h. Beginning at 0900 h, participants completed a performance testing session at hourly intervals. Following the 0900 h testing session, each participant was required to consume an alcoholic beverage consisting of 40% vodka and a non-caffeinated soft drink mixer, at hourly intervals. Twenty minutes

158 N. Lamond et al. after the consumption of each drink, blood alcohol concentrations (BAC) were estimated using a standard calibrated breathalyzer (Lion Alcolmeter S-D2, Wales), accurate to 0.005% BAC. When a BAC of 0.10% was reached, no further alcohol was given. Participants were not informed of their blood alcohol concentration during the experimental period. 2.3. Assessment of performance A 10-min visual Psychomotor Vigilance Task (PVT) was used to evaluate sustained attention. The PVT is reported to have a learning curve of 1 3 trials (Dinges et al. 1997), so participants were required to individually attend a short training session prior to the experimental period to familiarize themselves with the task and to minimize improvements in performance resulting from learning. For this report, two PVT performance metrics were evaluated: (1) increases in response times (RT), and (2) the number of lapses (RTs 5500 milliseconds). As RT data often have a proportionality between the mean and SD (Dinges and Barone Kribbs 1991), a reciprocal transformation was applied to the raw data before analysis. This transformation has the effect of substantially decreasing the contribution of very long lapses, and emphasizing slowing in the optimum and intermediate range of responses (Dinges et al. 1997). During test sessions, participants were seated in front of a blank wall in an isolated room free of distraction, and were instructed to complete the PVT task once. 2.4. Statistical analysis Due to one incomplete set, data from only fourteen of the participants were used in the analysis. To control for inter-individual variability in RT performance, test scores for each participant were expressed relative to the test score obtained at a BAC of 0.00%, as this was the only test period at which participants were neither (1) affected by a long period of prior wakefulness, or (2) intoxicated. Relative scores within each interval (hour of shift or 0.01% BAC intervals) were then averaged to obtain the mean relative performance across participants. Performance data in the alcohol intoxication condition was then collapsed into 0.02% BAC intervals. Evaluation of systematic changes in each of the PVT parameters (1) during the seven night shifts, and (2) with increasing blood alcohol concentrations were assessed separately by repeated-measures analysis of variance (ANOVA). As a repeated measures design was used, the Greenhouse-Geisser procedure (Winer 1971) was applied to produce more conservative degrees of freedom for all ANOVA analyses. In the alcohol condition, missing values were replaced by the group mean. Linear regression analysis was used to determine the line of best fit for the performance effects during the seven night shifts and the alcohol intoxication condition. The relationship between PVT performance and both time of shift and BAC are expressed as a percentage drop in performance for each hour on shift or each percentage increase in BAC, respectively. For each of the seven night shifts, the percentage drop in test performance was also equated with the percentage drop in test performance per 0.01% BAC, and the effects of night work on performance was expressed as a BAC equivalent. 3. Results 3.1. Simulated night shifts and performance Table 1 displays the results of the ANOVAs performed on the two PVT performance parameters for each of the seven simulated night shifts. For the first two nights, both

Adaptation of performance during simulated night work 159 parameters showed statistically significant (range of significant p values = 0.0391 0.0001) variation with time-of-shift. Across the shift, reaction times (RT) slowed and lapse frequency increased. The time-of-shift effect was associated with best performance at the beginning of the shift, and poorest performance at (or near to) the end of the shift. RT performance also significantly varied (range of significant p values = 0.0492 0.0001) during shifts 3, 4, 5 and 6. Again, RTs slowed across the shift, resulting in poorest performance at the end of the shift. In contrast, lapse frequency did not significantly vary across any of the remaining shifts (3 to 7). For each of the seven night shifts, the linear relationship between time-of-shift and PVT performance impairment was analysed by regressing performance against hour of shift. As is evident in table 2, there was a significant (range of significant p values = 0.0062 0.0001) linear correlation between time-of-shift and mean relative RT performance for all seven of the simulated night shifts. Similarly, regression analyses revealed a significant linear correlation (range of significant p values = 0.0260 0.0001) between lapse frequency and time-of-shift for the first six simulated night shifts. Overall, the decline in PVT performance was greatest on the first simulated night shift. As can be seen in table 2, the hourly decline in PVT performance decreased substantially across the shift week from 3.37% (RT) and 1.11% (lapse frequency) on the first night to 0.75% and 0.19%, respectively, by the final shift. 3.2. Alcohol intoxication and performance As can be seen in table 1, both PVT parameters showed statistically significant (range of significant p values = 0.0400 0.0001) variation as a function of blood alcohol concentration. As BAC increased, there was a concomitant increase in RT and lapse frequency, with poorest performance occurring at a BAC of 0.10%. The linear relationship between increasing BAC and performance impairment was analysed by regressing mean relative performance against BAC for each 0.02% Table 1. Summary of ANOVA results for PVT performance Simulated night shifts Alcohol intoxication F 7,91 P a F 5,65 P a PVT RT b 45.11 0.0001 shift 1 19.58 0.0001 shift 2 11.91 0.0001 shift 3 12.56 0.0001 shift 4 2.76 0.0492 shift 5 5.06 0.0161 shift 6 4.58 0.0087 shift 7 1.70 NS PVT Lapse total 4.26 0.0340 shift 1 4.00 0.0391 shift 2 4.79 0.0275 shift 3 2.03 NS shift 4 1.21 NS shift 5 1.29 NS shift 6 2.23 NS shift 7 1.67 NS a Corrected by Greenhouse-Geisser epsilon. b ANOVA conducted on transformed data (1/RT).

160 N. Lamond et al. interval. As is evident in table 3, there was a significant (range of significant p values = 0.0136 0.0034) linear correlation between BAC and mean relative performance for both parameters. For each 0.01% increase in BAC, the decrease in performance relative to baseline was 0.38% (lapse frequency) or 2.27% (RT). 3.3. Blood alcohol equivalents for each simulated night shift The primary aim of the present study was to express the effects of seven consecutive night shifts on performance as a blood alcohol equivalent. Figure 2 illustrates the comparative effects of alcohol intoxication and seven consecutive nights of work on the two PVT performance parameters. Performance decrements equivalent to those caused by a BAC of 0.05% were seen on the first shift before the 5th hour for both PVT RT and lapse frequency. Decrements equivalent to those caused by a BAC of 0.10% were observed at the 7th hour (RT) or just after the 5th hour (lapse frequency). On the second shift, impairment equivalent to that caused by a BAC of 0.05% was observed just after (RT) or just before (lapse frequency) the 6th hour. On the third shift, a comparable degree of impairment was observed just after the 6th hour (RT) or by the 4th hour (lapse frequency). For both of these shifts, impairment equivalent to that caused by a BAC of 0.10% was observed for lapse frequency at the 8th hour. Table 2. Linear regression analysis of PVT performance for the seven shifts F 1,6 P R2 % Decrease (per hour) PVT RT b shift 1 114.29 0.0001 0.95 3.37 shift 2 134.57 0.0001 0.96 2.57 shift 3 378.73 0.0001 0.98 2.47 shift 4 18.42 0.0051 0.75 0.99 shift 5 48.49 0.0004 0.89 1.54 shift 6 40.52 0.0007 0.87 1.16 shift 7 17.04 0.0062 0.74 0.75 PVT Lapses total shift 1 16.44 0.0067 0.74 1.11 shift 2 36.77 0.0009 0.86 0.53 shift 3 178.38 0.0001 0.97 0.54 shift 4 b 43.63 0.0012 0.90 0.20 shift 5 31.87 0.0013 0.84 0.38 shift 6 8.58 0.0263 0.59 0.19 shift 7 1.34 NS a Linear regression conducted on transformed data (1/RT). b Based on data from 2 8th h, DF = 1.5. Table 3. Summary of the linear regression analysis for the alcohol condition F 1,4 P R2 % Decrease (per 0.01% BAC) PVT RT 38.97 0.0034 0.91 2.27 PVT Total lapses 17.72 0.0136 0.82 0.38

Adaptation of performance during simulated night work 161 5 0 Response Latency -5-10 -15-20 0.05% BAC -25 0.10% BAC -30 11 9 Lapse Frequency 7 5 3 0.10% BAC 1 0.05% BAC Shift 1 Shift 2 Shift 3 Shift 4 Shift 5 Shift 6 Shift 7 Blood Alcohol Concentration (%) Figure 2. Mean relative performance levels for PVT response latency and lapse frequency in the simulated shift work (left) and alcohol intoxication condition. The equivalent performance decrement observed at a blood alcohol concentration (BAC) of 0.05% and 0.10% are indicated on the right axis. Values are mean + s.e.m. For the four remaining shifts, performance did not decrease to a level equivalent to the impairment observed at a BAC of 0.05% for either of the PVT variables, except on the fifth shift when impairment equivalent to that caused by a BAC of 0.05% was observed for lapse frequency at the 6th hour. 4. Discussion The results of this study extend the findings of previous work that has equated the effects of fatigue caused by sustained wakefulness and alcohol intoxication. For both PVT parameters, the first simulated night shift was associated with the greatest degree of performance impairment. In general, performance at the end of this shift was greater than that observed at moderate levels of alcohol intoxication. Although to a lesser degree, it was also significantly impaired during the second and third simulated night shifts. However, performance substantially improved over the final four shifts, suggesting that during a week of consecutive night shifts there is some adaptation.

162 N. Lamond et al. While previous research has found that some individuals tend to perform in a manner that is consistent with the expectation that they are intoxicated following alcohol consumption (Breckenridge and Dodd 1991), earlier studies that have used a protocol similar to that of this study indicated that the placebo beverage did not effect mean relative performance (Lamond and Dawson 1999). Moreover, the placebo condition generally did not create the perception of alcohol consumption, particularly when participants had already experienced the alcohol condition and thus the effects of alcohol on their subsequent behaviour and performance. As such, a placebo condition was not included in this study. As expected, increasing blood alcohol concentrations were associated with a significant linear decrease in performance. At a BAC of 0.10% mean relative performance was impaired by approximately 22.7% or 3.8% (PVT RT and lapse frequency, respectively). Overall, the decline in mean relative performance was either 0.38% or 2.27% per 0.01% BAC. These results are consistent with previous findings that suggest that alcohol produces a dose-dependent decrease in performance (Billings et al. 1991, Dawson and Reid 1997, Lamond and Dawson 1999). In line with previous studies of shiftworkers, it is probable that maximum sleepiness occurred on the first night shift due to extended prior waking (Tepas et al. 1981; A kerstedt 1988). Following a mean sleep period of 7.53 (SD = 0.63) h, the mean wake time for participants on the morning prior to the first shift was around 0900 h. As participants were explicitly instructed to remain awake and refrain from napping until the next sleep period, at the beginning of the first night shift they had been awake for approximately 14 h. Moreover, by the end of the first shift the amount of prior wakefulness was extended to over 20 h. Thus, it is not surprising that the greatest amount of performance impairment was observed on the first night. In general, performance on the first night shift decreased linearly, with poorest performance occurring at (or just before) the end of the shift. Similarly, performance decreased in a linear fashion during each of the remaining shifts. However, as the simulated night shift week progressed there was a noticeable reduction in the rate at which performance declined during each of the shifts. Using RT performance as an example, performance declined at a rate of 3.37% on the first night. On the second and third night, the rate at which performance declined dropped to 2.57% and 2.47%, respectively. Performance then improved further, and appeared to plateau for the remainder of the shifts (the mean decline over shifts 4 7 was 1.11%). A similar pattern was observed for lapse frequency. To quantify the performance impairment observed over the shift week, the degree of impairment associated with a BAC of 0.05% and 0.10% was used as a standard. Equating the effects of the two conditions (for each simulated night shift) indicated that by the end of the first night shift the degree of performance impairment was quite high. Indeed, the impairment was equivalent to (RT) or substantially greater than (lapse frequency) that observed at a BAC of 0.10%. This is rather concerning, given that this is twice the legal limit for driving in Australia. Blood alcohol equivalents for the remaining six shifts confirm that the degree of impairment observed across consecutive shifts substantially decreased. For both the second and third shifts, the RTs observed at the end of the shifts were slower than those observed at a BAC of 0.05%, yet not as slow as those observed at a BAC of 0.10%. Similarly, lapse frequency at the end of the second and third shifts was equivalent to but, unlike the first shift, not substantially greater than that observed at

Adaptation of performance during simulated night work 163 a BAC of 0.10%. Meanwhile, the performance decrements observed during the final four shifts were generally less than the impairment observed at a BAC of 0.05%. Overall, these findings suggest that an acute shift in the sleep-wake cycle initially produces performance decrements during the night shift that are greater than are currently acceptable for alcohol intoxication. It is presumed that these decrements reflect a combination of extended prior wakefulness on the first shift and desynchronization of the underlying circadian rhythms of alertness and performance due to the acute shift of the sleep-wake cycle. More importantly, the findings suggests that while night-time performance remains sub-optimal, over a week of consecutive night shifts there is a reasonable degree of adaptation. Indeed, it is clear that performance substantially improved over the final four shifts. By the fourth night (and for the remainder of the shift week) the performance decrements did not exceed those considered unacceptable when equated with alcohol intoxication. That is, the decrements were generally less than those observed at a BAC of 0.05%, the limit over which Australian laws mandate that individuals should be restricted from driving, working and/or operating dangerous equipment. It should however be noted that while performance improved over the series of simulated night shifts, it did not return to levels observed when individuals were (1) unaffected by a long period of prior wakefulness, (2) not performing at a circadian low point, and (3) not intoxicated (i.e. during the first test in the alcohol condition). Given that circadian adaptation is rarely complete (Colquhoun et al. 1968, Folkard et al. 1978, Knauth et al. 1978, Sack et al. 1992), this is not surprising. From a fatigue management perspective, an issue that usually needs to be taken into consideration when designing shifts schedules is the speed of shift rotation. That is, the number of shifts worked consecutively. There is much support for permanent night work to maximize adjustment, rather than rapidly rotating systems which are designed to minimize circadian disruption (reviewed by Wilkinson 1992). However, a primary argument against slow rotation or permanent night work is the evidence suggesting that the circadian rhythms of shiftworkers never completely adjust to shiftwork (Knauth et al. 1978, Dahlgren 1981, Czeisler et al. 1990), thereby forcing shift workers to continue to work during periods when the underlying circadian rhythm of performance is lowest (Folkard 1975, Czeisler et al. 1980, Folkard et al. 1985, Monk et al. 1997). In addition, as a result of insufficient circadian adaptation, there is the potential for cumulative sleep loss, a salient problem among night workers (Aaanonsen 1959, Moore-Ede and Richardson 1985). Consequently, it has been argued that shift schedules that require long sequences of night shifts may be more detrimental than only working one or two nights. However, the data presented in this paper suggests that this is not necessarily the case. Rather, the findings of the current study suggest that in rapidly rotating systems involving two to three consecutive night shifts, the performance impairment experienced at work will continually be greater than is currently acceptable for alcohol intoxication. In contrast, for individuals working a week of night shifts, there may be a significant degree of performance adaptation such that the impairment observed during the latter half of the week is comparatively reduced. Acknowledgements Research supported by a Large Australian Research Council grant.

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