Working night shifts is not linked to an increased risk of breast cancer, a major prospective UK analysis has found.

Wednesday 29 May 2019      Our research
Night shift sign

In a new ten-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, published today in the British Journal of Cancer, 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.

There was a statistically significant trend found specifically with average night hours worked per week, but no association observed between any other variables relating to night shift work and breast cancer risk, and the finding on hours per week alone is not supported by previous evidence or any proposed biological explanation.

The findings also add to a recent Breast Cancer Now Generations Study analysis – by the same authors and also published in the British Journal of Cancer – which suggested that exposure to light at night while sleeping does not increase a woman’s risk of developing breast cancer. 

Since the 1970s, it has been suggested that a potential link between shift work and an increased breast cancer risk could be explained by exposure to light at night disrupting the body’s internal clock, which could cause suppression of the sleep hormone melatonin and raise oestrogen levels in the body.

Co-author Dr Michael Jones, Staff Scientist in Genetics and Epidemiology at The Institute of Cancer Research, London, said:

A possible link between exposure to electric light at night and an increased risk of breast cancer was first proposed more than 30 years ago, but research has so far been inconclusive. Large-scale studies like the Generations Study can play a vital role in teasing out the subtle effects of lifestyle on breast cancer risk – or in this case in finding no evidence of association after all.

In our new study, we found no overall link between women having done night shift work in the last ten years and their risk of breast cancer – regardless of the different types of work they did involving night shifts, and the age at which they started such work.

Although night shifts may have other effects on people’s health, and we still don’t know the effect of a person’s body clock being disturbed for very long periods of time, it is reassuring to see more evidence suggesting that night shifts are not linked with a higher risk of breast cancer.

Baroness Delyth Morgan, Chief Executive at Breast Cancer Care and Breast Cancer Now, which funded the study, said:

We hope these findings will help reassure the hundreds of thousands of women working night shifts that it’s unlikely their job patterns are increasing their risk of breast cancer.

This question has been widely debated in recent decades and has understandably caused concern, and it’s encouraging that the evidence now suggests night shift work has no impact on breast cancer risk. We now await the International Agency for Research on Cancer’s review of the global evidence to gain even more clarity on this issue.

We know that breast cancer risk is affected by a combination of our genes, lifestyle choices and events throughout life, and there is never one single cause of the disease. But, with many contributing factors, it’s vital we support more women to do what they can to help shift the odds in their favour.

Whilst there are some things we can’t change, there are steps all women can take to lower their breast cancer risk, such as maintaining a healthy weight, keeping physically active and drinking less alcohol. Even small changes are a great start.

The Breast Cancer Now Generations Study is a landmark prospective study of the causes of breast cancer that is following over 113,000 UK women for over 40 years.

The Study – based at The ICR – has already led to a number of significant discoveries into the interlinked causes of breast cancer, including clarifying that women taking combined HRT are 2.7 times more likely to develop breast cancer than non-users, and that smoking is associated with an increased risk of breast cancer particularly if started during adolescence.

Breast Cancer Care and Breast Cancer Now thanks M&S for their generous support of the Breast Cancer Now Generations Study.