UC Berkeley

This dissertation focuses on assessing the effect of shift work on worker health, specifically identifying and evaluating selected working time characteristics as risk factors for incident hypertension and Type II diabetes mellitus. Shift work is a common occupational exposure across many sectors of the economy with a prevalence of near 20% of the US workforce. Shift work has also been consistently associated with many adverse health outcomes, including cancer. While most research has focused on the associations of night shift work and female breast cancer, recent evidence suggests that shift work may also cause an increased risk of reproductive disorders, gastrointestinal ulcers and cancers, prostate cancer, hypertension and Type II diabetes. In order to protect worker’s health while maintaining the efficiency of 24-hour work, it is important to identify the potentially harmful characteristics of shift work.

In chapter 1 we discuss definitions for shift work and present current hypotheses for how shift work impacts human health. We also discuss the challenges facing environmental health scientists and epidemiologists as they define and characterize a complex exposure, such as shift work. An effective assessment of the impact of shift work on human health is dependent on development of metrics of shift work that are designed to capture the biological impact of shift work, rather than workplace schedule variation.

In chapter 2 we develop and characterize the prevalence of selected working hour characteristics that may impact circadian rhythms in the American Manufacturing Cohort. In this first description of shift work in a US manufacturing workforce, we demonstrate that working hour characteristics such as shift type, duration, intensity, rotational direction, and social aspects of work need to be considered concurrently. Furthermore, these working hour characteristics vary by annual shift schedule. We identify, as expected, that permanent day workers have the lowest percentage of quick returns and rotations (as a permanent schedule would imply). However, working the day shift does not provide absolute protection from potentially disruptive characteristics of working hours since long work hours, as well as quick returns and rotations occurred when workers switched from morning to afternoon shifts (both considered day work). Notably, we also demonstrate that older workers are more likely to work permanent day schedules, while racial minorities such as African American workers are more likely to perform work with rotations.

In chapter 3 we present an analysis examining the impact of recent night and rotation work exposure and risk of hypertension. We apply the same definitions of night and rotational work described in chapter 2, and present evidence that combinations of recent night and rotational work increase the risk of hypertension. In particular, we identify higher risk of hypertension among all levels of recent night work compared with non-night workers. The highest risk of hypertension were among those with 95-100% night work, workers who would normally be considered ‘permanent night workers’. This suggests that permanent night workers are experiencing circadian rhythm disruption even though their work schedules are not rotating. Furthermore, we observe elevated hazard ratios for all combinations of night and rotational work compared to non-night workers. In particular, the hazard ratio was almost 4-fold for workers with mostly night work and frequent rotations compared with non-night workers. Even those workers with mostly non-night work and infrequent rotations had a 2-fold risk of hypertension, indicating the potential importance of recent shift work and hypertension risk.

In chapter 4 we analyzed the association between cumulative months of night work and average percentage of night shifts over follow-up using Cox proportional hazard models. The results from the Cox proportional hazard models provide some modest evidence that night work exposure may be associated with an increased risk of diabetes. This association was seen when night work exposure is classified as either the cumulative number of months of night work or the average percentage of night shifts over follow-up.

Chapter 5 concludes the dissertation with a summary of the results from each chapter, the strengths and limitations of the current work, and a discussion of the next steps in shift work research.