Grundy et al1 provided a diligent study into possible links between long-term shift work and incident breast cancer. Their investigation contributes to the evolution of research in a field of considerable relevance to occupational and environmental health. This is particularly true with regard to reducing ‘exposure’ classification errors by capturing diverse shift patterns and temporal details of the latter. Intriguingly, from a chronobiological perspective, there may be room for further improvements of classifying ‘exposure’; to examine this notion, we must answer a key question: what is the exposure-of-interest in this and …