Measurements of fatigue
Fatigue is often considered to be a generic term of which sleepiness is one of the major sub-components. Measuring fatigue is a complex task and there is no standardized methodology for the recording of fatigue
or sleepiness, in individuals driving a vehicle. However, techniques based on recordings of EEG (electrical brain waves) and, in most cases, EOG (eye movements), have been commonly used. Many studies have utilized blink measurements as a way of characterizing
the level of sleepiness. Another technique refers to subjective ratings which is the simplest way of measuring driver sleepiness. However, it is also of great interest to know more of how drivers perceive sleepiness and whether their perception correlates with objective indices of sleepiness.
Collecting data
The data collection in this study was performed during visits on board 13 cargo vessels of different types, such as bulk carriers, car
carriers and tankers. 30 participants were included in the study which focused on two separate watch systems: the 2-watch system – six hours on and six hours off – and the 3-watch system – four hours on and eight hours off.
Four types of instruments were used for data collection during the on board visits: questionnaires and diaries, eye movement measurements, activity meters and reaction time tests. These were chosen carefully to
ensure as useful objective and subjective measures as possible without disturbing the normal routines of the participants.
The questionnaire included questions about home conditions, health issues, sleep habits, satisfaction with the working situation etc. Sleep quality was measured by handing out sleep diaries to be filled in after
each period of sleep, including sleep on daytime off-duty periods. Also, subjective sleepiness estimations were performed by the participants once every hour using the Karolinska Sleepiness Scale (KSS). KSS is a nine-graded scale where 1 is very alert and 9 is very sleepy. The participants were at the same time told to rate their level of experienced stress on a similar 9-point scale.
Results from the questionnaire
The questionnaires from the two watch systems were compared and no significant difference was found. However, tendencies indicate that participants in the 3-watch system are more satisfied with their working hours and working situation than participants in the 2-watch system. Tendencies also suggest that participants in the 2-watch
system often get less than six hours sleep a day, more often nod off and fight against sleep than participants in the 3-watch system.
When asked to judge how much sleep the participants need the answers for both watch systems are very similar. The mean values for each watch system suggest that the participants in the 2- watch system think they need slightly less sleep (7.03 h) than the participants in the 3-watch system (7.43 h).
KSS and time of day
The mean values of KSS and stress values from the diaries, for each participant and every hour, were calculated both from the on board ratings and for the at home ratings. These were then compared for the
two watch systems. The values are generally not very high, but tendencies suggest that the participants in the 2-watch system are a
bit more tired, especially in the early morning and in the afternoon, whereas the stress level is about the same. The mean values also indicate that both watches are less sleepy and less stressed at home than on board. The highest mean KSS scores were recorded in the late night and early morning hours. Night time hours induced higher tiredness than day time work.
Analysing only high KSS scores for watch system and time of day reveals that 2-watch led to a larger amount of high KSS scores. Figure 2 shows the percentage of KSS scores higher than 7 (when the first
signs of sleepiness appear) for both shift systems. The average percentage of KSS scores over 7 was 3.46 for 2-watch and 1.29
for 3-watch. In 2.7 per cent of the scores participants had difficulties in staying awake (8 or 9) and in 82 per cent of these the participant worked in 2-watch.
Participants in the 2-watch system think they need less sleep than participants in the 3-watch think. A possible explanation is that the 2-watch mates are used to sleeping less and do not remember the feeling of being totally rested. We have heard comments like: “I did not know how tired I was until I changed from 2-watch to 3-watch” or
“After a few days at home I know what alert is” and “It was worth the lower pay”.
Most ratings over 7 come from 2-watch participants and most from the night shifts. During 39 occurrences the participants
had difficulties in staying awake. Whether these numbers are scored on watch or not, one should consider that seafarers are always “on call” and can be called upon to perform safety-critical tasks.
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Stress data, reaction times and EOG
The 2-watch system had a mean stress value of 3.23 and the 3-watch system 2.84. No significant effect of watch system was showed,
but a significant interaction with shift number – the more shifts the higher the stress scores. The reaction times after a night shift had much higher variance than after a day shift. After the night shift more scorings with long reaction times were present suggesting difficulty to focus and to concentrate for long periods of time. This kind of variance is seen when testing after alcohol. Generally it can be said that participants scored higher reaction times after a night shift than after a day time shift. The mean value after night shift was 0.35 seconds and after day shift 0.31 seconds (only a slight tendency to higher values for the 2-watch was found). This has been shown before in many domains, and here, again, it is shown that night work is not what people are made for. The results from the EOG did not add much useful information to the study and could be left out completely in similar studies.
Conclusions
Besides the analysis presented here the project Fatigue at Sea also consists of a literature review, an interview series with shipping
companies and a study of marine pilots, boatmen and other personnel. Also, a simulator study has been conducted and will be analysed later this year.
However, from the on board study a number of conclusions could be drawn. These are, in short
• Mates in the 3-watch are more satisfied with the working times and the working situation.
• Mates in the 2-watch system are sleepier, especially in early morning and afternoon.
• Most mates think they need at least eight hours of sleep a day.
• All mates are less sleepy and less stressed at home.
• KSS values are highest at night shifts.
• The most sleepy participants work in the 2-watch system.
• Reaction times are longer for all participants after a night shift.
• Variation in reaction times is larger for participant in the 2-watch system the suggesting them to be more unfocussed.
• 2-watch participants are more fatigued at the end of shift than 3-watch participants.
There is a lack of “hard data” to support stakeholders in their decision-making, and a lack of recommendations tuned to the needs and constraints of the shipping business. Finding solutions to this problem is constrained from many perspectives: suggestions must not lead to significant extra costs for the concerned parties and stakeholders.
The challenge is to find solutions that are effective, economical and easy to use.
Birgitta Thorslund and Albert Kircher, VTI, Sweden
Margareta Lützhöft, VTI & Chalmers, Sweden
Contact
Margareta Lützhöft
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