Work hours, sleep patterns and fatigue among merchant marine personnel

Transcription

1 J. Slwp Rex (I 997) 6, I Work hours, sleep patterns and fatigue among merchant marine personnel THOMAS F. SANQUIST', MIREILLE RABY', ALICE FORSYTHE' and ANTONIO B. CARVALHAIS' ' Battelle Seattle Research Centrc, Seattle, WA and ' United States Coast Guard Research and Development Centre Groton, CT, USA Accepted in revised form 13 October 1997; received 25 March 1997 SUMMARY A field study ofwork and sleep patterns among commercial merchant marine personnel is reported. Data collected over a d period from 141 subjects aboard eight ships included information concerning work-rest schedules, sleep timing, alertness on the job and critical fatigue. The data indicate that watchstanders on the 4-on, 8-off schedule show considerable disruption in their sleep. The average sleep duration for all mariners is 6.6 h; watchstanders obtain their sleep in fragmented periods that are frequently less than 5 h in duration. Analysis of critical fatigue shows an incidence of /0 across personnel and measures. Of particular concern are the watchstanders on the schedule, who sleep less than 4 h per 24-h period 22% of the time. Potential countermeasures, including changes in scheduling and staffing are proposed. KEYWORDS alertness, fatigue, sleep disruption, watchstanding, work-rest schedules INTRODUCTION Accidents related to work schedule and fatigue are a major detriment to transportation safety. A number of studies across various modes of transportation show that fatigue underlies a significant percentage of accidents (Lauber and Kayten 1988); further, many of the accidents appear to be a result of sleep disruption based on a requirement to work throughout a 24- h period. The role of fatigue in shipping accidents is wellillustrated by the grounding of the Exxon Vuldez in 1989 (National Transportation Safety Board 1990). Prior to the grounding, which occurred shortly after midnight, the watch mate had slept as little as 5 or 6 h, split between afternoon and early morning periods. The resulting fatigue contributed to poor navigation performance and the consequent grounding. The standard work schedule of watchstanders on merchant marine vessels involves the 4 h on, 8 h off. 4 h on watch schedule. This is usually a fixed schedule of work in American ships, with the first officer standing the and watch, the second officer standing the midnight to and watch, and the third officer standing the and watch. Additional and overtime duties are performed during the off-watch hours. A considerable C;,,.rc,.s~~(,orit/~/i[,[,: T. F. Sanquist. Battelle Seattle Research Centre NE 41 Street. Seattle. WA. USA. Tel.: ; fax: t ; c-mail: ~anquist(n:battelle.org body of sleep research indicates that fragmented work-rest scheduling of this type may be a predominant contributor to fatigue, duration of work during the 24-h period notwithstanding (Gillberg 1995). Studies of traditional shiftworkers indicate that the timing of the shift influences the amount of sleep obtained (Akerstedt et a/. 1982; Tepas and Carvalhais 1990). Sleep duration and quality in at-sea operations have been evaluated in several European studies, including Rutenfranz et al. (1988) and Fletcher et al. (1988). They found that watchkeepers had a lower average sleep duration than dayworkers, and that third officers ( watch) reported the lowest quality and amount of sleep. Compensation for sleep loss is not possible because of a lack of days off. Because shipping is a 24-h per day operation with unpredictable port arrival times, establishing fixed work-rest times is not feasible. Instead, it may be possible to develop guidelines for work-rest scheduling based on detailed analysis of mariner work-rest patterns. Existing regulatory approaches to maritime work-rest scheduling do not address the need for a continuous rest period. The United States (US) Oil Pollution Act of 1990, and the work-rest guidelines of the International Maritime Organization simply limit the total number of hours that mariners may work to a maximum of I5 h per day. In practice, operators continue to utilize the standard 4-on, 8-off schedule. The purpose of this report is to extend the previous work European Sleep Rescarch Society 245

2 246 7: E: Sunquist et al. conducted aboard ships to a larger and more homogenous sample, and to specifically quantify the incidence of sleep and work patterns that lead to fatigue in mariners. This information can be used to develop potential countermeasures and regulatory approaches to maritime work-rest scheduling. METHOD The primary tool used to gather data for extended periods (10-30 d) was a mariner logbook. This logbook was developed to collect the following data: sleep timing and quality for up to three sleep episodes per day, alertness before and after three work periods by means of a visual analogue scale (VAS), sea state, and whether the ship was at sea or in port. Sleep timing data recorded included time in bed, time asleep, time awake and time got up in hours and minutes. Sleep quality was rated on a 1-5 scale on the dimensions of ease of falling asleep, ease of arising, sleep period sufficiency, sleep depth and restedness. The minimum and maximum values of the scale were verbally anchored with the terms least and most. The booklet was pocket-sized and provided a sufficient number of pages for entering 10d of data. For ships engaged in longer data collection periods, multiple booklets were used. Mariners were instructed to fill out the booklets as close to the time of the sleep or work period as possible. The total time required to fill out the logbook on any one day was approximately 2 min. The background information inventory (BII) consisted of a standard survey form that mariners filled out during their free time. The survey contained questions related to sleep behaviour on the ship and at home; questions related to chronic fatigue; various personality scales; and questions related to general health, work habits, and means used to reduce fatigue. There were 72 questions in the BII; the survey was self-administered and took approximately 60min to complete. The third instrument was the retrospective alertness inventory (RAI). This tool was developed by Folkard and colleagues as a means of rapidly gathering alertness ratings over a 24-h period (Folkard et al. 1995). The RAI consists of a single page and rates each hour of the 24-h period on a scale of 1 (very alert) to 9 (very sleepy), with a score of 0 if the respondent is usually sleeping at that hour (the scale was reversed in analysis). Respondents provided a single estimate of their alertness, in contrast to the daily ratings obtained with the logbook. Previous research has shown the ratings on this instrument to correlate very highly with ratings obtained on a daily basis and with human performance data. In the present research, mariners provided retrospective alertness ratings for the following scenarios: (1) at home (2) at sea (3) beginning of sea tour and (4) end of sea tour. Procedure for ship selection and on-board research protocol Because of the desire for a relatively homogeneous sample of ships on a defined trade route, our focus was on tankers and freight ships on US West Coast runs, recruited with the assistance of the US Coast Guard. Researchers met the ship at a designated port to introduce the research protocol. On any particular ship there was either one or two researchers, who rode the ship between two ports, usually for a total voyage duration of approximately 5 d. Mariners who volunteered for the study were compensated $50 for their participation. Introducing the protocol to 20f crew members generally took about 3d following port departure. The following 2d were used to address any further questions that crew members had, and to periodically reinforce the need to fill out logbooks in a timely manner. Data were retrieved from the ship by meeting the vessel in the next port following completion of the data collection period (at least 10 d and 2 ports). Description of ship and subject samples The research protocol, including the first pilot-test ship, involved data collection from a total of 141 individual mariners on eight different ships. The average age of the participants was During the time periods under study, the ships encountered no significant heavy weather that would adversely affect alertness. Because mariners work round-the-clock schedules, we focused our effort on sampling from all categories of worker - watchstanders, command personnel (master and chief engineer), dayworkers (e.g. able-bodied seamen and unlicensed engineers), and the steward department. Each of these classes of worker has different work-rest schedule constraints by the nature of their job. Table 1 provides a summary of the sample in terms of work category and number of subjects. Freight ships (n=2) and tankers (n=6) were selected for the study because in the US these types of ships are subject to different work hour regulations. Additionally, the operational aspects of freight ships and tankers are different, which may have a bearing on sleep patterns. Across the eight ships in the sample, the average rate of participation was 67% of the crew members (range =42-920/0). The BlI was collected from all participating mariners; the logbook was collected on ships two through eight yielding 2038 logbook days, and the RAI on ships three through eight. Data analysis Analysis of Variance (ANOVA) was used to evaluate the impact of the principal factors of watch type, ship type and sleep type Table 1 Number of subjects in each mariner category and ship type Watch type Ship type Totals Tanker Freighter Command (master and chief engineer) 5 Operational day workers Steward Total n= European Sleep Research Society, J. Sleep Rex, 6,

3 Watch 1, !2 120 g 80 9" 160 9" 120 % 200 % g 280 -I 320 rn Sleep and futigue in merchant marine personnel 241 Command WOO Watch 2, Daywork In 80 In 80 > 9" n y n rn I Watch 3,8-12 Steward : 120 p 80 d Y 's B I -I Figure 1 Sleep patterns over a 24-h period for individual logbook days. Black indicates sleep periods. on the dependent measures of sleep duration, sleep quality and alertness on watch. Analysis of critical fatigue indicators was based on prior research illustrating impaired performance and/or imminent sleep in association with low alertness levels (Akerstedt and Folkard 1995) and sleep reduction (Gillberg 1995). RESULTS Sleep behaviour at home vs. at-sea To determine the extent to which sleep disruption induces fatigue among mariners, it was necessary to evaluate whether mariners differ in the amount and timing of sleep they obtain when not working aboard ship. The BIT indicated that the average reported sleep duration at home is h, and the average go-to-bed time is 23:17 hours+ 16min. Separate one factor (watch type, six levels) ANOVAs showed no differences between watches in at-home sleep duration (F= 0.406, d.f. = 5, 130, D0.844) and sleep onset time (F= 1.43, d.f. = 5, 130, P>0.217). A two factor repeated measures (watch type by time of day) ANOVA on at-home RAI values showed no differences between watch categories (F= 1.52, d.f. = 5, 83, D O. 19). The Greenhouse- Geisser correction for sphericity was applied to the repeated measures factors, and showed that there was a main effect of time-of-day on at-home alertness (F= , d.f. = 1, 498, P<O.OOl) but no interaction between watch type and time of day (F= 1.5, d.f. = 5, 498, D O. 10). The difference between the reported duration of sleep at home for individual mariners and the average sleep duration per 24-h period at sea obtained from the logbooks (sleep debt) was analysed. Mean sleep length at home was 7.9h (s.e.= 0.19); mean sleep length at sea was 6.6h (s.e.=0.09). A paired Student's t-test indicated a significant effect of sleep location, with a sleep debt of 1.3 h per night incurred aboard ship (t = 6.61, d.f.=95, P<O.OOl). Thirty-eight per cent of the mariners indicated that they experienced fatigue or decreased alertness during watch, although this did not differ between watch types (chi-square = 8.02, d.f. = 5, NS) European Sleep Research Society, J. Sleep Rex, 6,

4 248 7: I? Sanquist et al. ~~ ~ ~ ~ ~ Table 2 Average slccp duration and quality per 24-h period. Standard errors are in Wutck Ship type Averagc, sleep durution Average.xirep quality parentheses. - ~ Tanker 6.7 (0.21) 18.0 (0.76) Freighter 6.8 (0.39) 15.1 (1.43) Tanker 6.5 (0.19) 17.2 (0.67) Freighter 5.2 (0.3) 17.9 (2.04) Tanker 6.6 (0.16) 15.6 (0.30) Freighter 7.8 (0.46) 15.6 (2.14) Command Tanker 7.2 (0.16) 17.5 (0.97) Freighter 7.3 (0.54) 17.1 (0.79) Day workers Tanker 6.8 (0.28) 19.2 (0.98) Freighter 7.0 (0.45) 19.3 (1.14) Steward Tanker 5.8 (0.22) 16.4 (1.52) Freighter 6.2 (0.41) 19.1 (1.33) Mean 6.6 (0.68) 17.4 (0.32) Sleep behaviour aboard ship Regression analyses for individual subjects were computed for total sleep duration per 24-h period, using logbook day as a predictor. A Student s t-test indicated that the regression coefficients werc not significantly different from 0 (t =0.37, d.f. = 72; D0.7). Because no trends in sleep duration over time were observed, analyses of shipboard sleep were conducted on average sleep durations for a 24-h period; the averages are composed of data obtained over a d period for each mariner. Figure 1 shows the individual sleep records for all mariners in the different watch types across the 24-h period. A notable feature in this figure is the regularity of timing of the sleep episodes - generally in proximity to the work periods. For example, mariners on the watch sleep in two episodes, one of which is initiated at hours and the other taking place from to hours. A similar pattern is shown for the watch. The watch personnel go to sleep shortly after midnight, with a supplemental nap later in the day. Several of these mariners appear to take three sleep episodes per day. Command personnel sleep during what are considered conventional hours, but also show considerable variation for daytime sleep episodes based on the requirements to work all night in restricted waters. Dayworkers also sleep during conventional hours, and show similar variation in daytime sleep episodes. The steward department shows the most regular pattern of sleep/wake behaviour. A two factor ANOVA (watch type by ship type) indicated that watch type significantly influences sleep duration (F(5, 83) = 7.33; P<O.OOl), and interacts with the type of ship (F(5, 83)=3.03; P<0.014). A main effect of ship type was not observed (F(1, 83)=0.308; D0.58). Table 2 presents the average sleep durations for the different watch types aboard freighters and tankers. The interaction between ship type and watch type is seen in the and watches; on tankers, the watch obtained 6.5 h of sleep whereas 5.2 h were obtained on the freighter. The opposite pattern occurs for the watch: tanker personnel obtained 6.6 h whereas freighter personnel obtained 7.8 h. Post-hoc analysis of watches 2 and 3 indicate that these two watches account for 93% of the variance in the watch by ship type interaction (F(1, 27)= 22.44; P<0.001). Watches I, 4, 5 and 6 did not contribute substantially to the interaction (F(3, 56) = 0.062; NS). Average sleep duration shows a significant inverse correlation with average number of hours worked per day (r=-0.5, d.f.=91, P<O.OOl). Marine sleep quality The five separate sleep quality scales were factor analysed by principal components analysis. The solution indicated that a single factor accounted for 62.5% of the variance in the data. Each of four additional factors extracted accounted for less than 15% of the variance each, and were all correlated 0.65 or above with the first factor. Based on this result, we determined that a sum of the quality scaled would be most appropriate for analysis. The summed quality metric correlates highly (r= 0.95) with the factor scores obtained in principal components analysis. Regression analyses for individual subjects were computed for total sleep quality for each sleep episode, using logbook sleep episode number as a predictor. A Student s t-test indicated that the regression coefficients were not significantly different from 0 (t=0.35, d.f. =90; P20.726). Because no trends in sleep quality over time were observed, subsequent analyses were conducted on average sleep quality for each subject. A two-factor ANOVA (watch type by ship type) revealed no significant effects of watch type (45, 82) = 1.78; P>0.07), ship type (F(1, 82)=0.003; Pr0.95) or their interaction (F(5, 82)= 1.33; P>0.25). Table 2 shows the average sleep quality scores. Indicators of critical fatigue in mariners The preceding sections have documented the existence of substantial sleep disruption among mariners. In the following European Sleep Research Society, J. Sleep Res

5 ~ -. Sleep und jatigue in merchunt murine personnel 249 Table 3 Average percentages of critical fatigue values for each watch type. Standard errors are in parentheses WUtCll Command Day workers Steward Mean 7.4 (1 24) 22.0 (3.17) 1.0 (0.5) 4.4 (2.04) 2.6 (I 3) 9.1 (3.4) 7.4 (1.1) A1ertnes.i < (6.75) 11.0 (2.68) 7.1 (1.6) 5.4 (1.16) 3.6 (1.67) 8.0 (2.28) 11.0 (1.56) analyses we focus on the concept of critical fatigue (i.e. fatigue associated with impaired performance and imminent sleep). Of the data we have collected, there are two measures that can be assessed: (1) the proportion of 24-h periods in which total sleep was less than 4h, and (2) the proportion of logbook alertness ratings of 3 (sleepy) or less. For each subject, the proportion of logbook entries meeting the aforementioned criteria were determined by dividing the number of reports of lowered alertness or restricted sleep by the total number of logbook reports for that subject. The average proportion of critical fatigue values for each watch type is shown in Table 3. Separate one-factor ANOVAs were used to evaluate the effect of watch type upon the proportion of critical fatigue reports. For sleep duration of less than 4 h, a significant effect of watch type was observed (F (5, 89)- 12.1; P<O.OOl). Post-hoc analysis indicates that personnel on the watch report significantly more restricted sleep durations than personnel on other watches (Tukey h, P<0.05). Opportunities for recovery sleep (i.e. 2 8 h) were less than the proportion of restricted sleep periods - 5.2%. These episodes occurred almost exclusively for command, daywork and steward department personnel. For alertness levels equivalent to sleepy to fighting sleep, a significant effect of watch type was observed (F(5, 59) = 5.8; P<O.OOI). Post-hoc analysis indicates that personnel on the 24.0&04.00 watch report significantly more work periods with critically low alertness levels than personnel on other watches (Tukey /I, P<0.05). Relationship of watch duration and alertness The visual analogue alertness scales were divided into nine equal intervals, and assigned a numeric score based on the position of the rating mark. This procedure was used to permit comparison of the retrospective and daily ratings. Regression analyses for individual subjects were computed for logbook alertness for each work period, using a logbook work period number as a predictor. A Student s t-test indicated that the regression coefficients were not significantly different from 0 for either before work (t = 0.01, d.f. = 75, PB0.99) or after work alertness (t= 1.36, d.f.=71, P>O.17). Subsequent analyses were conducted on average alertness values for each subject. Figure 2 presents data from the RAI and logbook that illustrate the average fluctuations in mariner alertness over the time course of each watchstanding period. The first feature to note from this figure is the similarity of data obtained from the logbook and the RAI. For the and watch, the average patterns are remarkably similar. Correlations were computed on the group average data shown in Fig. 2. Using all three watch groups, Y =0.74, d.f. = 11, P< A three-factor ANOVA (watch by time of day (a.m., p.m.) by time-into-watch (hour 1, 4)) on logbook alertness scores yielded a main effect of time of day (F(1,49) =5.82, P<0.02), an interaction between watch and time of day (F(2, 49)= 4.47, P<0.02), an interaction between time of day and timeinto-watch (F(1, 49) =27.2, P<O.OOI) and an interaction between watch, time of day and time-into-watch (F(2, 49) = 4.73, P<O.OI). The main effects of watch (F(2, 49)=0.72; 3.0 I 1.o 2.0 ~ Time of [By Figure 2. Time course of alertness over the watch period for midnight to and watchstanders, obtained from logbook and RAI European Sleep Research Society, J. Slwp Rex, 6,

6 250 7: F: Sunyuist et al. Table 4 Results of multiple regression analysis Predictor Ak~rtnc.r.s at.~turt Alcriness ut end of work period oj work period B B R2 B B R' --~ Sleep quality Sleep duration Constant D0.S) and time-into-watch (F(1, 49) =0.39; P0.5) were not significant. Each watch type shows a different alertness profile over the time course of the watch, and this profile is different from morning to afternoon periods. For example, the watch shows relatively little change in alertness in the to period, whereas the watch shows a dramatic increase in alertness and the watch shows a moderate increase in alertness during their first watches of the day. In the afternoon, the patterns reverse: the personnel show a modest decline in alertness, the personnel show a modest increase, and the personnel show a substantial drop in alertness. Sleep duration and quality and alertness during work periods Multiple regression computations were made on individual subject alertness values in the work period immediately following a sleep period (i.e. values obtained from the logbook). This analysis was carried out using the individual sleep and work period records from the logbook for each subject - a total of 2834 sleep episodelwork period combinations. Two variables, (I) the total duration of the sleep period and (2) the total quality reported for that sleep, were evaluated as predictors of alertness in the subsequent work period by a stepwise multiple regression analysis. The analysis was carried out for reports of alertness at the start and end of the work period. The resulting regression coefficients are shown in Table 4. For alertness at the start of a work period, a significant multiple R of 0.61 was obtained (F(1, 2833)= , P<O.OOI). For alertness at the end of the work period, a significant multiple R of 0.43 was obtained (F(I, 2772)= 634. I; P<0.001). For both regression equations, sleep duration contributed only marginally to the variance explained. DISCUSSION Fatigue resulting from disrupted sleep appears to be a problem of fdirly widespread magnitude in the US maritime industry. On the basis of a simple survey question, 38% of the respondents reported feeling fatigued at some point during their watch or duty period. Analysis of mariner sleep durations per 24-h period indicate that the average sleep duration is 6.6 h per night, with some categories of worker averaging as little as 5.2 h per night. The sleep of watchstanders is fragmented because of work scheduling, which further reduces the restorative value of sleep. On measures of critical fatigue, 11% of the work periods reported are associated with critically low levels of alertness at some time during the watch, and 7.4% of all sleep periods are severely curtailed. The watchstanders account for the largest proportion of cases showing critical fatigue levels. The following discussion considers the results of this work in terms of sleep adequacy, impacts on alertness and potential fatigue countermeasures. Maritime schedules and sleep adequacy Several aspects of these data contrast with previous at-sea studies of sleep and work. In particular, the studies of Rutenfranz et al. (1988) reported average sleep durations of h, nearly an hour greater than the 6.6 h obtained in the current study. Similarly, their average work durations were between 9.4 and 10.4 h, in distinction to the average of 11.5 h in the current study. These differences may be attributed to the more homogeneous sample of ships use in the current study, and the increased work requirements as crew sizes decline. Current findings similar to previous work include the fragmented and lower quality sleep of the watchstanders. Rutenfranz et al. (1988) also observed that personnel on the watch were limited to a single sleep episode 20'% of the time. A similar percentage (22%) was obtained in this study; these sleep episodes resulted in less than 4 h of sleep in the 24-h period. It is noteworthy that the largest percentage of marine collisions and groundings occur during the watch (United Kingdom Protection and Indemnity Club 1992). The sleep patterns and durations observed in this work suggest that restorative sleep is not being obtained by a large percentage of mariners. Compensation opportunities are rare, and occur exclusively for personnel on daywork schedules. Analysis of critical fatigue indicators and interviews with mariners suggest that alternative work-rest scheduling would be desirable. Virtually all mariners we interviewed expressed interest in ways to obtain longer duration sleeps, rather than splitting their sleeps into separate episodes. Alertness during watch periods Measures of alertness over the watch period show (I) an increase in alertness during watch in the first half of the day (2) decreases in alertness during watch in the second half of the day, and (3) substantial alertness shifts as watches change Europedn Sleep Research Society. J Sleep R1.s,

7 Sleep and.fatigue in merchant marine perxmnel 251 The increases in alertness during watch in the early part of the day suggest that extending the watch duration may be an appropriate means of maintaining higher alertness levels during watch and for providing longer periods of rest. Similarly changes in workhest scheduling may help to reduce the number of episodes of critically reduced alertness on the and watch. The goal of alertness management strategies for maritime watchstanders would be to maintain alertness at the higher end of the scale during watchstanding and other work periods, and to preclude dropping to critical levels where sleep precursors such as slow eye movements occur (Akcrstedt and Folkard 1995). Potential fatigue countermeasures Alternative work schedules appear to be a productive avenue for intervention in the maritime environment. Disrupted sleep patterns and insufficient sleep periods are directly linked to the 4-011, 8-off work schedule of the watchstanders. Previous research with alternative watch schedules (Fletcher et al. 1988) indicates that longer duration sleeps can be obtained when watchstanding is compressed into a close-watch system of 6hon, 2-0ff, and 2-on. Variations on this type of arrangement could result in a workhest system that would provide watchstanders with a minimum uninterrupted 10 h rest period. Maintaining sleep patterns once they are established is another potential countermeasure that can potentially offset disrupted sleep patterns. Port calls can result in dramatic shifts in the timing and pattern of sleep for a 1 or 2-d period. Strategic use of shore relief personnel may help to alleviate this problem. Finally, employing non-watchstanders to provide breaks during extended watch periods may help to reduce the incidence of critically reduced alertness during the early morning and late afternoon hours. CONCLUSIONS The data obtained in this field study indicate that sleep disruption and fatigue is a substantial problem among mariners, particularly watchstanders on the 4-011, 8-off work schedule. This results in critical fatigue levels among watchstanders as often as 24%) of the time. Among various intervention strategies, the one likely to have the most profound effect is a schedule change. Because of the complex interactions of watch schedule with other work activities, this approach must be developed on a rational, task-structured basis, and be able to accommodate a variety of perturbations including port calls, bad weather, and staff shortages. The proposed countermeasures are a first step in the direction of better workhest scheduling in the maritime environment. ACKNOWLEDGEMENTS This work was supported by contract number DTCG D-E00777 from the United States Coast Guard Research and Development Centre. REFEERENCES Akerstedt, T. and Folkard, S. Validation of the S and C components of the three-process model of alertness regulation. Sleep, 1995, 18 (I): 1-6. Akerstedt, T., Torsvall, L. and Gillberg, M. Sleepiness and shiftwork: Field studies. Slecy, 1982, 5 (Suppl. 2): Fletcher, N., Colquhoun, W. P., Knauth, P., DeVol, D. and Plett, R. Work at sea: A study of sleep, and of circadian rhythms in physiological and psychological functions, in watchkeepers on merchant vessels - VI. A sea trial of an alternative watchkeeping system for the merchant marine. Int. Arch. Occup. Environ. Health, 1988, 61: Folkard, S., Spelten, E., Totterdell, P., Barton, J. and Smith, L. (1995). The use of survey measures to assess circadian variations in alertness. Sleep, 1995, 18 (5): Gillberg, M. Sleepiness and its relation to the length, content and continuity of sleep. J. Sleep Rrs., 1995, 4 (Suppl. 2): Lauber, J. K. and Kayten, P. J. Sleepiness, circadian dysrhythmia, and fatigue in transportation system accidents. Sleep, 1988, 11 (6): National Transportation Safety Board. Marine Accident Report - Grounding oj the U. S. Tunkship Exxon Vuldez on Bligh Ree$ Prince Willium Sound, Neur Vuldez. Alusku. Murch 24, NTSBIMAR , Washington, DC, Rutenfranz, J., Plett, R., Knauth, P., Condon, R., DeVol, D., Fletcher, N., EickhoH; S., Schmidt, K. H., Donis, R. and Colquhoun, W. P. Work at sea: A study of sleep, and of circadian rhythms in physiological and psychological functions, in watchkeepers on merchant vessels Sleep duration, and subjective ratings of sleep quality. Int. Arch. Occup. Environ. Health, : 331 ~339. Tepas, D. I. and Carvalhais, A. B. Sleep Patterns of Shiftworkers. Occup. Med.: Stute ojthe Art Rrviebis, 1990, 5 (2): United Kingdom Protection and Indemnity Club. Anulysis of Major Cluirns. United Kingdom Mutual Steam Ship Assurance Association (Bermuda) Limited, London, UK, European Sleep Research Society, J. Sk!l.l, Res., 6,