ELECTRONIC PAPER

The impact of a week of simulated night work on sleep, circadian phase, and performance

N Lamond, J Dorrian, G D Roach, K McCulloch, A L Holmes, H J Burgess, A Fletcher, D Dawson

The Centre for Sleep Research, The Queen Elizabeth Hospital, South Australia

Correspondence to:
Correspondence to:
Dr N Lamond
The Centre for Sleep Research, 5th Floor, Basil Hetzel Institute, The Queen Elizabeth Hospital, Woodville Rd, Woodville SA 5011, Australia; nicole.lamond{at}unisa.edu.au

Aims: To investigate factors that may contribute to performance adaptation during permanent night work.

Methods: Fifteen healthy subjects participated in an adaptation and baseline night sleep, directly followed by seven simulated eight-hour night shifts (2300 to 0700 hours). At the end of each shift they were taken outside and exposed to natural light for 20 minutes. They then slept from approximately 0800 hours until they naturally awoke.

Results: There was a significant increase in mean performance on a visual psychomotor vigilance task across the week. Daytime sleep quality and quantity were not negatively affected. Total sleep time (TST) for each of the daytime sleeps was reduced, resulting in an average cumulative sleep debt of 3.53 hours prior to the final night shift. TST for each of the daytime sleep periods did not significantly differ from the baseline night, nor did TST significantly vary across the week. There was a significant decrease in wake time after sleep onset and sleep onset latency across the week; sleep efficiency showed a trend towards greater efficiency across the consecutive daytime sleeps. Hours of wakefulness prior to each simulated night shift significantly varied across the week. The melatonin profile significantly shifted across the week.

Conclusions: Results suggest that under optimal conditions, the sleep debt that accumulates during consecutive night shifts is relatively small and does not exacerbate decrements in night-time performance resulting from other factors. When sleep loss is minimised, adaptation of performance during consecutive night shifts can occur in conjunction with circadian adaptation.

Keywords: neurobehavioural performance; adaptation; prior wake; shiftwork; sleep debt

Abbreviations: ANOVA, analysis of variance; DLMO, dim light melatonin onset; EEG, electroencephalogram; NREM, non-rapid eye movement sleep; PSG, polysomnographic; PVT, psychomotor vigilance task; REM, rapid eye movement sleep; RT, response time; SOL, sleep onset latency; SPT, sleep period time; SSS, Stanford Sleepiness Scale; SWS, slow wave sleep; TST, total sleep time; WASO, wake time after sleep onset

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