Sleep Medicine 13 (2012) Contents lists available at SciVerse ScienceDirect. Sleep Medicine. journal homepage:

Transcription

1 Sleep Medicine 13 (2012) 72 Contents lists available at SciVerse ScienceDirect Sleep Medicine journal homepage: Original Article Subjective and objective sleepiness among oil rig workers during three different shift schedules Siri Waage a,b,, Anette Harris b,c, Ståle Pallesen d,e, Ingvild B. Saksvik d, Bente E. Moen a,f, Bjørn Bjorvatn a,e a Department of Public Health and Primary Health Care, University of Bergen, Norway b Uni Research, Bergen, Norway c Research Center for Health Promotion, Faculty of Psychology, University of Bergen, Norway d Department of Psychosocial Science, University of Bergen, Norway e Norwegian Competence Center for Sleep Disorders, Haukeland University Hospital, Norway f Department of Occupational Medicine, Haukeland University Hospital, Norway article info abstract Article history: Received 21 December 2010 Received in revised form April 2011 Accepted 8 April 2011 Available online 2 October 2011 Keywords: Shift work Night work Sleepiness Reaction time Karolinska Sleepiness Scale Accumulated Time with Sleepiness Objectives: Examine sleepiness in three different shift work schedules (within-subject design) in the offshore oil industry. Methods: Sleepiness was measured in 19 oil rig workers, using subjective (Karolinska Sleepiness Scale; Accumulated Time with Sleepiness) and objective measures (reaction time). The work schedule consisted of two weeks of 12 h day work (day shifts), two weeks of 12 h night work (night shifts), and two weeks of swing shift work (one week of night work followed by one week of day work). Results: Sleepiness was highest during the first days of night and swing shifts, and also in the middle of the swing shift work period, but gradually decreased as the days on the night shift progressed. While at home following the two-week work period, the workers reported more subjective sleepiness after night shift than after day or swing shifts. Reaction time tests during the work period showed no significant differences between the shift schedules. There was a significant shorter reaction time the last day compared to the beginning or middle of the work period. Conclusions: Subjective sleepiness was higher during the first days of night work compared to day work, and also when the swing shift workers changed from night work to day work in the middle of the twoweek work period. Subjective sleepiness was increased at home following night shifts compared to after day and swing shifts, suggesting that swing shift workers adapted their circadian rhythm during their second period of work, during the day shift week, offshore. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Night work is associated with both sleepiness and reduced performance [1,2]. This is caused by circadian misalignment, where night shift workers both work and sleep at the wrong circadian phase [3,]. The circadian rhythm of most shift workers does not adapt to night work [5], even after 1 2 weeks of consecutive night work [3,]. Much knowledge exists about the impact of shift work in general, but much less research has been conducted within the offshore workplace [7]. The most commonly used work schedule in the Norwegian offshore oil industry consists of two weeks of working followed by four weeks off. The shift schedule used varies between companies, but the most common are fixed shift (day shift Corresponding author at: Department of Public Health and Primary Health Care, University of Bergen, Kalfarveien 31, N-5018 Bergen, Norway. Tel.: ; fax: address: siri.waage@isf.uib.no (S. Waage). and night shift every other work period) and swing shift. Swing shift involves one week of night shifts followed by one week of day shifts during the same work period. On the rotating day, the night shift normally ends at 000 h or 000 h and the day shift starts 8 h later at 1200 h or 100 h. Many offshore workers prefer swing shift because they feel readapted to a normal circadian rhythm when they start their four weeks off work period at home [7]. Previous studies have shown quicker adjustment to night work in offshore workers than what is seen in other work populations, with a gradual decrease in sleepiness during the first week of the work period [8,9]. However, it has been found that offshore shift workers had more problems readapting to a normal rhythm at home after night shift work [8]. Few studies have specifically examined swing shifts. Swing shift workers have reported good adaptation to night shifts, but large individual variations have been found in readaptation to the subsequent day shifts [10]. Swing shifts have also been reported to be associated with greater problems of alertness and performance as compared to night work [11] /$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.101/j.sleep

2 S. Waage et al. / Sleep Medicine 13 (2012) 72 5 The environment on offshore oil rigs offers good opportunities to study the effects of shift work. The work period involves long work hours, mostly indoors with little exposure to sunlight and with no domestic commitments. Studies from this environment might thus be regarded as laboratory-like in terms of sleep conditions and domestic interference, and are among the few to show that sleepiness related to shift work improves after consecutive night shifts [8,9]. The aim of the present study was to examine sleepiness in relation to different shift work schedules measured using both subjective and objective methods. Sleepiness has been defined as a drive toward sleep [12]. Sleepiness is most commonly measured subjectively, although the phenomenon clearly includes pronounced behavioral and psychological elements [13]. Standardized questionnaires have been designed to assess sleepiness, e.g., the Karolinska Sleepiness Scale. In addition, reaction time tests are considered an objective measure of performance, and have been shown to be sensitive to circadian changes of sleepiness in healthy subjects [1]. The majority of shift workers are reported to experience sleepiness in connection with night shift work, while day shift work is associated with only minor or no sleepiness [13]. Ageing is associated with a decreased ability for circadian adjustment to night work and with an increased tendency for sleep disturbances [15,1]. Inter-individual differences in circadian amplitude, circadian phase, and mean performance level are reported in the literature [17]. Individual differences in circadian rhythms are documented both under stable circadian conditions and in response to disturbances such as in shift work [18]. The present study is unique in being the first to investigate sleepiness in the same shift workers using a within-subjects design during three different shift schedules (day shifts, night shifts, and swing shifts). The repeated-measure design allows investigation of changes over time within the same individuals. Such temporal changes are of interest, and may enable statistically powerful comparisons of different conditions within the same individuals [19]. Our study specifically investigated the following hypotheses: (1) Night work, i.e., night shifts and swing shifts, is associated with increased levels of sleepiness as compared to levels experienced in working day shifts. (2) Swing shifts are associated with increased levels of sleepiness after the rotation from night work to day work in the middle of the work period. (3) Workers having completed two consecutive weeks of night shifts report more sleepiness when returning home as compared to workers who have completed two weeks of day or swing shifts. 2. Methods 2.1. Subjects and design All 32 workers in the processing area of one Norwegian North Sea oil rig were asked to take part in the present study. The work schedule normally used on this oil rig consisted of alternating two-week work periods of either day or night shift work. At the employees request for a swing shift schedule, the company agreed to implement this and to evaluate the effects of swing shift. It is important to evaluate sleepiness in relation to shift work on oil rigs, as alertness is of great importance to avoid accidents. In all, 28 workers agreed to participate in the study (87.5%). Nineteen subjects (13 men and women) were assessed during all three shift schedules. Out of the nine subjects who did not complete the study, two withdrew from the study, two were transferred to onshore work in the company, and five worked only day shifts. Some workers had incomplete data for one or more than one week and were, due to analysis of repeated measures, therefore excluded from the analyses. Consequently, the number of participants in the different analyses varies from 8 to 15. There was no consistent pattern concerning the missing data between the subjects, except for one worker who did not register data during night shift work. The work schedule studied consisted of two weeks of work offshore followed by four weeks off from work. The day shift started at 0700 h and ended at 1900 h and the night shift started at 1900 h and ended at 0700 h. The swing shift schedule consisted of one week of night shifts followed by one week of day shifts. On swing day, the workers ended their night shift at 000 h or 000 h and started the day shift at 1200 h or 100 h. The initial measurements for the study took place during fall 2007 when participants were working fixed shifts (alternating periods of day shifts and night shifts). Workers were measured again during the fall of 2008, when they had been working swing shifts for a minimum of 9 months, to ensure they had experience with the new schedule and also to ensure similar seasonal conditions. The workers were not given any information about factors that could help them to more easily phase shift their biological rhythms by administering, for instance, bright light or melatonin. Each measurement followed the same procedure. Participants were monitored for four weeks, including the week prior to the work period, the two-week work period offshore, and the first week at home after the work period. The workers kept sleep and wake diaries, supplied by the researchers, during this four-week period. They also received a reminder by phone to start completing the diaries. In addition, nursing staff at the oil rig handed out and collected supplementary diary pages to complete during the work period. Diaries kept at home were sent to the researchers by mail. Furthermore, workers performed six reaction time tests during the offshore work period. Offshore nursing staff organized these and recorded data on a computer belonging to the project group. The present study is a part of a larger project. Data concerning other variables (cortisol and sleep) are presented elsewhere [20] or in press [21]. 3. Instruments 3.1. Objective measures Objective ratings of sleepiness were obtained using a 10 min simple serial reaction time test on a Palm handheld computer (Palm Inc, Santa Clara, CA, USA) Ò. The test consists of black squares appearing on the screen at randomly distributed intervals. Participants were instructed to respond to the stimulus by pressing a key as soon as possible. The software registers reaction time in milliseconds (ms). If no response was given within 1750 ms, a new interval was started. If the participant pressed the key in advance or within 120 ms after the stimulus presentation the response was discarded and a warning signal was displayed. Mean and median reaction time as well as the number of lapses (>500 ms) were calculated for each test. The test has been validated as a measure of sleepiness and performance [22]. During the work period, workers performed the reaction time test six times: twice at day/night 1, twice at day/night 7 (day 8 when they were on swing shifts, after the rotation), and twice at day/night 13. The tests were performed at the start and end of these shifts (within the first or last 2 h, respectively). While performing the test, workers were asked to sit down in an undisturbed area, usually in the rest area or the nurse s office, away from potential distractors such as TV, radio, etc Subjective measures The Karolinska Sleepiness Scale (KSS) [23] and the Accumulated Time with Sleepiness (ATS) [2] were used as measures of subjective sleepiness (wake diaries). The KSS consists of a nine-point scale with sleepiness rated 1 = very alert, 3 = alert, 5 = neither alert or sleepy, 7 = sleepy, but no problems staying awake, and 9 = very sleepy, fighting sleep, effort to stay awake. The workers rated

3 S. Waage et al. / Sleep Medicine 13 (2012) 72 sleepiness every other hour while awake during the work period at the oil rig. The ATS scale is designed as a method for integrating subjective sleepiness over longer time periods. The workers were asked if certain symptoms of sleepiness had occurred during the period they were awake, and, if so, for how long, measured by the proportion of the wake period when the symptom was present, ranging from 0% to 100%. In the ATS, workers were asked: Did you experience any of the following symptoms: heavy eyelids, feeling gravel eyed, irresistible sleepiness, or reduced performance ATS ratings were registered every day before bedtime for four weeks during periods of day, night, and swing shifts Ethics The study was approved by the Regional Committee for Medical and Health Research Ethics of Western Norway (REK-West) and the Norwegian Social Data Service (NSD). Written, informed consent was obtained from all subjects. Participants were informed that no information which could potentially be used to identify individuals would be given to the oil company, and that the oil company would only get access to anonymous results as presented in this publication, in addition to in an internal report. 3.. Statistical analysis Statistical testing was based on 2-way and 3-way analyses of variance (ANOVA) with separate post hoc tests (LSD) for parameters that were found to be significant in the overall analyses. In the present study it was assumed that sleepiness would increase with night work as compared to with day work as well as after the changeover from night work to day work during swing shift, and, finally, that an increase in sleepiness would be seen at home after night work. These assumptions comprised directional hypotheses, permitting the use of one-tailed tests. A 3-way analysis of variance was conducted for the reaction time test, a 3-way analysis of variance was conducted. The three factors were shift schedule (three levels: day, night, and swing shifts), days (three levels: beginning, middle, and end of the work period) and test time (two levels: start and end of shift). Also, a 2-way analysis of variance was conducted to compare reaction times within the 12-h work shift. The two factors used were shift schedule (three levels; day, night and swing shifts) and test time (two levels; start and end of shift). For KSS data, a 2-way analysis of variance was conducted. The two factors used were shift schedule (three levels: day, night, and swing shifts) and days (13 levels: days). A mean KSS score for each day was calculated. We also conducted a 3-way analysis of variance to explore changes during the work day. For practical reasons, we chose to explore the days used for the reaction time test. The three factors used were shift schedule (three levels: day, night, and swing shifts), days (three levels: beginning, middle, and end of the work period) and time points (six levels: 8 h/20 h, 10 h/22 h, 12 h/2 h, 1 h/02 h, 1 h/ 0 h, 18 h/0 h). For ATS data, a 2-way analysis of variance was conducted. The two factors used in the ANOVAs were shift schedule (three levels: day, night, and swing shifts) and days (13 levels: days). When comparing the work periods, we eliminated arrival/departure days due to missing data. In addition, a 2-way analysis of variance was conducted in two sets of separate analyses to investigate adaptation to shift work and readaptation to a normal sleep pattern after shift work. The first analysis looked at adaptation to shift work by comparing the last seven days at home to the first seven days in the work period. The second analysis comprised readaptation after shift work and compared the last seven days of the work period to the first seven days at home. The two factors used in the ANO- VAs were shift schedule (three levels: day, night, and swing shifts) and days (1 levels). p-values were corrected for lack of compound symmetry using the epsilon correction according to The Huynh Feldt procedure. Alpha levels were set to.05 (one tailed test). In order to include as many subjects as possible in the analyses, missing data were replaced with careful estimates in line with previous studies [9]. For the KSS, missing time points were replaced with an average based on its pre- and subsequent time points. If the first time point was missing, the second time point was inserted. Similarly, when the last time point of the day or night was missing, the penultimate time point was inserted. When three or more time points in a row were found to be missing, no missing data were replaced. When missing all time points of a particular day or night, an average of the mean of the day or night before and after was inserted. Due to several missing data points on the days of arrival and departure at the oil rig, these days were excluded from the analyses. On the day of rotation, a mean score of the night and the day was calculated and used in the analyses. Missing data for the ATS were replaced in a similar fashion to that used for the KSS. If three or more consecutive days or nights were missing, no data were inserted and the case was excluded. The total percentage of missing data corrections was between 0.% (KSS) and 3.2% (ATS). All data were analyzed using PASW statistics version 17 for Windows (SPSS, Inc., Chicago, IL).. Results Workers mean age at the initial assessment was years (range 28 0 years). Five of the workers were below 0 years of age, nine of the workers were aged between 0 and 50 years, and five of the workers were between 50 and 0 years. Participants had worked shifts for an average of 18 years (range 35 years). Five of the workers had worked shifts for 10 years, seven of the workers years, and seven of the workers had been working shifts for more than 21 years. The mean duration of their offshore work experience was 12 years (range 1 25 years). During the work period, the workers reported their total intake of coffee, tea, and coca cola in cups per day. During day shift the mean intake was 3.2 cups (range 0 9 cups per day), during night work the mean intake was 3.8 cups (range 0 8 cups per day), and during swing shift the mean intake was 3. cups (range 0 8 cups per day). The workers mean bed time and wake-up time are presented in Table 1. However, we have no information about second jobs, travels, or daily doings during their free period or overtime during their work period that would affect their bed- and wakeup-times..1. Objective measures.1.1. Reaction time There was no significant difference in objective sleepiness as measured by the mean reaction test time between the three shift schedules (F 2,18 = 0.1, p =.27). There was a significant difference between days (F 2,18 = 9.05, p <.01), with the post hoc test showing a significantly faster reaction time on the last day of the work period than in the beginning (p =.01) or middle (p =.02) days. Fig. 1 shows reaction time patterns across the work period for all three shift schedules. As shown in the figure, there were small, nonsignificant differences in reaction time between the first (beginning of the work shift) and second (end of the work shift) tests, as well as across work days during day shifts. During night shifts there was a gradual improvement in reaction time over the work period. There was a slight increase in reaction time across the work period during swing shifts, most likely caused by the change from night shifts to day shifts (Fig. 1). Measurements for the first 12-h shift

4 S. Waage et al. / Sleep Medicine 13 (2012) 72 7 Table 1 Mean bedtimes and wake-up times in the oil rig workers during the day shift ( h), night shift ( h), and swing shift (one week of h/one week of h). Data are provided from the week before offshore work, the two week work period, and the first week at home after offshore work. Home week before offshore work Offshore week 1 Offshore week 2 Home week after offshore work Bed time Wake-up time Bed time Wake-up time Bed time Wake-up time Bed time Wake-up time 00:12 08:09 22:5 0:05 22:50 0:0 00:27 08:1 00:2 08:21 08:00 15:28 08:20 15:5 00:13 08:09 Swing shift 23:59 08:33 07:7 15:19 22:50 05:7 00:03 08:30 30 Mean reaction time in milliseconds Day/Night 1 Start - End Day/Night 7 Start - End Day/Night 13 Start - End Fig. 1. Mean reaction time in milliseconds, tested at the start and at the end of the shift at the beginning of the work period, middle of the work period, and end of the work period (n = 10). found that mean reaction time was significantly slower at the end than at the beginning of the shift (F 1,1 = 9.83, p <.01). The results of the median reaction time were similar to the results of the mean reaction time (data not shown). There were no significant differences in the number of lapses, neither between the three shift schedules, nor between days or tests (data not shown)..2. Subjective measures.2.1. KSS There were no significant overall differences in subjective sleepiness as measured by the KSS between the three shift schedules (Table 2). There was, however, a significant difference in sleepiness between days (Table 2). The post hoc test showed a significantly higher degree of sleepiness for the first days of each work period compared to most other days (p-values between.001 and.03), and also an increase in sleepiness in the middle of a work period (p-values between.002 and.03) compared to most other days. There was a significant interaction effect of shift days (p =.03), with sleepiness being fairly stable during day shifts, but with increased levels during the first days of night and swing shifts as well as in the middle of the swing shift work period. Fig. 2 shows the mean values of the KSS on all the three shift schedules. There was higher sleepiness during night work than during day work at the beginning of the work period, but the sleepiness scores on the night shifts gradually decreased after a few days. As shown in Fig. 2, sleepiness while on the swing shift increased on the day after the rotation and the following day compared to the previous week. Fig. 3 shows the KSS ratings at different time points for three different work days; day 1 (at the beginning of the work period), day 7 (day 8 for swing shift, themidpointoftheworkperiod),andday13(theendpointofthe work period). We found a significant difference between the six time points (F 5,5 = 10.51, p <.001). The post hoc tests showed that workers reported significantly more sleepiness toward the end of a work day than at its beginning. In addition, there were significant interactions between shift type and time points (F 10,90 = 7.79, p <.001), indicating an increase in sleepiness during night and swing shifts, but not during day shifts. Also, the interaction between days and time points was significant (F 10,90 =.23, p <.001), indicating that sleepiness increased more during the first shift of each work period than it did at its midpoint or end (Fig. 3). Sleepiness scores for day 13 were similar in all three shift schedules (Fig. 3) ATS Table 2 presents results from the ANOVAs on sleepiness measured by the ATS during each work period. There was a significant difference between work schedules in heavy eyelids (Table 2). The post hoc test showed that a feeling of heavy eyelids was more common during swing shifts than during day shifts (p =.01). There were significant changes in heavy eyelids, irresistible sleepiness, and reduced performance across the work days. Furthermore, there were significant interaction effects of shift days on the variables of heavy eyelids, feeling gravel eyed, and irresistible sleepiness. Sleepiness was fairly stable during day shift periods, whereas there were increased levels of sleepiness during the first days of night and swing shifts compared to most other days, as well as in the middle of the work period during swing shifts (Table 2 and Fig. 2).

5 8 S. Waage et al. / Sleep Medicine 13 (2012) 72 Table 2 Subjective sleepiness as measured by the Karolinska Sleepiness Scale (KSS) and Accumulated Time with Sleepiness (ATS), across the two work weeks, adaptation to shift work and readaptation to normal day rhythm after shift work, results from 2-way analysis of variance, F-values with corresponding degrees of freedom. Sleepiness across two work weeks Type of shift Days Interaction shift X days F(df) F(df) F(df) KSS mean 0.1(2,28) 3.75(12,18) ** 1.18(2,33) * ATS Heavy eyelids 2.95(2,28) * 3.77(12,18) ** 2.30(2,33) * Feeling gravel eyed 1.57(2,28) 1.5(12,18) 1.95(2,33) * Irresistible sleepiness 0.3(2,2) 2.3(12,15) * 2.01(2,312) * Reduced performance 0.13(2,28) 2.37(12,18) * 1.15(2,33) Adaptation to shift work ATS Heavy eyelids 1.02(2,2) 2.00(13,15) 1.5(2,312) Feeling gravel eyed.5(2,2) * 2.99(13,15) * 1.7(2,312) Irresistible sleepiness 0.8(2,2) 1.27(13,15) 1.20(2,312) Reduced performance 0.53(2,2) 1.9(13,15) 0.90(2,312) Readaptation after shift work ATS Heavy eyelids 8.1(2,18) ** 5.3(13,117) **.53(2,23) ** Feeling gravel eyed 1.07(2,1) 2.03(13,10) 1.3(2,208) Irresistible sleepiness 1.27(2,1) 3.9(13,91) * 2.78(2,182) * Reduced performance 3.0(2,18) 7.07(13,117) **.8(2,23) ** * p <.05. ** p <.01. In addition to measuring sleepiness during the work period, ATS ratings were also recorded during the weeks spent at home immediately before and after each work period. This provided us with the opportunity to examine adaptation to shift work and readaptation to a normal day rhythm after a shift work period in terms of subjective sleepiness. Table 1 summarizes the results from the 2- way ANOVAs. In adaptation to shift work, there was a significant difference in sleepiness as indicated by feeling gravel eyed between the three shift schedules (Fig. ). Post-hoc tests showed that feeling gravel eyed was significantly more common in the adaptation period to swing shifts (p =.01) compared to in day shifts. In readaptation to normal day rhythm after shift work, there was a significant difference in terms of heavy eyelids between the three shift schedules (Table 2 and Fig. ). Post-hoc tests showed that heavy eyelids were significantly more common after night shifts compared to following either day (p <.01) or swing shifts (p =.01) (Fig. ). There was also a significant difference in heavy eyelids between days as well as an interaction between shift and days (p <.01) (Table 2 and Fig. ). Post-hoc tests showed that heavy eyelids increased in particular during the first days at home after a night shift period. Similarly, there were significant differences in irresistible sleepiness and reduced performance between days, as well as significant interactions between shift and days for both reduced performance (p <.001) and irresistible sleepiness (p =.0) in readaptation after shift work, with patterns similar to heavy eyelids (Table 2). 5. Discussion There were few overall differences in subjective and objective measures of sleepiness between the three different shift schedules in this study. Only sleepiness as measured by heavy eyelids showed significant overall differences between day, night, and swing shifts. However, there were significant interactions for sleepiness between shift days, as measured both by the KSS and several of the ATS scores. Increased levels of sleepiness were found during the first days of both night and swing shift periods, with levels of sleepiness also rising in the middle of the swing shift period. Significant interactions may be of greater interest than the overall main effects. Statistically, a significant interaction between two independent variables implies that one variable modifies the effect of the other on the dependent variable. For instance, in the present context, the significant shift days interaction implies that differences in sleepiness values across days depended on the shift schedule, as expected. The significant interactions for subjective ratings support our first hypothesis, namely that night work is associated with increased levels of sleepiness as compared to day work. Our results also show that the adaptation to night work happens quickly according to subjective measures, where KSS and ATS values for night and swing shifts were similar to the values of day shifts within a few days. This supports earlier findings by our group [8], and is also in line with findings from other isolated settings like Antarctica and British sector offshore oil installations [5,25]. Such adaptation may be explained by lack of conflicting bright light exposure and absence of domestic obligations, as suggested previously [9,25]. Adaptation to night work in the offshore oil industry is shown to take place fairly quickly [8,9], but changing to day work after adaptation to night work is shown to negatively affect sleep [9]. Our second hypothesis was that swing shift work would be associated with increased levels of sleepiness after rotating from night to day work in the middle of the work period. Sleepiness is known to be correlated with sleep duration, i.e., the shorter the sleep duration, the greater the sleepiness [2]. As shown, there were no significant differences in reaction time between the three shifts, but there were significant interactions seen in shift days for the subjective sleepiness ratings (both KSS and ATS). Post-hoc tests showed increased levels of sleepiness in the middle of the swing shift work period, when changing from night to day shift, as we predicted. This gives support to our second hypothesis. In line with this, the sleep data from the same population also showed that total sleep time as measured by sleep diaries was longer during night and day shift than during swing shift in the second week of work, i.e., after the rotation [21]. Our finding is also in accordance with results found in another offshore population where the return to day work after one week of night work led to a marked increase in subjective sleepiness [9]. In addition, cortisol analyses for the same workers showed a similar pattern, giving further support to the hypothesis. When workers were on swing shifts, the cortisol rhythm the first day after rotation was completely flat, indicating a disturbed circadian rhythm [20]. The majority of the workers in the present study preferred swing shifts to fixed day and night shift work [20]. This is in accordance with a previous study [11]. A swing shift schedule gives the

6 S. Waage et al. / Sleep Medicine 13 (2012) KSS value Day in work period 20 Mean ATS score "Heavy eyelids" Day in work period Fig. 2. Mean KSS values (n = 15) and mean scores of heavy eyelids (n = 13) across work days during day shift, night shift, and swing shift. workers the opportunity to return home with a normal day rhythm after the two week work period. When returning home readjusted to a normal rhythm the workers are able to take greater part in daily life activities and social demands from the first day of their free period compared to offshore workers returning home exhausted and not having adjusted their circadian rhythm to a normal day rhythm [7]. It is also likely to assume that it is easier and takes shorter time to readapt to a normal day night rhythm after only one week of night work compared to after 1 consecutive days of night work. We assume that the workers value the advantage of returning home readjusted to a day rhythm more than the disadvantages of sleepiness during the work period. During the study, the workers were not given any recommendations for facilitating the phase shift when changing from night to day rhythm. The mean bedtimes and wakeup-times during the swing shift also show that the workers did not try to gradually delay their rhythm by drifting their sleep time later from day to day. Our last hypothesis stated that sleepiness would be increased at home after night shifts as compared to after day or swing shifts. In order to examine readaptation to a normal day night rhythm after shift work in terms of sleepiness, we included ATS ratings at home during the week following each two-week work period. Results clearly showed that subjective sleepiness was increased during the first days at home after night shifts as compared to after day or swing shifts. Moreover, levels of sleepiness measured after two weeks of night work were nearly twice the levels when starting night shift work. This supports our third hypothesis that sleepiness is increased at home after 1 consecutive night shifts compared to day or swing shifts. Previous studies have shown that adaptation to night work is easier than readaptation back to a normal day rhythm [8,9]. One reason for this is that it is easier to phase delay than to phase advance the circadian rhythm. The sleepiness results

7 70 S. Waage et al. / Sleep Medicine 13 (2012) 72 Mean KSS score Mean KSS score Mean KSS score /20 10/22 12/2 1/02 1/0 18/0 Time points Day/Night 1 08/20 10/22 12/2 1/02 1/0 18/0 Time points Day/Night 7 08/20 10/22 12/2 1/02 1/0 18/0 Time points Day/Night 13 Fig. 3. Mean KSS values between the different time points (every other hour) during work, for three shift schedules (n = 10). are also supported by the cortisol analyses. The workers had a low cortisol response after awakening as well as high evening values, indicating that they had not yet recovered one week after a twoweek period of working night shifts [20]. These results, however, were not corroborated by the sleep data, as there were no significant differences between the three shift schedules concerning readaptation in terms of sleep [21]. Sleepiness at work is one of the most common reported complaints related to shift work, and is associated with an increased risk of errors and injuries [1]. Caffeine has been documented to have profound effect on sleep and wake function. Studies have suggested that caffeine counteracts alertness and performance impairments instigated by sleepiness [27]. However, the workers in our study reported very similar amounts of caffeine intake during all three shifts and with small variations from day to day during the work period, suggesting that the use of caffeine does not affect the results of the sleepiness measures. Several strengths and limitations of this study should be noted. The workers in the present study had been working in shifts for many years (mean 17 years), and they seem to reflect a selected healthy population coping fairly well with shift work. The offshore oil industry has strict regulations according to health and the use of medication. All oil rig workers are screened for health problems every other year and all use of drugs is regularly controlled by health personnel on the oil rig. The present sample is in accordance with another offshore shift work sample in terms of demographics and health [28], and probably reflects the healthy worker effect, leaving individuals who prefer and cope well with shift work in the shift work population over time [29]. In the offshore industry it is claimed that there is a negative relationship between age and health, consequently that health problems arise with the combination of high age and long offshore exposure [30]. However, another study by our group found that age was not associated with sleep, sleepiness, or health indicators among oil rig workers [31]. This gives support for a present healthy worker effect. Our results may be influenced by the low number of participants, limiting the study s statistical power. We found no significant differences between the three shift schedules in terms of reaction time as an objective measure of sleepiness. In studies with small group sizes (e.g., n = 20), there is a possibility that non-significant results may be due to insufficient power. However, both subjective measures used in the study showed an increase in sleepiness in relation to night work, giving support to the hypotheses. On the other hand, because of the clear directional assumptions of the hypotheses, one-tailed statistical tests were used, granting the corresponding advantage of increased statistical power [32]. The repeated measure design is a method used to investigate changes over time [19], but the statistical method is vulnerable to missing data points. In our study the number of valid observations in the analyses varied between eight and 15, whereas the total number of subjects was 19. The study is also a controlled field study and should therefore be regarded as a supplement to findings from laboratory studies where a low N is common. Our results are based on a small sample of workers in a controlled and confined shift work environment and may therefore not be generalized to other working conditions or populations. Still, results from studies in well-controlled environments such as the offshore industry can give indications for other shift work populations, by suggesting that social and domestic factors are highly influencing the shift workers. Also, shift work is an integral part of many offshore jobs and concerns a great many workers. The offshore oil industry represents a unique environment that needs to be addressed specifically rather than managed as direct extensions of routine onshore shift work [7]. Future studies within the offshore oil industry are needed to make firm recommendations for work schedules and safety. Our analyses were based on a within-subject design where the same workers were tested under different work schedules. This is a major strength and makes the study unique. In addition, the use of both subjective and objective measurements strengthens the design of the study. However, in future research, other objective measures of sleepiness such as the Multiple Sleep Latency Test (MSLT) or measures of accidents should be considered. Also in future studies, samples of endogenous melatonin should be considered as a measure of circadian rhythms.

8 S. Waage et al. / Sleep Medicine 13 (2012) Mean score in "feeling gravel eyed" % of the day At home First week offshore Adaptation to shift work, last week at home and first week at work 30 Mean score in of "heavy eyelids" % of the day Last week offshore Athome Readaptation after shift work, last week at work and first week at home Fig.. Adaptation to shift work illustrated by feeling gravel eyed (n = 13) and readaptation to day rhythm after shift work illustrated by heavy eyelids (n = 10). In conclusion, increased levels of subjective sleepiness were seen during the first days of night and swing shift work, and also in the middle of the swing shift work period. After returning home, the workers reported markedly more subjective sleepiness after night shift than after working day or swing shifts. No differences were found regarding reaction time between the different shift work schedules. Conflicts of Interest The ICMJE Uniform Disclosure Form for Potential Conflicts of Interest associated with this article can be viewed by clicking on the following link: /j.sleep Acknowledgments The study was supported by funding from the oil company Statoil. We would like to thank all the participants at the oil rig for their contributions, as well as the medical staff on location for their help and support. References [1] Akerstedt T. Sleepiness as a consequence of shift work. Sleep 1988;11:17 3. [2] Akerstedt T. Shift work and disturbed sleep/wakefulness. Occup Med (Lond) 2003;53:89 9.

9 72 S. Waage et al. / Sleep Medicine 13 (2012) 72 [3] Burgess HJ, Sharkey KM, Eastman CI. Bright light, dark and melatonin can promote circadian adaptation in night shift workers. Sleep Med Rev 2002;: [] Smith MR, Fogg LF, Eastman CI. Practical interventions to promote circadian adaptation to permanent night shift work: study. J Biol Rhythms 2009;2: [5] Barnes RG, Deacon SJ, Forbes MJ, Arendt J. Adaptation of the - sulphatoxymelatonin rhythm in shiftworkers on offshore oil installations during a 2-week 12-h night shift. Neurosci Lett 1998;21:9 12. [] Czeisler CA, Johnson MP, Duffy JF, Brown EN, Ronda JM, Kronauer RE. Exposure to bright light and darkness to treat physiologic maladaptation to night work. N Engl J Med 1990;322: [7] Ross JK. Offshore industry shift work health and social considerations. Occup Med (Lond) 2009;59: [8] Bjorvatn B, Kecklund G, Akerstedt T. Rapid adaptation to night work at an oil platform, but slow readaptation after returning home. J Occup Environ Med 1998;0:01 8. [9] Bjorvatn B, Stangenes K, Oyane N, Forberg K, Lowden A, Holsten F, et al. Subjective and objective measures of adaptation and readaptation to night work on an oil rig in the North Sea. Sleep 200;29: [10] Gibbs M, Hampton S, Morgan L, Arendt J. Adaptation of the circadian rhythm of -sulphatoxymelatonin to a shift schedule of seven nights followed by seven days in offshore oil installation workers. Neurosci Lett 2002;325:91. [11] Parkes KR, Clark MJ, Payne-Cook E. Psychosocial aspects of work and health in the North Sea oil and gas industry-part III. Sleep, mood and performance in relation to offshore shift rotation schedules. Health and Safety Executive Offshore Technology Report; [12] Dement WC, Carskadon MA. Current perspectives on daytime sleepiness the issues. Sleep 1982;5:5. [13] Akerstedt T. Work hours, sleepiness and the underlying mechanisms. J Sleep Res 1995;: [1] Mathis J, Hess CW. Sleepiness and vigilance tests. Swiss Med Wkly 2009;139:21 9. [15] Akerstedt T, Torsvall L. Shift work shift-dependent well-being and individual-differences. Ergonomics 1981;2: [1] Harma MI, Hakola T, Akerstedt T, Laitinen JT. Age and adjustment to night work. Occup Environ Med 199;51: [17] Van Dongen HP, Dinges DF. Circadian rhythms in sleepiness, alertness, and performance. In: Kryger MH, Roth T, Dement WC, editors. Principles and practice of sleep medicine. Elsevier Saunders; p [18] Kerkhof GA. Inter-individual differences in the human circadian system: a review. Biol Psychol 1985;20: [19] Van Dongen HP, Olofsen E, Dinges DF, Maislin G. Mixed-model regression analysis and dealing with interindividual differences. Methods Enzymol 200;38: [20] Harris A, Waage S, Ursin H, Hansen AM, Bjorvatn B, Eriksen HR. Cortisol, reaction time test and health among offshore shift workers. Psychoneuroendocrinology 2010;35: [21] Saksvik IB, Bjorvatn B, Harvey AG, Waage S, Harris A, Pallesen S. Adaptation and readaptation to different shift work schedules measured with sleep diary and actigraphy. J Occup Health Psychol 2011;1:331. [22] Lamond N, Dawson D, Roach GD. Fatigue assessment in the field: validation of a hand-held electronic psychomotor vigilance task. Aviat Space Environ Med 2005;7:8 9. [23] Akerstedt T. Psychological and psychophysiological effects of shift work. Scand J Work Environ Health 1990;1:7 73. [2] Gillberg M, Kecklund G, Akerstedt T. Relations between performance and subjective ratings of sleepiness during a night awake. Sleep 199;17: [25] Thorne H, Hampton S, Morgan L, Skene DJ, Arendt J. Differences in sleep, light, and circadian phase in offshore 18:00 0:00 h and 19:00 07:00 h shift workers. Chronobiol Int 2008;25: [2] Van Dongen HP, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep 2003;2: [27] Roehrs T, Roth T. Caffeine: sleep and daytime sleepiness. Sleep Med Rev 2008;12: [28] Waage S, Moen BE, Pallesen S, Eriksen HR, Ursin H, Akerstedt T, et al. Shift work disorder among oil rig workers in the North Sea. Sleep 2009;32: [29] Costa G. Shift work and occupational medicine: an overview. Occup Med (Lond) 2003;53:83 8. [30] Ringstad AJ, Bakke Å, Arvesen B, Skaftun JE. Aldring og helse kartleggingsstudie. Report in Norwegian; [31] Waage S, Pallesen S, Moen BE, Bjorvatn B. Shift work and age in petroleum offshore industry. Int Maritime Health 2010;2: [32] Kecklund G, Akerstedt T. Sleep in a truck berth. Sleep 1997;20:1 9.