Depression and insomnia are common and frequently co-morbid. Both are associated with impaired occupational functioning. The objective of this historical cohort study was to compare their relative impact upon medically certified disability pension award.

Data from a population-based health survey in Nord-Trøndelag County in Norway (HUNT-2) was linked with a comprehensive national social security database. Participants within working age (20-66) not already claiming disability pension were included in the study (N=37 308).

We compared insomnia and depression as predictors of disability pension award between 18-48 months after the health survey. Both insomnia and depression approximately doubled the risk of disability pension award after adjustment for multiple health and sociodemographic factors. Co-occurrence was less prevalent (2.1%) than expected and produced an additive risk for pension award. 25% of the 3800 participants with insomnia had no other health condition. Due to higher prevalence, insomnia predicted more work-related disability than depression in terms of population attributable fractions.

Depression is consistently recognized as a major contributor to work disability and is frequently the eliciting diagnosis in disability pension award. Our results suggest that insomnia has an equally important and independent role, particularly amongst the younger group, but rarely found in official registries of disability pension causes. This suggests that this potentially treatable factor has considerable economic impact, and should receive more attention in clinical and public health management.