Environmental factors are factors that can occur in our surroundings that are associated with increased or decreased risk of breast cancer.
An environmental factor that may be associated with an increased risk of breast cancer include exposure to light at night through shift work.
The evidence is indicative of an association between the factor and increased or decreased risk of breast cancer, but there is not sufficiently strong evidence to be more certain.
Some studies have suggested that shift work that disrupts the normal sleeping cycle may be associated with an increased risk of breast cancer. The evidence is stronger for an increased risk of breast cancer after 20 years or more of shift work.
Shiftwork is work during hours other than standard daylight hours. It can be permanent (regular work on one shift only), continuous (all days of the week) or discontinuous (interruption on weekends).
Working at night, in light, might reduce the production of the hormone ‘melatonin’ which is normally produced during the night under dark conditions. Melatonin might play a role in reducing DNA damage so reducing its production might increase the risk of breast cancer.
Evidence classification: Suggestive
The evidence is suggestive of an association between shift work that disrupts the circadian rhythm and an increased risk of breast cancer. However, there are some limitations to the evidence. The supportive evidence is mostly from case-control studies rather than the more robust cohort studies. There is some evidence of a dose-response relationship. The evidence is stronger for an increased risk of breast cancer either after over 20 years of night shift work or after shorter periods with many consecutive shifts.1
Shift work can disrupt the circadian system, with hormonal effects, including on melatonin. Melatonin is produced with a regular circadian rhythm, and disruption to its levels indicates circadian disruption. Melatonin has anti-proliferative effects on cultured human cancer cells, and there is some evidence of an anti-oestrogenic effect.2 There is also evidence from animal models that melatonin inhibits or reduces DNA damage by free radicals.2 However, data from clinical trials on the role of melatonin in lowering the risk of breast cancer are lacking.
The International Agency for Research on Cancer (IARC) has indicated that ‘shiftwork involving circadian disruption’ is ‘probably carcinogenic to humans’ (Group 2A).3 It concluded that there was limited epidemiological evidence in humans for a link between night shift work and cancer; most of the human epidemiological evidence was for breast cancer.
Some, but not all, recent meta-analyses support an association between night shift work and breast cancer risk. This association is seen mostly in case–control studies rather than the more robust cohort studies. There is some evidence of a dose-response relationship. The evidence is stronger for an increased risk of breast cancer after more than 20 years of night shiftwork.4
The Nurses’ Health Study (NHS) found an association between 20 or more years of rotating shiftwork and increased risk of breast cancer among women who were younger at recruitment.5 A second report from the NHS II reported a dose-response relationship with increased risk of breast cancer among premenopausal but not postmenopausal women who had increasing exposure to outdoor light at night and worked night shifts.6
- Hansen J (2017). Night shift work and risk of breast cancer. Current Environmental Health Reports 4 (3):325–339
- International Agency for Research on Cancer (2010). Painting, firefighting, and shiftwork, IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, volume 98, IARC Working Group on the Evaluation of Carcinogenic Risks to Humans, Lyon.
- Straif K, Baan R, Grosse Y, et al. (2007). Carcinogenicity of shift-work, painting and fire-fighting. Lancet – Oncology 8:1065–1066.
- Lin X, Chen W, Wei F, et al. (2015). Night-shift work increases morbidity of breast cancer and all-cause mortality: a meta-analysis of 16 prospective cohort studies. Sleep Medicine 16:1381–1387.
- Wegrzyn LR, Tamimi RM, Rosner BA, et al. (2017). Rotating night-shift work and the risk of breast cancer in the Nurses’ Health Studies. American Journal of Epidemiology 186(5):532–540.
- James P, Bertrand KA, Hart JE, et al. (2017). Outdoor light at night and breast cancer incidence in the Nurses’ Health Study II. Environmental Health Perspectives 125(8):087010.