The joint contribution of pain and insomnia to sickness absence and disability retirement: a register-linkage study among Norwegian and Finnish employees.

BACKGROUND: Pain and insomnia are both independently associated with work disability. Although pain and insomnia often co-occur, their joint associations with subsequent sickness absence and disability retirement have not been studied. We aimed to examine these associations in two prospective occupational cohorts while considering key covariates.
METHODS: Norwegian Hordaland Health Study (n = 6892, 59% women) and Finnish Helsinki Health Study (n = 6060, 78% women) data were used. Those with only pain, only insomnia or both conditions at baseline were compared with those with no pain and no insomnia. Work disability outcomes were derived from national and employers' register data. Medically certified sickness absence spells lasting 2 weeks or more and all-cause disability retirement were examined. Register-based follow-up was 4 years for sickness absence and 5 years for disability retirement. Covariates were sex, age, marital status, education, smoking, alcohol use, body mass index and blood pressure. Poisson and Cox regression models were fitted.
RESULTS: Both pain and insomnia were associated with subsequent sickness absence and disability retirement, but the associations were stronger for those reporting co-morbid pain and insomnia with support for a synergistic interaction effect, particularly regarding disability retirement. The associations were largely similar in both cohorts and remained after full adjustments.
CONCLUSIONS: This study is the first to report the separate and combined effects of pain and insomnia on objective health outcomes. Common patterns observed in two separate cohorts suggest that the combination of pain and insomnia might be particularly relevant for subsequent disability retirement.