Shift work in the offshore vessel fleet: circadian rhythms and cognitive performance.
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- Institutt for biologi 
Workers on offshore vessels operate day and night under a wide variety of environmental conditions. Shift-work affects human physiology and it is known to cause fatigue and other health-related issues. Shift work is common in the offshore vessel fleet, and the shift work schedules in widespread use are 6-h shifts, 12-h shifts, and the 8-8-4-4-h shift (8 on, 8 off, 4 on, 4 off). Of these three shifts, the 6-h shift is by far the most widely used. Until now, the implications of these schedules have remained relatively unexplored, and the first aim of this thesis was to investigate these three shift schedules in some detail. The second aim was to pay special attention to the 12-h shift, and investigate physiological adaptations to this shift in further detail. The third aim was to investigate the potential of physical exercise as a means of reducing the fatigue and reduced cognitive performance experienced by shift workers. The methods employed included questionnaires, objective and subjective sleep quality, cognitive testing and physiological measures such as 6-sulphatoxymelatonin levels, cortisol levels, blood pressure, heart rate and core body temperature. Papers I and II investigated various aspects of sleep disturbances in offshore fleet shift workers on 12-h shifts, 6-h shifts and 8-8-4-4-h shifts. The 6-h shift schedule was hypothesised to be inferior to both the 12-h and the 8-8-4-4-h shift schedule on various parameters related to sleep and sleep disturbances. The results speak in favour of the 12- h shift and the 8-8-4-4-h shifts. Sleep disturbances are more common among 6-h shift workers than in 12-h shift workers, and noise and shift-work itself are regarded more of a problem by this group. There are regional differences in most of the measured variables. The 8-8-4-4-h shift workers enjoy better sleep efficiency, more continuous sleep, more sufficient sleep and fewer daytime naps than the 6-h shift workers. Papers III and IV looked at 12-h shifts specifically in order to improve our understanding of the physiological adaptation to 12-h night shifts. It was hypothesised that the rate of adaptation and total phase shift in 6-sulfatoxymelatonin levels are slower than those of other nightshift workers in the offshore industry, primarily due to lower light exposure and environmental conditions. It was also hypothesised that after seven days of working night shifts there will be an increase in core body temperature, blood pressure, and heart rate. Measures of cognitive performance, objective and subjective sleep quality were also predicted to show improvements. The results show that offshore fleet shift workers adapt to 12-h night shifts primarily due to nightly light exposure, and secondarily due to caffeine consumption, food intake and physical activity at night. This adaptation is less than that seen in other branches of the offshore industry, due to exposure to lower light intensities. Core body temperature also adapts in the course of seven days on the night shift, but blood pressure and heart rate remains unchanged. Subjective sleep quality remains relatively stable during periods at sea, but shows inconclusive results during seven days at sea. Cognitive performance improves significantly after a week at sea. Paper V investigated the potential of physical exercise as a measure to improve cognitive performance and subjective sleepiness. It was hypothesized that cognitive performance declines and subjective sleepiness can be reversed by performing physical exercise at night. The results show that cognitive performance and subjective sleepiness improve after performing physical exercise at night. All improvements from physical exercise are transient in nature, and return to resting levels in the course of a few hours. All in all, this thesis shows that 12-h shifts and 8-8-4-4-h shifts are preferable to 6-h shifts. Offshore vessel workers on 12-h night shifts adapt to their working schedule which is primarily due to increased light exposure, and secondarily due to physical activity, caffeine and food intake. The adaptation, however, is slower than in other sectors of the offshore industry due to exposure to lower light intensities. Physical exercise can counteract the effects of reduced cognitive performance induced by nighttime working hours, and thus counteract the lower light intensities experienced by this group of workers.