Is night-shift work a risk factor for breast cancer?

Night-shift work and breast cancer risk, a controversial topic

Is night-shift work a risk factor for breast cancer? Experimental research just published gives the molecular basis for asserting that it may well be. Researchers at Paris-Saclay University studied the association between night work and increased tumour aggressiveness. They created a simulated jet-lag situation for mice that reproduced a change in the rhythm between day and night. The collaborative teams observed how disturbances in the rhythm had an impact on breast cancer stem cells and the development of breast tumours. In particular,  they detected an increased spread of cancer cells and the formation of metastases. They speculated that this is related to increased inflammation. 
Several mechanisms have been postulated before to explain an association between night work and breast cancer, with inhibition of the hormone melatonin by the use of artificial light at night widely seen as the most likely.

The topic is controversial. Some studies in the last year found a significantly higher risk of cancer among people working at night. However, other studies have found little or no correlation at all. Exactly a year ago a cohort study concluded that night shift work does not increase the risk of breast cancer.

The evidence from this new study provides important additional evidence to support the proposition that such a link does exist, and about the molecular mechanism involved. Risk factors for breast cancer are multiple: hereditary (mostly BRCA1 and 2 genes), behavioural (smoke, pollution, parity, lactation) or related to poor nutrition or alcohol consumption. Hormonal factors play an obvious role. There is still a long way to go to understand the influence of time-shifts and work-related stress.