Sleep disturbances and fatigue: independent predictors of sickness absence? A prospective study among 6538 employees

Background: Although sleep disturbances and fatigue are common conditions, frequently shown to be associated with sickness absence, only a few studies have prospectively investigated their independent effects on sickness absence, while adjusting for depressive symptoms. This study aims (i) to examine whether sleep disturbances and fatigue are independently related to the onset of register-based sickness absence of epsilon 3 weeks during a 1-year follow-up in a representative sample of the Danish workforce and (ii) to determine if possible associations are gender-specific. Methods: Data were used from the Danish Work Environment Cohort Study and linked with sickness absence data from the Danish National Register of Social Transfer Payments. A total of 6538 employees, 3178 men and 3360 women, were included in the analyses. Results: Sleep disturbances predicted risk of sickness absence after adjustment for covariates, but lost statistical significance after further adjustment for depressive symptoms. Fatigue among men predicted risk of sickness absence [Hazard ratio (HR) = 1.25, 95% confidence intervals (CI) 1.00-1.56] after adjustment for covariates, depressive symptoms and sleep disturbances. Conclusion: Sleep disturbances in both genders and fatigue in women did not predict sickness absence after depressive symptoms were taken into account. In men, fatigue was significantly related to future sickness absence, also when adjusted for depressive symptoms and sleep disturbances. Further prospective studies are needed to explore the pathways from fatigue to sickness absence in more detail. The study suggests that early detection and treatment of fatigue in men should be high on the stakeholder's agenda.