Objectives Exposure misclassification is a common challenge in shift-work epidemiology. This study used a large national survey of Canadian nurses to examine shift-work’s effects on depression; multiple exposure definitions with varying levels of specificity were applied to illustrate the impacts of exposure assessment.

Methods The analytic sample (n=11,450) was obtained from the 2005 National Survey of the Work and Health of Nurses. Logistic regression was used to assess relationships between shift-work and depression for high, moderate, and low specificity definitions of shift-work exposure. The low and moderate specificity definitions described shift timing (day/shift and day/evening/night/rotating, respectively); the high specificity definition described both shift timing (day/evening/night/rotating) and frequency of rotation (slow/medium/rapid/undefined). All model estimates were bootstrapped and adjusted for the potential confounding effects of sociodemographic, health, and work variables.

Results The high specificity shift-work definition model showed the strongest relationships, with increased odds of depression in the rapid rotating shift group (OR=1.51, 95% CI=0.91–2.51) and in the undefined rotating group (OR=1.67, CI=0.92–3.02), relative to the regular day group. Odds of depression were decreased in the slow rotating group (OR=0.79, 95% CI=0.57–1.08). For the low and moderate specificity exposure definition models, weak relationships were observed for all shift categories (OR range 0.95 to 0.99).

Conclusions This study’s findings support associations between shift-work and depression, and the need for specific and hypothesis-driven exposure assessment in future studies to correctly identify exposure-response relationships and to appropriately target health interventions.