Long term night-shift work raises a woman’s risk for breast cancer, researchers found.

Those with a 30-plus year history of night-shift work had a 2.21-fold greater risk for breast cancer compared with the general population (95% CI: 1.14-4.31), Kristan Aronson, PhD, of Queens University Cancer Research Institute in Kingston, Ontario, and colleagues reported in Occupational and Environmental Medicine.

Previous studies have shown an association between night-shift work and increased breast cancer risk among women in the military and those in healthcare professions, such as nursing, but not in the general population.

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Aronson and colleagues found a particularly strong association between those with a 30-plus year history of night work in health occupations vs. those with no history of night work (OR=3.11). The odds were also elevated in those working nights in nonhealth jobs (OR=2.25), but the findings were not statistically significant (95% CI: 0.92-5.52).

“Long-term night-shift work in a diverse mix of occupations is associated with increased breast cancer risk and not limited to nurses, as in most previous studies,” the researchers wrote.

The study included 1,134 breast cancer cases from the British Columbia Cancer Registry and 1,179 age-matched controls, who had no history of cancer other than nonmelanoma skin cancer, and who participated in a mammography screening program in British Columbia or Kingston, Ontario.

Approximately one-third of participants in both groups had worked a job in which at least 50% of their time was spent working evening or night shifts, including both rotating and permanent night-shift schedules.

The proportion of breast cancer cases were similar among women who reported night-shift work duration of 3 months to 14 years and 15 to 29 years, but there was a notably higher proportion of cancer cases among those who had worked nights for 30 years or more (2.5% vs. 1.1%), the researchers found.

Shorter duration of night-shift work did not significantly correlate with breast cancer risk. The overall association for long-term night-shift work showed no interaction by tumor hormone receptor status.

“As shift work is necessary for many occupations, understanding of which specific shift patterns increase breast cancer risk, and how night-shift work influences the pathway to breast cancer is needed for the development of healthy workplace policy,” the researchers wrote.

Although the biological mechanism is not yet understood, the researchers hypothesized it involves melatonin, which is known to regulate circadian rhythm based on light exposure and may also have properties that help protect against cancer.

“While increased light exposure during night shifts is thought to decrease production of melatonin, thereby increasing cancer risk, other mechanisms are also possible,” they wrote.

Study limitations included lack of data on the number of consecutive night shifts participants worked, and the possibility for recall and selection bias.


  1. Grundy A et al. Occup Environ Med. 2013; doi:10.1136/oemed-2013-101482.