No link between night shift work and breast cancer

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Working night shifts is not linked to an increased risk of breast cancer, a major prospective UK analysis has found. In a new 10-year study of more than 100,000 women, funded by Breast Cancer Now, researchers examined extensive details of women’s night shift work, finding those who worked night shifts were no more likely to develop breast cancer than those who had not.

The results build on the conclusions of a 2016 meta-analysis that suggested shift work had little or no effect on breast cancer incidence – a study that had been challenged due to the older average age of participants and because it had limited detail on the nature of women’s shift work.

The findings come as the worldwide evidence on night shift work and cancer, including any possible impact on breast cancer risk, is set to be reviewed by the International Agency for Research on Cancer (IARC) in the summer of 2019.

Breast cancer is the UK’s most common cancer, with around 55,000 women and 350 men being diagnosed each year. For decades, it has been suggested that night shift work may increase a woman’s breast cancer risk, with the IARC concluding in 2007 that shift work disrupting the body’s sleep-wake cycle was ‘probably carcinogenic’. However, the evidence has been inconclusive and recent research has suggested there may be no impact on breast cancer risk after all.

In a comprehensive new analysis, scientists at The Institute of Cancer Research, London, studied data from 102,869 women from the Breast Cancer Now Generations Study – following participants for a median of 9.5 years to identify who went on to develop breast cancer.

At recruitment, detailed information was collected on any occupations these women had within the last ten years that regularly involved working between 10pm and 7am. The researchers investigated a wide range of variables including job type, the age at which women started and ended shift work, the nights they worked per week, the average hours worked per night and whether night shift work was started before first pregnancy.

The median age of participants at recruitment was 45 years, and 17.5% of participants (17,981 out of 102,869 women) reported being a night shift worker within the last ten years.

Data were also gathered on known breast cancer risk factors such as obesity (BMI), levels of physical activity, alcohol consumption, family history, age at first period and menopause, HRT use, number of children and age at their births, and duration of breastfeeding – allowing the study to control for a wide range of potential confounding factors.

The team then followed-up with repeat questionnaires six years later to update the night shift data.

The study observed that 2,059 out of 102,869 women went on to develop invasive breast cancer, but found no overall association between night shift work and the likelihood of developing breast cancer.

The researchers – led by Dr Michael Jones and Professor Anthony Swerdlow at The Institute of Cancer Research (ICR) – also found no significant difference in risk in relation to the type of night shift work, the age at which women started night shift work or whether night shift work was started before or after first pregnancy.

Background: It is plausible that night shift work could affect breast cancer risk, possibly by melatonin suppression or circadian clock disruption, but epidemiological evidence is inconclusive.
Methods: Using serial questionnaires from the Generations Study cohort, we estimated hazard ratios (HR) and 95% confidence intervals (95%CI) for breast cancer in relation to being a night shift worker within the last 10 years, adjusted for potential confounders.
Results: Among 102,869 women recruited in 2003–2014, median follow-up 9.5 years, 2059 developed invasive breast cancer. The HR in relation to night shift work was 1.00 (95%CI: 0.86–1.15). There was a significant trend with average hours of night work per week (P = 0.035), but no significantly raised risks for hours worked per night, nights worked per week, average hours worked per week, cumulative years of employment, cumulative hours, time since cessation, type of occupation, age starting night shift work, or age starting in relation to first pregnancy.
Conclusions: The lack of overall association, and no association with all but one measure of dose, duration, and intensity in our data, does not support an increased risk of breast cancer from night shift work in women.

Michael E Jones, Minouk J Schoemaker, Emily C McFadden, Lauren B Wright, Louise E Johns, Anthony J Swerdlow

Institute of Cancer Research material
Journal of the National Cancer Institute meta-analysis
British Journal of Cancer abstract

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