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.
| Identifer | oai:union.ndltd.org:ADTP/173371 |
| Date | January 2001 |
| Creators | Roach, Gregory D |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | © 2001 Gregory D Roach |
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