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Résumé (anglais)

BACKGROUND Shift work is common with wide-ranging implications for worker health. It is also complex, presenting challenges for exposure assessment in epidemiological studies and the development of strong evidence to inform health interventions and policies. This dissertation generated new information on the measurement, assignment, and determinants of shift work exposure, in order to address important limitations in this field of epidemiology. METHODS In Chapter 2, 152 full-shift personal light-at-night measurements were collected from 102 shift workers in emergency services and healthcare to investigate exposure variability and different exposure metrics. In Chapter 3, multiple exposure indicators were constructed for a national survey of nurses (n=11,450) to demonstrate the impacts of exposure assignment on observed relationships between shift work and depression. In Chapter 4, interviews were conducted with 88 employers in one Canadian province to examine determinants of workplace-level shift work policies and practices. RESULTS In Chapter 2, average light-at-night exposures varied across occupations and settings; between-group variance exceeded between-worker and within-worker variance, and all exposure metrics were moderately-to-highly correlated. In Chapter 3, the strongest relationships between shift work and depression were observed in the model with highest exposure precision, defined by shift timing and rotation intensity, whereas weak relationships were observed in models with lower exposure precision, defined by shift timing or presence/absence of shift work. In Chapter 4, long duration shifts varied by industry and were more likely in large workplaces; shift work education/training was more likely in large workplaces and those without seasonal shift work; and nighttime lighting policies were more likely in workplaces reporting that maintenance, client service needs, or prior nighttime incidents affected shift work. CONCLUSIONS This dissertation can inform future epidemiological studies of shift work. Chapter 2 identified high-level exposure indicators (e.g., occupation groupings) and flexibility in the choice of highly correlated metrics for light-at-night exposure studies. Chapter 3 showed that increasing the precision of exposure assignment reduced measurement error and effect attenuation for the outcome of depression. Chapter 4 identified determinants of workplace-level shift work policies and practices (e.g., industry, employer size, temporary work, and employer motivations) to consider in future research and interventions. Lay Summary One third of Canadians perform shift work (something other than a regular daytime schedule). Shift work is linked to various physical and mental health outcomes, however strong evidence to clarify these relationships and inform health interventions has been slow to develop. One reason is that shift work “exposure” is complex and difficult to assess in health research, where risks are evaluated by comparing groups of people with differing amounts of exposure to a hazard. This dissertation addressed three evidence gaps concerning the assessment of shift work exposure in health research. Chapter 2 measured and characterized light-at-night exposure levels of shift workers; Chapter 3 assessed how different definitions of shift work exposure affects researchers’ ability to detect health effects; and Chapter 4 identified workplace characteristics that may determine certain types of shift work policies and practices. These findings should be considered in future studies of shift work and health.