Shift work and breast cancer risk: is there a link?

Several studies, mostly among female nurses, have suggested that night shift work is a risk factor for breast cancer. One of the main mechanisms underlying this association is the light induced suppression of melatonin, a hormone that protects against cancer that is normally produced at night while we sleep. Other mechanisms include circadian disruption (disruption of daily biological rhythms) and sleep deprivation.

Even so, there are still some doubts about the validity of some of the existing studies. It is remarkable that 4 systematic reviews and meta-analyses were published in 2013 with inconclusive results. For this reason, researchers at CREAL, a research centre of ISGlobal alliance, Manolis Kogevinas and Kyriaki Papantoniou, comment in an editorial published in the journal Occupational Environmental Medicine on the Grundy et al paper published in the same issue f this journal and also reflect on needs of future research in this field in order to confirm the effects of night shift work and circadian disruptionin breast cancer and other tumors.

As they say in this article, the experimental evidence on exposure to light at night and breast cancer is very strong. Evidence from epidemiological studies is mostly positive despite the inconsistencies and the lack of a clear dose-response. One of the main limitations of existing literature is that shift work was defined in different ways across studies. In the study by Grundy et al, a refined exposure assessment (exposure (night shift work) was carefully assessed based on individual lifetime occupational histories.) was used and a two-fold increase in breast cancer risk was found after long-term (>30 years) exposure to night shift work.

In addition, researchers remind us that this research on circadian disruption and cancer was stimulated by the 2007 evaluation by the International Agency for Research on Cancer (IARC) of shift work that involves circadian disruption as probably carcinogenic to humans

Papantoniou K, Kogevinas M. Shift work and breast cancer: do we need more evidence and what should this be? Occup Environ Med 2013.