“Women who work long-term night-shift jobs … are twice as likely to develop breast cancer,” is the story in The Independent, as well as a number of other newspapers.
The report is based on a study which found that women who worked night shifts for 30 years or more were twice as likely to develop breast cancer.
Other studies have previously suggested a link between shift work and breast cancer, but they have mainly been confined to nurses. In this study, researchers looked at women working in many different roles.
One hypothesis discussed in the study involves the hormone melatonin – which has been suggested as having cancer protective properties. Exposure to light is known to reduce production of melatonin. So night shift workers going from a day environment to an artificial light environment at night would have lower levels of this hormone.
The results may sound worrying, but women who work nights are advised not to panic. It’s worth noting that no link was found between higher breast cancer risk and periods of night work which were shorter than 30 years. It is also possible that the lifestyle associated with night shifts – such as lack of exercise – may contribute to the risk.
Further research is needed on this important topic, in particular into whether disruption of the sleep cycle can affect the hormones involved in some breast cancers.
Where did the story come from?
The study was carried out by researchers from Queen’s University and University of British Columbia, Canada, and Drexel University in the US. It was funded by the Canadian Institutes of Health Research.
The study was published in the peer-reviewed medical journal: Occupational and Environmental Medicine.
It was covered fairly, if a little sensationally, in the media. Some of the headlines did not make it clear that only long-term night shift working – 30 years or more – was associated with an increased breast cancer risk.
What kind of research was this?
This was a case-control study looking at the relationship between night shift work and breast cancer among more than 2,300 women, living in Vancouver, Canada. In this retrospective type of study, a group of participants who have a particular outcome (in this case breast cancer) are matched with a group who do not have that outcome. Researchers then look at whether specific factors (in this case a history of night shift work), are associated with the outcome in question.
The researchers point out that night shift work has been suggested as a risk factor for several types of cancer, including breast cancer, as we saw with a 2012 news story claiming (with little evidence) that “Night shifts ‘cause 500 breast cancer deaths a year’”.
One hypothesis is that the sleep hormone melatonin is disrupted by night shift work and that this may in turn increase the production of another hormone, oestrogen. Oestrogen is involved in the development of two in every three cases of breast cancer.
The authors say that previous studies on this topic may not have been accurate in defining night shift work and also, that studies have been mainly limited to nurses. They wanted to examine women in different occupations and to look at different shift patterns. They also looked at the type of tumours involved and whether their growth was influenced by the hormones oestrogen or progesterone. This is called a tumour’s hormone receptor status.
Treatment for hormone-receptor-positive cancers involves hormone therapies such as the medication tamoxifen.
What did the research involve?
The researchers recruited women with breast cancer and a control group from two areas of Canada. For the breast cancer group, women were aged between 20 and 80, with a diagnosis of either in-situ or invasive breast cancer and no previous cancer history (other than non-melanoma skin cancer).
The control group were recruited from women who had had normal mammogram results or a diagnosis of benign breast disease, in cancer screening tests, and were matched by age to the breast cancer group.
In total, 1,134 women who had breast cancer and 1,179 controls were recruited.
All the women were sent a study questionnaire with questions on potential confounders for breast cancer risk, including:
- medical and reproductive history
- family history of cancer
- history of tobacco and alcohol intake
- physical activity
- occupation – past and present
- places of residence
They either completed it themselves or gave their responses in a telephone interview. They also provided blood samples and granted access to their medical records concerning breast health.
The information about occupation was used to categorise each job as either night shift or non night shift. For the main analysis, night shift jobs were those where 50% or more of time was spent on evening and/or night shifts, whether rotating or permanent. Researchers also used other information in their analysis, such as the start and end times of shifts.
The duration of night shift work was classified into four categories – none, 0-14 years, 15-29 years, and 30 or more years. Type of job was also classified into one of 10 categories according to a Canadian national occupation classification.
For all breast cancer cases, researchers collected data from hospital and medical records on the type of tumour. They looked at each cancer’s hormone receptor status – whether tumour growth was influenced by production of the hormones oestrogen or progesterone.
They also assessed the relationship between night shift work and breast cancer risk according to whether women had been through the menopause.
They analysed their results using standard statistical methods and adjusted the findings for the confounders listed above.
What were the basic results?
About one third of all the women – both cases and controls – had done night shift work. Women who had done night shift work for either 0-14 or 15-29 years were at no higher risk of breast cancer than those who had not.
However, women who had worked night shifts for 30 years or more were at more than twice the risk of breast cancer (odds ratio [OR] 2.21, 95% confidence interval [CI] 1.14 to 4.31) compared to other groups.
The results were similar for both health and non-health care workers. No association was found between night shift work and the hormone status of cancer.
How did the researchers interpret the results?
The researchers say that long-term night shift work in a range of occupations is associated with increased breast cancer risk and not limited to nurses, as in most previous studies.
Disruption of the sleep hormone melatonin could influence the risk of cancer by its effect on oestrogen production, they argue. However, the results of this study were inconclusive as to whether there was an association between night shifts and hormone status of the cancer.
Since shift work is necessary for many occupations, more research is needed into how specific shift patterns might influence breast cancer risk, to formulate the development of healthy workplace policies, they argue.
This study appears to support previous research suggesting a link between long term night shift work (in this study, of 30 years or more duration) and higher breast cancer risk. It looked in detail at patterns of night shift work across a number of occupations.
One limitation is that it relied on participants recalling their occupations and patterns of shift work, sometimes over lengthy periods, which might have led to inaccuracies. A study which followed women forward in real time rather than looking at outcomes retrospectively (an inception cohort study), would be more reliable.
It is also possible that lifestyle factors related to night shift work may contribute to a higher breast cancer risk, although researchers tried to take account of these in their analysis.
If the researchers’ hypothesis proves correct and night shift working does increase breast cancer risk, then it should be possible to compensate for this increase by taking a number of preventative steps. These include quit smoking if you smoke, maintain a healthy weight, eat a healthy balanced diet, moderate your consumption of alcohol and take regular exercise.