Background Rheumatoid Arthritis (RA) is a chronic inflammatory disease, with symptoms varying with the circadian rhythm (i.e. stiffness and pain are increased in the mornings, and associated with increased inflammatory cytokine production). Shift work, defined here as working outside regular day shifts, can cause circadian dysfunction, and is considered to be a potential contributory factor to a range of clinical and pathological conditions, including inflammation. There is limited data on the relation between shift work and RA, but one previous cohort study (1) reported an association between shift work and RA development in women.

Objectives To further investigate the relation between shift work and the risk of subsequent development of RA in women and men separately.

Methods Among participants in a population-based health survey (N=30447), incident cases of RA were identified by linking the cohort to local and national RA registers, followed by a structured review of the medical records. For each validated case, four controls, matched for sex, year of birth and year of screening, who were alive and free of RA when the index person was diagnosed with RA, were selected. Life style related factors and other exposures had been assessed in the survey using a questionnaire. Individuals who responded yes to the question “Does your work mean inconvenient working hours or shift work?” were classified as shift workers. The impact of shift work on the risk of RA was examined in conditional logistic regression models, stratified by sex.

Results A total of 172 patients (136 women/36 men, mean age at RA diagnosis 63 years) were diagnosed with RA after inclusion in the health survey. The median time from inclusion to RA diagnosis was 5 years (range 1–13). Among pre-RA cases, 55 individuals, 47 women (38%) and 8 men (27%), were classified as shift workers. The corresponding figures for the controls were 28% among women and 25% among men. Shift work was associated with an increased risk of subsequent development of RA in women (odds ratio (OR) 1.60; 95% confidence interval (CI) 1.01–2.57). There was no significant association between shift work and RA in men (OR 1.04; 95% CI 0.35–3.05). The estimated impact of shift work on the risk of RA in women was similar in separate models adjusted for level of formal education (OR 1.56; 95% CI 0.97–2.50) and smoking (OR 1.53; 95% CI 0.95–2.45).

Conclusions In this nested case-control study, women with inconvenient working hours or shift work had an increased risk of a future diagnosis of RA compared to those with regular working hours. This association did not appear to be explained by potential confounders, i.e. level of formal education or smoking. This suggests that exposures associated with shift work in women, possibly including circadian dysfunction, may contribute to the development of RA.

1. Puttonen et al. Ann Rheum Dis 2010; 69:779–80

Disclosure of Interest None declared