Rheumatoid arthritis (RA) is a chronic inflammatory disease with both environmental and genetic determinants.1 Shift work, defined here as working outside regular day shifts, has been found to induce changes in the biomarkers of systemic inflammation such as leucocyte count2 3 and T cell responses.4 5 We examined whether shift work also predicts development of RA.

Participants were drawn from the Finnish Public Sector Study,6 a prospective cohort study of 70 376 local government employees who responded to a survey in 1997–2004. We excluded those who had a history of RA at baseline (n=939) or had missing data on age, sex, or self-reported work schedule (n=2035), leaving a final sample of 67 402 participants aged 18–65 years, 80% women. RA was identified based on registered data if one or more of the following criteria were met: medically certified sick leave due to RA; hospital treatment of RA; special reimbursements for the treatment and rehabilitation of RA; prescription and purchasing of medication specifically for RA (gold); and disability pension due to RA. Shift work was assessed by the direct question: “Do you work regular day shifts?” (yes/no). Covariates were sex, age, body mass index (kg/m²), physical activity (MET h/week), alcohol use (g/week), psychological distress (General Heath Questionnaire; cut-off point >4), smoking (never, ex-smoker or current smoker) and socioeconomic position (non-manual or manual).

During the mean follow-up of 4.2 years, new RA was diagnosed in 229 participants (1-year incidence per 100 000 was 91 in women and 39 in men). Shift work was associated with a 1.33-fold (95% CI 1.01 to 1.75) increase in age-adjusted risk of RA in women (table 1). The association remained after adjustment for all covariates. In men, shift work was not associated with RA.

These data show an excess risk of developing RA among female shift workers. This association was not accounted for by socioeconomic factors, health behaviours or psychological distress. A corresponding excess risk was not seen in men, but this should be interpreted cautiously given the low number of incident cases in men (n=22). Our study benefits from large sample size, prospective study design and utilisation of several national health registers. However, we had no information on the length of exposure to shift work or type of shift work (two shifts, three shifts or permanent night shift). Health records included only the severe forms of RA, so early or less severe cases were missed. It is possible that some cases of RA were not incident, but this is an unlikely source of major bias given that reverse causality from RA to shift work is not plausible.

This study is the first to report an association between shift work and RA in women, raising the hypothesis that shift work may increase the risk of RA. Further large-scale studies are needed to test whether our findings are generalisable to other populations and to determine whether adverse changes in the immune system mediate the association.