Research on circadian disruption and cancer was stimulated by the 2007 evaluation by the International Agency for Research on Cancer of light at night and circadian rhythm disruption as probably carcinogenic to humans.1 Circadian rhythm disruption ultimately affects a large part of the population in modern societies and not only those in shift work. The experimental evidence on light at night and breast cancer is very strong, clearly showing the potential for light at night and circadian rhythm disruption to cause cancer. The evidence from epidemiological studies is mostly positive notwithstanding inconsistencies and lack of a clear dose–response. Since the first meta-analysis by Megdal et al,2 several systematic reviews and meta-analyses have been performed, three of them published in 2013, attempting to summarise the association between night shift work and breast cancer risk. Two3 ,4 of the three recent reviews concluded that the epidemiological evidence is overall weak or insufficient, whereas the third review5 supported the hypothesis. Even though inconclusive, all reviews highlighted the methodological limitations of the existing studies and the heterogeneity between studies in methods including basic definitions of exposure. So, do we need more evidence and what should this be?

The paper by Grundy et al6 provides more evidence supporting the association between shift work and breast cancer risk, as most case–control studies have done so far. In this well-conducted large case–control study, a twofold risk of breast …