[Shift work and breast cancer].

The International Agency on Research on Cancer (IARC) has recently classified "shiftwork that involves circadian disruption" as "probably carcinogenic to humans" (Group 2A) on the basis of "limited evidence in humans for the carcinogenicity of shift-work that involves nightwork", and "sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)". The epidemiologic evidence of a relationship between shift and night work and breast cancer in women is based upon nine studies, six of which suggest a moderately increased risk to develop breast cancer after prolonged exposure to shift and night work. The aim of this paper is to summarize the possible physio-pathological mechanisms (internal disruption of biological circadian rhythms and clock genes, melatonin suppression through light by night, sleep deprivation) and the problems connected with a proper risk assessment of the risk for breast cancer risk in women shift workers.