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Fischer, Dorothee (2015): Of 'Islands and Pancakes': the shape of sleep in shift work. Dissertation, LMU München: Faculty of Medicine



Circadian disruption, the mismatch of internal (i.e. chronotype) and external time (i.e. shift-associated working time), is one potential mechanism underlying shift work-associated diseases. Shift work involving circadian disruption was classified as probably carcinogenic to humans by the IARC in 2007. Despite postulating circadian disruption as the causal link between shift work and diseases, the IARC did not provide a clear definition. The scarcity of theoretical frameworks as well as quantitative measures impedes the systematic investigation of its causes and consequences. Any quantification of circadian disruption, however, needs to take into account individual internal time, as otherwise true effects will be over- or underestimated. This dissertation describes three projects studying circadian disruption in real shift workers. In Project One, the sleep-wake behaviour of 35 shift workers in a 12-h rotational schedule is examined using the MCTQShift 165 and wrist-recorded actimetry data. The field study demonstrates the importance of chronotype and shift sequence for circadian misalignment and sleep duration setting the groundwork for Project Two. A method to quantify circadian disruption of the sleep-wake cycle is proposed called ‘mid-sleep deviations’ that integrates two crucial aspects of sleep: internal time and sleep history. The measure uncovers a unique, distinct and chronotype-specific geometry of actimetry-based sleep-wake behaviour in 53 shift workers. Comparison with existent measures of circadian disruption confirms the validity of ‘mid-sleep deviations’ and highlights its additional information value. In Project Three, the proposed measure of ‘mid-sleep deviations’ is applied to evaluate sleep log data from 97 shift workers employed in seven different shift schedules. The results of mixed model analyses challenge current guidelines on night and shift work showing that the number of consecutive (night) shifts beneficial for an individual depends strongly on chronotype.