Advancing the Measurement and Understanding of Composite Sleep Health in Shift-Working Populations

Abstract

Introduction: Sleep is a fundamental biological process that supports mental and physical functioning. Its restorative value depends not only on sufficient duration, but also on sleep being consolidated, efficient, regular, and timed appropriately within environmental and physiological rhythms. These dimensions can be understood through the framework of sleep health, which conceptualises sleep as a multidimensional construct. This framework is particularly relevant for shift-workers, whose work schedules can disrupt circadian alignment, constrain sleep opportunity, and alter the distribution of behaviours across the 24-hour day. However, existing research has tended to examine individual sleep metrics or subjective sleep outcomes, with limited attention to objectively measured sleep health in real-world shift-working populations. This thesis investigated the measurement, distribution, and behavioural context of objectively measured composite sleep health (CSH) in shift-workers.

Methods: This thesis comprised five interconnected studies using accelerometery and diary data from free-living adult workers. An initial agreement study compared sleep metrics derived from wrist, mid-thigh, and low-back accelerometers. Subsequent studies applied a five-dimension CSH framework, comprising duration, variability, efficiency, timing, and regularity, to examine differences across work-pattern groups, associations with self-reported health and well-being outcomes, relationships with health behaviours, and associations with 24-hour time-use composition.

Results: Mid-thigh accelerometery produced sleep estimates broadly comparable to wrist-derived estimates, supporting its use for group-level CSH assessment. Objectively measured CSH was generally poor across the cohort, with night shift-workers showing the poorest multidimensional sleep health. CSH showed limited associations with self-reported psychosocial and health outcomes, although subjective sleeping troubles showed the clearest relationship. Most self-reported health behaviours explained little variance in CSH. The 24-hour time-use profile of shift-workers was dominated by sedentary time, followed by sleep, with smaller proportions of the day spent in light and moderate-to-vigorous physical activity. In compositional analyses, greater relative sedentary time was associated with lower CSH, while greater relative sleep was associated with higher CSH.

Conclusions: Objective CSH provides a practical framework for multidimensional sleep health, moving beyond single sleep metrics to show how sleep duration, variability, efficiency, timing, and regularity combine within real-world working conditions. By linking objectively measured sleep health with work-pattern differences, self-reported health outcomes, health behaviours, and 24-hour time use, this thesis demonstrates that poor sleep health in shift-workers is not simply a sleep-duration problem, but part of a broader behavioural and occupational context. These findings support the value of objective, multidimensional sleep assessment for identifying sleep-health burden in shift-working populations and for informing future research focused on feasible, context-sensitive strategies to improve sleep health.