Major study finds no link between night shift work and breast cancer

Cancer

"Night shifts do not increase breast cancer risk, study concludes," reports The Guardian.

This reassuring headline follows analysis of information about more than 100,000 women in the UK over a 10-year period.

A link between night shift work and breast cancer risk was first proposed 30 years ago.

This was because of fears that working with artificial light at night could disrupt the body clock or the production of the hormone melatonin.

In turn, these imbalances could increase the risk of breast cancer.

But studies since then have been inconclusive.

Some studies reporting that there could be a link were criticised as they did not allow for differences according to the length of time people had worked shifts, whether they worked shifts before or after having children, or different risks for different types of breast cancer.

This study was designed to answer these more detailed questions.

The study found no evidence that having worked night shifts within the past 10 years affected a woman's risk of any kind of breast cancer during the 9.5 years of follow-up.

The results should reassure women who work night shifts that their hours of work are unlikely to affect their chances of getting breast cancer.

Where did the story come from?

The researchers who carried out the study were from the Institute of Cancer Research in the UK.

The study was funded by the charity Breast Cancer Now and the Institute of Cancer Research.

It was published in the peer-reviewed British Journal of Cancer on an open access basis, so the study is free to read online.

The study was reported on in an accurate and balanced way by most of the UK media.

Several news stories mentioned that shift working had previously been identified as a probable cancer risk by the International Agency for Research on Cancer in 2007, but the agency was expected to revise its advice this year.

What kind of research was this?

This was a cohort study. Cohort studies are a good way to see whether there are links between possible risk factors (in this case night shift work) and outcomes such as breast cancer.

They're more reliable if, as happened in this case, the figures are adjusted to take account of other possible risk factors and look at changing risk factors over time.

What did the research involve?

Researchers recruited 113,700 women aged 16 or older from the UK, starting in 2003 and finishing in 2014.

Women were asked to fill in detailed questionnaires:

at the start of the study
2.5 years after the start of the study
6 years after the start of the study
9.5 years after the study (for those who'd been in the study long enough)

Women were asked:

whether they'd been diagnosed with breast cancer
whether they'd worked night shifts (10pm to 7am) during the previous 10 years
how many hours they'd worked on average each night and week
how long they'd worked night shift jobs
what type of occupation they worked in
how old they'd been when they started working nights
whether they'd had children before starting night shifts

Other questions covered potential confounding factors linked to breast cancer risk, including:

close family history
use of hormonal contraceptives or hormone replacement therapy (HRT)
smoking and alcohol use
body mass index (BMI)
whether they had children and at what age
physical activity levels
age they started their periods
whether they'd entered the menopause

The researchers looked at results from the 102,869 women who did not have breast cancer (or any other type of cancer) at the start of the study to see whether women who'd worked night shifts were more likely to be diagnosed with cancer during the follow-up period.

They adjusted their figures to take account of women's ages and other potential risk factors identified in the questionnaires.

What were the basic results?

During the study, 2,059 cases of breast cancer were diagnosed (around 2% of women in the study).

The researchers found 17.5% of women in the study had worked night shifts during the previous 10 years, and 214 (1.2%) of them developed breast cancer.

Overall, women who said they'd worked night shifts during the previous 10 years were no more likely to be diagnosed with breast cancer during follow-up than women who had not worked night shifts (hazard ratio [HR] 1.0, 95% confidence interval [CI] 0.86 to 1.15).

The researchers found no difference in risk related to:

type of work done on night shift
age women started working nights
working nights related to time of having first child
time since last night shift work
average hours worked per night
average nights worked per week
number of years worked at night
combined number of hours worked at night
menopausal status
type of breast cancer measured by receptor status or cell type

They did find 1 possible link between the average number of hours worked per week and breast cancer.

But they said this may have been down to chance, since all the other measures of length of time worked on night shift and breast cancer were negative, as was the overall association.

How did the researchers interpret the results?

The researchers said: "We found no evidence for an overall increase in risk of breast cancer for women who had been night shift workers within the last 10 years, or by hours worked per night, nights worked per week, average hours worked per week, cumulative years of employment, cumulative hours or time since cessation of such work."

Conclusion

The findings of the study suggest that working night shifts is unlikely to increase the risk of breast cancer.

The study looked in detail at shift work and time spent working shifts, and at different types of breast cancer.

It found no convincing evidence that night shift work increases the risk of any type of breast cancer.

But the study has a couple of limitations to be aware of.

The researchers only looked at night shift work done or started in the 10 years before the start of the study. The average age of the women in the study was 45.

So if night shift work affected a woman's long-term risk of breast cancer when she was young, but less so when she was older, it's possible that this study would not have picked that up.

The other limitation is that there were fewer women doing longer or more frequent night shifts, so there's slightly less confidence in these results.

We do not know whether breast cancer can be prevented, as the causes are not fully understood.

But there are some steps you can take to reduce your risk of breast cancer: