With a recent study in this journal, Hansen and Lassen1 followed a promising path to answer pressing questions with regard to postulated causal links between shift work and cancer.2 By employing diurnal preference to characterise the individual's chronobiological propensity to work and sleep or rest, the authors may have used a marker of susceptibility to stress and strain experienced by individuals who work at chronobiologically unusual times. This novel approach may have captured information relevant to the undefined link, namely ‘circadian disruption’, in the ‘probable’ chain of cancer causation in shift workers which experts of the International Agency for Research on Cancer (IARC) identified in 2007.3 Here is why and how we could diligently follow-up on this thought-provoking case-control study.

First, some remarkable observations: In this study of night-shift work among women in the Danish military, diurnal preference was associated with differential breast cancer risks. For those with 884 or more cumulative night shifts, the OR for morning types was 3.9 (95% CI 1.6 to 9.5) and 2.0 (95% CI 0.7 to 5.8) for similarly exposed evening types; in the so-called intermediate types no increased risks were detected. The information on night work was collected before IARC's aforementioned classification, which lends methodological weight to the observations of this case-control study.

Second, some necessary information: When Hansen and Lassen refer to ‘morning type’ or ‘evening type’ preference, this corresponds colloquially to ‘larks’ and …