Night shifts 'linked to increased breast cancer risk'

“Irregular sleeping patterns have been ‘unequivocally’ shown to lead to [breast] cancer in tests on mice, a study suggests,” BBC News reports. Scientists are concerned a similar effect may occur in women working night shifts.

This study looked at mice genetically modified to develop breast cancer. They were exposed to either a normal cycle of 12 hours of light and 12 hours of darkness or an inverted cycle. It found mice in the inverted group gained more weight and developed breast cancer sooner.

These findings appear to support previous research suggesting a link between night-shift work and breast cancer, which we discussed in 2012 and 2013.

Further human research will be required to determine if shift working does increase the risk and what measures can be taken to minimise this.

The researchers go as far as recommending that women with a known predisposition to breast cancer (such as having genetic mutations linked to breast cancer) should avoid shift work. But, obviously, not everyone has the luxury of picking and choosing what hours they work.

If you work night shifts, you can offset your risk of breast cancer and other cancers by quitting smoking if you smoke, maintaining a healthy weight, eating a healthy, balanced diet, moderating your consumption of alcohol and taking regular exercise. 

Where did the story come from?

The study was carried out by researchers from the National Institute for Public Health and the Environment (RIVM), Erasmus University Medical Center and Leiden University Medical Center, all in the Netherlands, and Ludwig-Maximilian University in Germany.

Funding was provided by the RIVM Strategic Programme and the Dutch Ministry of Social Affairs and Employment.

It was published in the peer-reviewed medical journal Current Biology on an open access basis, so it is free to read online or download as a PDF.

The BBC’s reporting of the study was accurate and made it very clear at the start of the story that the research involved mice, so the results may not necessarily apply to humans.  

What kind of research was this?

This laboratory research assessed the effects of alternating light cycles in mice. This involved conducting two related studies – a randomised longitudinal study and a cross-sectional study – to assess breast cancer risk with alternating light cycles.

Findings from animal studies are useful for making discoveries to be investigated further in humans.  

What did the research involve?

This study aimed to investigate the causal links between chronic circadian rhythm disturbance (CCRD) and increased cancer risk. CCRD is a term used to describe persistent disruption of the body clock – the normal sleep-wake cycle. 

Breast cancer-prone mice were placed in a cycle of 12 hours of light and 12 hours of dark. At the end of every week, the light or dark phase was extended to 24 hours to invert the light-dark cycle.

Two studies were performed on the mice, controlling for other possible factors contributing to cancer risk:

• all mice were melatonin deficient
• neither group was exposed to sunlight
• there was no vitamin D difference between groups

The only other possible difference between the groups was the timing of food intake, which may be disruptive to metabolism.

Longitudinal study

When the mice reached eight weeks old they were randomly assigned to remain under a normal 12:12-hour light-dark cycle or undergo a weekly alternating 12:12-hour light-dark cycle. Measurements of body weight and tumour-free survival were taken.

Cross-sectional study

Mice who did not take part in the longitudinal study were analysed further in a cross-sectional study. These mice stayed under the light-dark cycle or weekly light-dark inversion, representing circadian rhythm disturbance. After 18 cycles, blood and tissue samples were collected for analysis. 

What were the basic results?

In the longitudinal study, mice exposed to weekly light-dark inversions saw a larger increase in body weight compared with those kept in the normal light-dark conditions.

This was not the result of food intake as a significantly smaller amount of food was consumed by mice in the inversion group. This effect could be seen at six weeks, but it only became significant at week 24.

There was no significant difference in body weight gain in the cross-sectional part of the study, perhaps because there were fewer light-dark inversions.

Mice exposed to the weekly inversions showed a decrease in tumour suppression. This is carried out by certain genes that try to prevent normal cells turning cancerous.

The time taken to breast tumour development was reduced by 17% in the inversion mice compared with the control mice, at 42.6 weeks compared with 50.3 weeks.

The cross-sectional study indicated the circadian rhythm was still disrupted seven days after the light-dark pattern was switched. 

How did the researchers interpret the results?

The researchers concluded that, “Animals exposed to the weekly light-dark inversions showed a decrease in tumour suppression. In addition, these animals showed an increase in body weight.

“Importantly, this study provides the first experimental proof that CRD increases breast cancer development.” 

Conclusion

This study in mice appears to support previous research suggesting a link between night-shift work and breast cancer. It looked at an inverted pattern of light and dark to assess whether this is linked to greater risk.

The researchers found mice exposed to weekly light-dark inversions saw a larger increase in body weight and quicker tumour development.

One limitation with this study is it is an animal study, which reduces the generalisability of findings. However, as there are a number of studies that have drawn similar conclusions – some in humans – these findings do add to the research in this area.

Shift working can disturb what is known as the circadian rhythm – the internal body clock. This can disrupt the normal workings of a hormone called melatonin and lead to poor sleep and chronic fatigue.

Rotating shift work and a persistent lack of quality sleep can also disrupt the production of insulin, which may increase the risk of someone developing type 2 diabetes. It has also been linked to a range of chronic conditions, such as obesity, depression, diabetes and heart disease.

The Health and Safety Executive has some useful and practical advice for people who work night shifts:

• take extra care if you drive to and from work as your concentration may be impaired – if possible, it may be a better idea to use public transport
• identify a suitable sleep schedule of at least seven hours a day – you may find it useful to keep a diary to assess what sleep times suit you best
• try to create an environment that promotes good sleep – for example, heavy curtains or an eye mask may help you sleep during the day
• make changes to your diet to improve alertness and digestion – smaller healthy snacks during your shift may be a better idea than one big meal
• limit your use of stimulants such as caffeine or energy drinks, as well as sedatives such as alcohol – while they may bring short-term benefits, they are unlikely to help in the long term
• try to get regular exercise of at least 30 minutes a day

If these findings are correct and shift working does increase breast cancer risk, it is even more important to modify other lifestyle factors known to increase the risk of several types of cancer.

These include quitting smoking if you smoke, maintaining a healthy weight, eating a healthy, balanced diet, moderating your consumption of alcohol and taking regular exercise.

Links To The Headlines

Poor sleeping patterns link to cancer. BBC News, July 21 2015

Links To Science

Van Dycke KCG, Rodenburg W, van Oostrom CTM, et al. Chronically Alternating Light Cycles Increase Breast Cancer Risk in Mice. Current Biology. Published online July 20 2015