A cohort study examined the association between shift work and myocardial infarction among 5,000 factory workers in India. The crude relative risk was 1.8 (95% CI: 1.5–2.1). After stratifying by smoking status and adjusting for age, the stratum-specific relative risks were: smokers 1.2 (95% CI: 0.9–1.5) and non-smokers 1.1 (95% CI: 0.8–1.4). What is the most likely explanation for this change in the association?

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