Of 'Islands and Pancakes'

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.