Quantifying cumulative circadian disruption from shift work and associations with health outcomes in a large cohort of nurses.

Study objectives: Night shifts are commonly used as proxy for circadian disruption (CD) in epidemiological studies. However, other shift types can also cause CD if they interfere with a worker's biological night. We quantified and compared cumulative CD to night shift exposure and assessed their associations with health-related outcomes.

Methods: Shift work exposure was derived from questionnaire data for 42,119 nurses for the period 2012-2017. Cumulative CD was estimated as the total overlap (hours) between shift work and preferred sleep-wake times. Pearson's correlation (r) assessed relationships between cumulative CD and night shift exposure. Associations with sleep disturbances, medication use, and overweight were analyzed using Poisson regression.

Results: The median cumulative CD among shift workers was 1,674 hours over six years (interquartile range=432-3,153). High CD (≥2,809 hours) was associated with increased prevalence of sleep problems (incidence rate ratio [IRR]=1.10, 95% confidence interval [CI]=1.07-1.13), melatonin use (IRR=1.86; 95%CI=1.70-2.04), sleep medication use (IRR=1.15; 95%CI=1.01-1.32), and overweight (IRR=1.04; 95%CI=1.02-1.07). The number of performed night shifts strongly correlated with cumulative CD (r=0.93), and using night shifts as proxy for CD gave similar results. However, among shift workers who did not perform night shifts, high CD was still associated with increased sleep problems and melatonin use.

Conclusion: Cumulative CD is associated with sleep- and health disturbances, even among shift workers who do not perform night shifts, underlining its potential role in disease development. While night shifts remain a practical proxy in large-scale studies, our study highlights the importance of using more nuanced, individualized measures of CD.