Sleepiness and Sleep in a Simulated “Six Hours On/Six Hours Off” Sea Watch System

Abstract

Ships are operated around the clock using rapidly rotating shift schedules called sea watch systems. Sea watch systems may cause fatigue, in the same way as other irregular working time arrangements. The present study investigated subjective sleepiness and sleep duration in connection with a 6 h on/6 h off duty system. The study was performed in a bridge simulator, very similar to those found on ships. Twelve officers divided into two groups participated in the study that lasted 66 h. Half of the subjects started with the 06:00-12:00 h watch and the other half with the 12:00-18:00 h watch. The subjects alternated between off-duty and on-duty for the remainder of the experimental period. Approximately halfway through the experiment, the 12:00-18:00 h watch was divided into two 3 h watches/off-duty periods. The effect of this was to reverse the on-duty/off-duty pattern between the two groups. This enabled all subjects to work the four possible watches (00:00-06:00 h, 06:00-12:00 h, 12:00-18:00 h, and 18:00-24:00 h) in an order that was essentially counterbalanced between groups. Ratings of sleepiness (Karolinska Sleepiness Scale; KSS) were obtained every 30 min during on-duty periods and if subjects were awake during off-duty periods. The subjectively rated duration of sleep was recorded after each off-duty period that preceded watch periods when KSS was rated. The results showed that the average level of sleepiness was significantly higher during the 00:00-06:00 h watch compared to the 12:00-18:00 h and 18:00-24:00 h watches, but not to the 06:00-12:00 h watch. Sleepiness also progressed significantly from the start toward the end of each watch, with the exception of the 06:00-12:00 h watch, when levels remained approximately stable. There were no differences between groups (i.e., the order between watches). Sleep duration during the 06:00-12:00 h off-duty period (3 h 29 min) was significantly longer than during the 12:00-18:00 h period (1 h 47 min) and the 18:00-24:00 h period (2 h 7 min). Sleep during the 00:00-06:00 h period (4 h 23 min) was longer than all sleep periods except the 06:00-12:00 h period. There were no differences between groups. In spite of sufficient opportunities for sleep, sleep was on the average around 1-1 h 30 min shorter than the 7-7 h 30 min that is considered "normal" during a 24 h period. This is probably a consequence of the difficulty to sleep during daytime due to the alerting effects of the circadian rhythm. Also, sleepiness during the night and early mornings reached high levels, which may be explained by a combination of working close to or during the circadian trough of alertness and the relatively short sleep periods obtained. An initial suppression of sleepiness was observed during all watches, except for the 06:00-12:00 h watch. This suppression may be explained by the "masking effect" exerted by the relative high levels of activity required when taking over the responsibility of the ship. Toward the end of watches, the levels of sleepiness progressively increased to relative...

A systematic review of the sleep, sleepiness, and performance implications of limited wake shift work schedules

“…The studies examined in this review employed a range of schedules, which can be broadly divided into: (i) work/rest schedules, such as 4 hours-on/8 hours-off, 6 hours-on/6 hours-off and 8 hours-on/8 hours-off sched- The last seven days at sea were used in the analyses 6h-on/6-off : TST was similar for both watch rotations 4h-on/8h-off: TST was higher for team one followed by team three then team two watch rotations TST was shorter, frequency of nodding while on duty and excessive daytime sleepiness was higher on 6h-on/6-off compared to 4h-on/8h-off watch systems (24); and (iii) work/ rest schedules that incorporate a swing shift, "dog" shift, layover or disruption, typically in the middle of a roster schedule (22,23,29,33,37). Five studies examined 8 hours-on/8 hours-off roster schedules, all of which were conducted with Australian train drivers (22,25,29,36,37), five examined 6 hours-on/6 hours-off shift schedules, predominantly in ship bridge officers and offshore fleet workers (17,23,30,34,39), and nine examined 4 hours-on/8 hours-off shift schedules all among maritime watch keepers (17, 26-28, 30-33, 35).…”

“…Eight of the studies identified in this review used a within-subject's design (18,22,23,25,29,(36)(37)(38), ten a between-subject's design (17,24,26,27,30,31,34,35,39,40), and four employed a mixed within-and between-subjects design (19,28,32,33). Seventeen studies were field-based (17-19, 22, 24-32, 34-37) and five used laboratory simulations (23,33,(38)(39)(40).…”

“…Five studies examined 8 hours-on/8 hours-off roster schedules, all of which were conducted with Australian train drivers (22,25,29,36,37), five examined 6 hours-on/6 hours-off shift schedules, predominantly in ship bridge officers and offshore fleet workers (17,23,30,34,39), and nine examined 4 hours-on/8 hours-off shift schedules all among maritime watch keepers (17, 26-28, 30-33, 35). The remaining LWSW studies employed less common schedules.…”

“…Only primary, peer-reviewed articles published in English were included, while studies examining pharmacological interventions were excluded. From this search, we identified 22 papers which examined LWSW (17)(18)(19)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40), two pairs of which overlap (25,29,36,37), leaving a total of 20 independent studies. The identified studies are summarized in table 1.…”

“…Seventeen studies were field-based (17-19, 22, 24-32, 34-37) and five used laboratory simulations (23,33,(38)(39)(40).…”