Shiftwork, particularly that involving night work and/or transmeridian flight, forces a mismatch between the sleep/wake cycle and the endogenous circadian timing system. Specifically, shiftworkers are often required to sleep at a phase in the circadian cycle when they would usually be active, and to work at a phase in the circadian cycle when they would usually be asleep. The current thesis describes a series of five studies designed to examine the disruption and adaptation associated with shiftwork, with an emphasis on night work and, to a lesser extent, transmeridian flight. The first study (Chapter 3), conducted in the field, was designed to examine the effects of break duration and time of break onset on the amount of sleep that shiftworkers obtain between consecutive work periods, and to consider the role that pineal production of melatonin may play in this process, through its regulation of sleep. Not surprisingly, total sleep time (TST) increased with break duration for breaks that began at similar times of day. Importantly though, TST was greater for breaks that occurred during the night-time than for breaks that occurred during the daytime. These results indicated that the minimum-length break requirements contained in prescriptive duty hours regulations might not necessarily protect shiftworkers from being exposed to unacceptable levels of fatigue. In addition, there was a temporal relationship between the circadian rhythms of sleep duration, sleep quality, and 6-sulphatoxymelatonin excretion, such that sleep was longer and of better quality when melatonin production was relatively high. This data did not prove a causal link, but it did provide further indication that melatonin may be involved in the regulation of sleep. The aim of the second study (Chapter 4) was to examine the effects of time of day, shift duration, and prior sleep length on self-assessed alertness and neurobehavioural performance of shiftworkers in a real work setting. Cosinor regression models fitted to the data indicated that time of day had a significant effect on alertness and performance, with both reaching nadirs in the early morning. Indeed, the cosinor regression lines of best fit explained more than 90% of the within-subjects variability in both the alertness and performance measures. In addition, alertness declined as shift duration increased and rose as prior sleep length increased, and there was a decline in performance across work periods that was greater for extended shifts. However, the results indicated that time of day was the most important determinant of subjective alertness and neurobehavioural performance. Consequently, the fatigue associated with night work can never be eliminated, only minimised through the application of risk management strategies. The aim of the third study (Chapter 5) was to quantify the effects of fatigue on performance in a simulated work environment, i.e. a rail simulator, and to compare them with the effects of alcohol intoxication. Reaction time (RT) performance on a visual psychomotor vigilance task (PVT) was also assessed. Rather than cause a general decline in performance as was hypothesised, fatigue impaired some safety and efficiency measures (i.e. number and duration of extreme speed violations increased, average speed reduced, brake use increased), but not others (i.e. fuel use, inter-train forces, and minor and moderate speed violations were unaffected). The reduction in safety and consequent increase in risk due to fatigue reached levels equivalent to those associated with moderate levels of alcohol intoxication (i.e. -05?-10%). The results indicated that fatigue caused participants to disengage from operating the simulator such that safety was traded off, not necessarily deliberately, against some aspects of efficiency. RT performance on the PVT was also significantly impaired by fatigue, similar to the magnitude of impairment associated with moderate levels of alcohol intoxication (i.e. -05?-10%). However, the PVT results could not predict the complex relationship between simulator safety and efficiency measures. This indicated that the effects of fatigue on performance in the workplace cannot necessarily be derived on the basis of simple performance measures such as RT. The fourth study (Chapter 6), conducted in the laboratory, was designed to assess adaptation to a simulated night work schedule using salivary dim light melatonin onset (DLMO) as the circadian phase marker. Participants worked seven consecutive simulated 8-hour night shifts (i.e. 23:00?07:00h). This resulted in a mean total phase delay in DLMO of 5.5 hours, equivalent to an average delay of 0.8 hours per day. In addition, baseline DLMO was significantly correlated with mean wake time over the previous seven days. These results indicated that partial circadian adaptation occurred in response to the simulated night work schedule, and that baseline DLMO was reliably predicted by the mean wake up time for the preceding week. The radioimmunoassay used proved to be a sensitive measure of melatonin concentration in saliva for the determination of DLMO, and thus provides an alternative phase marker to core body temperature. The last study (Chapter 7) was designed to examine the adaptation of a RAAF aircrew to several small time zone transitions using salivary melatonin onset as the marker of circadian phase. In addition, the effects of the aircrew?s work schedule on their sleep/wake patterns and subjective alertness were assessed. During the first six days of a routine surveillance patrol (SURPAT), the aircrew travelled eastward and melatonin onset occurred progressively earlier (i.e. phase advanced). During the second six days of the SURPAT, the aircrew travelled westward but melatonin onset did not significantly shift. Night-time sleep duration was shorter prior to work days than prior to rest days, and subjective alertness was not significantly affected by either the duration of night-time sleep prior to work, or the duration of flight. The melatonin onset results indicated that participants? body clocks adapted well to several small time zone transitions when initially travelling eastward, but did not adapt to a similar pattern of time zone transitions when subsequently travelling westward. This was contrary to expectations based on studies of single acute time zone transitions, which indicate that adaptation to westward flight is more rapid than adaptation to eastward flight. Taken together, the results of these five studies confirm that shiftwork provides a considerable source of disruption to shiftworkers? sleep/wake patterns. Whilst this disruption to shiftworkers? sleep may impair subjective alertness, the greatest influence on alertness and performance is exerted by time of day. Furthermore, the combined effects of sleep disruption and time of day may result in a level of performance impairment in a simulated work environment similar to that associated with moderate levels of alcohol intoxication. Finally, night work and transmeridian flight provide a source of circadian disruption, the adaptation to which can be assessed in both laboratory and field settings by examining changes in the timing of nocturnal melatonin onset. / thesis (PhD)--University of South Australia, 2001.

Extended duration and irregularly planned overnight shifts are becoming frequent features of current working time arrangements. However, there is great concern about the detrimental effect that these shifts are likely to have on the sleep, performance and fatigue levels of workers. To date, the empirical evidence regarding the effects of extended and irregular overnight shifts is sparse and workers have rarely been followed up longer term to assess possible changes over time. Because these types of overnight shifts are being introduced into transport settings such as aviation and shipping where safe and productive operations are paramount, there is a real imperative to examine and trial workplace-based methods aimed at counteracting night shift related fatigue and performance deficits. Taking short duration naps on the nightshift at work is one potential countermeasure that has shown promise in the laboratory but has yet to be examined in any real-world setting under conditions of extended duration or irregular night work. This thesis takes up these issues in a series of four empirical workplace-based studies. The impact of the introduction of regularly planned 12 h day and overnight shifts was examined on three occasions via questionnaire in a group of aircraft maintenance engineers, once before the change, and twice after the change from 8 h shifts. The results showed that high levels of acute fatigue on the 12 h night shifts and high chronic fatigue levels were common, and were strongly associated with poor overall job performance, poor safety of job performance, and longer recovery times, effects that persisted over time. Of major concern was the finding that over half of the engineers reported having nodded off while driving home from their extended overnight shifts. The impact of irregularly planned 8 h overnight shifts worked by cargo handlers in a shipping container terminal was also examined via self-report questionnaire. The findings revealed significant detrimental impacts on sleep, fatigue levels and performance at work. High fatigue levels were strongly associated with poor self-rated performance output and safety of performance at work on the overnight shifts. The primary focus of this thesis was to evaluate the effect of a scheduled 20 min nap opportunity taken in these two fully operational workplaces during the night shift on sleep, performance and alertness. Sleep was achieved by 50 to 59% of workers. Speed of performance in 24 male aircraft maintenance engineers on a vigilance task was significantly faster at the end of a 12 h overnight shift that had contained sleep during the nap compared to the control condition. There was no such effect of the nap on the performance of 22 cargo handlers working impromptu 8 h shifts in a shipping container terminal. However, the duration of sleep obtained by the 8 h workers was shorter in comparison and was likely to have been undermined by the high noise levels around the nap facility and time pressures related to the organisation of work. Actigraphy data recorded from all subjects showed that subsequent daytime sleep was not disrupted by the night shift nap opportunity. Overall, the most interesting findings were that simply being tired and presented with a sleep opportunity during the night shift did not guarantee either sleep, or recovery but that sleep, as opposed to rest, was necessary for the benefits to performance to be fully realised. These findings have shown for the first time that management of fatigue associated with extended duration night work can be achieved in some instances by the use of a short duration nap taken in the workplace on the night shift. However, operationalizing in-situ napping on the night shift has a range of organisational influences that must be considered for the potential of such a strategy to be fully harnessed.

Thesis (M. Phil.)--University of Hong Kong, 1988.

Background Night shift work is inevitable for maintaining continuous services in different sectors e.g. healthcare, financial, transport and service sectors. Night shift work increases exposure of light at night. Exposure of light at night suppresses production of a neurohormone melatonin. Melatonin has shown potential cancer protective action in animal experiments. Melatonin deficiency is suggested to be related to the development of various cancer especially breast cancer. Breast cancer incidence in Hong Kong is rising. Particular concern about association between night shift work and breast cancer is raised. Objective To collect evidence from studies of other countries with study populations of different professions and to evaluate the relationship between night shift work and breast cancer Method MEDLINE was searched to identify publications, limited to English articles, from 1973 to May 2011. Search terms include (circadian rhythm or night work or night shift or shift work or work schedule tolerance) and (cancer or neoplasm or neoplasia) and (risk or rate or incidence). No restriction was set to the publication type. Results Altogether 343 titles retrieved from MEDLINE search. Finally, 8 primary observational studies that met inclusion criteria were identified for this review. Of these, two were prospective cohort studies, one was retrospective cohort study, two were nested case-control studies and three were case-control studies. Most of the study had crude exposure assessment of night shift work, in which four studies relied on group level of exposure probability instead of individual exposure information. Six of eight studies showed positive results on the association of night shift work and breast cancer in women. Three studies found that risk of breast cancer was increased significantly for those who had engaged in night shift work in a long duration i.e. more than 20-30 years, but they were all conducted in populations of same occupational group i.e. nurse and only a moderate increase of breast cancer risk was found. The results were subject to confounding and bias. No consistent results were found for effect of shorter duration of night shift work on risk of breast cancer. Conclusion Based on the studies included, there is suggestive evidence of an association of night shift work and breast cancer. Further studies on this are needed. Involvement of population of different occupational groups, controlling confounder of hormone use and conducting exposure assessment with high reliability using individual information instead of that from group are suggested. / published_or_final_version / Community Medicine / Master / Master of Public Health

Not available / text

Thesis (M.Phil.)--Murdoch University, 2003. / Thesis submitted to the Division of Social Sciences, Humanities and Education. Bibliography: leaves 309-378.

Thesis (M.S. in Operations Research)--Naval Postgraduate School, December 2004. / Thesis advisor(s): Nita Lewis Miller. Includes bibliographical references (p. 87-89). Also available online.

Thesis (M.S.)--West Virginia University, 2000. / Title from document title page. Document formatted into pages; contains iii, 48 p. : ill. (some col.). Vita. Includes abstract. Includes bibliographical references (p. 43-45).

Thesis (M. Sc.)--Memorial University of Newfoundland, 1997. / Blbliography: leaves 65-69.

Mental fatigue has been proven to be highly prominent during shift work, due to long, irregular working hours and disruption of the circadian rhythm. Measuring mental fatigue has been a challenge for many years, where commonly cognitive test tasks are used to assess mental fatigue. Moreover, these test tasks do not isolate where fatigue is occurring during human information processing. The human information processing system consists of four core stages, each of which requires numerous cognitive functions in order to process information. The Human Kinetics and Ergonomics Department at Rhodes University has developed six cognitive test tasks where each isolates a cognitive function: an accommodation test task, a visual detection test task, a reading test task, a memory test task, a tapping test task and a neural control test task. The cognitive functions include: eye accommodation, visual discrimination, visual pattern recognition, memory duration, motor programming and peripheral neural control. General task-related effect can also be examined for each of these cognitive test tasks which include choice reaction time, visual detection, reading performance, short-term memory, motor control and tracking performance. Additionally, a simple reaction time test task has been developed to analyse simple reaction time. This test task does not isolate a cognitive function. One or more parameters can be examined for each cognitive function and task-related effect. The first aim of this study was to validate numerous cognitive test tasks for mental fatigue in a simulated shift work laboratory setting. The second aim was to assess the validated cognitive test tasks in Phase 1 in a field-based rotational shift work setting. Parameters revealing sensitivity to mental fatigue would be validated for mental fatigue applied to rotational shift work and would be inserted into an assessment tool. In the laboratory setting, the seven cognitive test tasks were examined on four different types of shift work regimes. The first regime was a standard eight-hour shift work system, and the other three were non-conventional shift work regimes. Participants (n = 12 per regime) were required to complete one day shift followed by four night shifts, where testing occurred before and after each shift and four times within each shift. The cognitive test tasks revealing sensitivity to fatigue included: visual detection test task, reading test task, memory test task, tapping test task, neural control test task and simple reaction time test task. The testing of Phase 2 was conducted in three different companies, where each performed a different type of rotational shift work. The six cognitive test tasks validated for mental fatigue in Phase 1 were tested before and after work for each shift type within the rotational shift work system adopted by each company. Company A (n = 18) and Company B (n = 24) performed two-shift rotational shift work systems, where the shift length of Company A was 12-hours and the shift length of Company B was irregular hours. Company C (n = 21) performed an eight-hour three-shift rotational shift work system. Nine parameters revealed fatiguing effects and were inserted into the assessment tool, five of which provided information on a specific cognitive function: error rate for visual discrimination, processing time for visual pattern recognition, error rate for visual pattern recognition, impact of rehearsal time on memory recall rate for memory duration and the high-precision condition for motor programming time. The remaining four parameters provided information on general task-related effects: reading speed for reading performance, recall rate for short-term memory, reaction time for motor control and simple reaction time. Therefore, an assessment tool comprising nine parameters was validated for mental fatigue applied to rotational shift work, where five of the parameters were able to isolate exactly where fatigue was occurring during human information processing and the other four parameters were able to assess fatigue occurring throughout the human information processing chain.