Objectives Shift work entailing circadian rhythm disruption is linked to chronic disease. One suggestion is that obesity mediates the relationship, yet research investigating the link between shift work and obesity report mixed findings, with a propensity towards a positive association. Since a paucity of research in this area has been conducted in Canada, this study examined the association between shift work and obesity within two Canadian studies; one of Ontario females, and the other, a highly educated nation-wide sample.

Method Healthy subjects from the Canadian Study of Diet, Lifestyle, and Health cohort study (CSDLH; 1447 males, 2170 females), and the Ontario Women’s Diet and Health case-control study (OWDHS; 3474 female controls) were analyzed. Overweight was defined as BMI≥25, <30, and obesity: BMI≥30.

Reported occupation was linked to shift work exposure assessment, defined as regular evenings, nights, or rotating work, derived from Survey of Labour and Income Dynamic data. The association between shift work (tertiles), and obesity, separated by sex and dataset, was determined through polytomous and logistic regression controlling for potential confounders.

Results In adjusted regression (reference=lowest exposure), intermediate shift work was negatively associated with overweight for CSDLH females (OR: 0.51, 95% CI: 0.26–0.99), yet positively associated with obesity for OWDHS females (OR: 1.71, 95% CI: 1.08–2.71). For males, high shift work exposure was negatively associated with increased weight (OR: 0.57, 95% CI: 0.35–0.92).

Conclusions The association between shift work and obesity is multifaceted and depends on population factors. Further investigations within a highly educated and diversified workforce are warranted.