Cumulative sleep deprivation is often associated with work patterns involving night shift or early morning shifts. Adaptation of the circadian system to the shift pattern is reported to promote improved duration and quality of sleep and a concurrent improvement in performance. The current study followed twenty-nine operators at a live-in mining operation working to a seven-day, seven-night shift pattern who collected saliva samples for melatonin measurement, recorded sleep using activity monitors and diaries, and underwent performance testing (psychomotor vigilance task) for one complete roster cycle. The time of onset of melatonin secretion changed significantly (P=0.022) across the week of both Day and Night shifts (2104 h ± 16 min versus 2130 h ± 16 min, respectively), but the small magnitude of the change indicates a lack of true circadian rhythm adaptation to the lifestyle. Total sleep time was longer following the seventh Day shift (associated with a period of 24 h off prior to the commencement of Night shifts). There were no other changes in total sleep time. Further, there were no improvements in sleep onset latency or sleep efficiency on Day or Night shifts. However, reaction times recorded at the end of the shifts slowed across the seven Day and seven Night shifts indicative of impairments in psychomotor performance (F(6,168)=6.087, P<0.001). The results suggest that previous reports of adaptation to consecutive night shifts cannot necessarily be applied to onshore or Australian environments. Adaptation is dependent on factors such as light exposure, environmental conditions, shift parameters such as wake-up, work start and work end times and individual characteristics.
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