Background: Uncertainty remains whether associations for psychological distress and sickness absence (SA) observed between and within individuals differ, and whether age, gender and work-related factors moderate these associations.

Methods: We analyzed SA records of 41,184 participants of the Finnish Public Sector study with repeated survey data between 2000 and 2016 (119,024 observations). Psychological distress was measured by the General Health Questionnaire (GHQ-12), while data on SA days were from the employers' registers. We used a hybrid regression estimation approach adjusting for time-variant confounders-age, marital status, occupational class, body mass index, job contract type, months worked in the follow-up year, job demand, job control, and workplace social capital-and time-invariant gender (for between-individual analysis).

Results: Higher levels of psychological distress were consistently associated with SA, both within- and between-individuals. The within-individual association (incidence rate ratio (IRR) 1.68, 95% CI 1.61-1.75 for SA at high distress), however, was substantially smaller than the between-individual association (IRR 2.53, 95% CI 2.39-2.69). High levels of psychological distress had slightly stronger within-individual associations with SA among older (>45 years) than younger employees, lower than higher occupational class, and among men than women. None of the assessed work unit related factors (e.g. job demand, job control) were consistent moderators.

Limitations: These findings may not be generalizable to other working sectors or cultures with different SA policies or study populations that are male dominated.

Conclusions: Focus on within-individual variation over time provides more accurate estimates of the contribution of mental health to subsequent sickness absence.