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 2 population-based, case–control studies, 1 with incident cases (1,343 cases, 2,900 controls) and 1 with prevalent cases (5,129 cases, 4,509 controls). 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.
Results: In both studies, there was a significant association between working shift at a young age and occurrence of MS (odds ratio (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 (95% CI, 1.2–3.6) among those who had worked shifts for 3 years or longer before age 20 years, compared with those who had never worked shifts. The OR for the corresponding comparison in the prevalent study was 2.1 (95% CI, 1.3–3.4).
Interpretation: The observed association between shift work at a young age and occurrence of MS in 2 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 proinflammatory responses and may thus be part of the mechanism behind the association.