Objective Environmental factors play a prominent role in MS aetiology. The aim of this study was to investigate the potential association between shift work and MS risk, which has previously never been investigated.

Methods This report is based on two population-based, case-control studies, one with incident cases (1343 cases, 2900 controls) and one with prevalent cases (5129 cases, 4509 controls). Using logistic regression, the occurrence of MS among subjects who have been exposed to shift work at various ages was compared with that of those who have never been exposed by calculating the odds ratio with a 95% confidence interval.

Results In both studies, there was a significant association between working shift at a young age and occurrence of MS (OR 1.6, 95% CI: 1.2–2.1 in the incidence study, and OR 1.3, 95% CI: 1.0–1.6 in the prevalence study). In the incident study, the OR of developing MS was 2.0 (1.2–3.6) among those who had worked shifts for three years or longer before age 20, compared with those who had never worked shifts. The OR for the corresponding comparison in the prevalent study was 2.1 (1.3–3.4).

Interpretation The observed association between shift work at a young age and occurrence of MS in two independent studies strengthens the notion of a true relationship. Consequences of shift work such as circadian disruption and sleep restriction are associated with disturbed melatonin secretion and enhanced pro-inflammatory responses and may thus be part of the mechanism behind the association.