Sleep duration and mortality, influence of age, retirement, and occupational group.

ABSTRACT

Previous work has shown that both long and short sleep duration is associated with increased mortality, with lowest risk around 7 hr. This has had widespread impact on views on the optimal sleep duration. However, age, being employed/retired, and blue-/white-collar status, may influence the time available for sleep and thus, confound the association. We investigated the role of these factors on the association between sleep duration and mortality. We used employed and retired participants (N = 25,430) from the Swedish National March Cohort and Cox proportional hazards regression to model the shape of the association. We found a significant U-shaped association in a multivariable model with a hazard ratio (HR) of 1.24 (95% confidence interval [CI] 1.10, 1.39) for <5-hr sleep duration, and a HR of 1.30 (95% CI 1.12, 1.51) for ≥9-hr sleep duration, with the lowest HR for 7 hr, but with a span of low HRs from 5 to 8 hr. Unadjusted values showed a pronounced U-shape. Adjusting for age accounted for most of the attenuation in the multivariable model. Stratification into five age groups showed a significant U-shape only in those aged >60.3 years at baseline. The shape of the association did not differ between blue-/white-collar workers, nor between employed and retired groups. We conclude that the U-shaped association between sleep duration and mortality is present only in older individuals.