- World Health Organisation categorised shift work as carcinogenic in 2007
- Found unsocial working hours disrupts circadian rhythm or ‘body clock’
- Oxford University tested theory on data from more than a million women
- It found the risk was no different to women who have never worked shifts
Kate Pickles For Mailonline
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They are known to impact on the health of employees, affecting sleep patterns, meal times and people’s ability to exercise.
But, despite the rumours, night shifts do not cause breast cancer, a new Oxford University study found.
In 2007, the World Health Organisation categorised shift work as carcinogenic because it was claimed the unsocial working hours disrupted circadian rhythm or the internal ‘body clock’.
The conclusion was based on animal experiments and at the time there was only limited evidence about breast cancer risk in humans.
Night shifts may impact on diet and exercise but do not increase the risk of breast cancer, according to a study by the University of Oxford
In this study, by Oxford’s cancer epidemiology unit and commissioned by Cancer Research UK, the theory was tested on more than a million women.
Three large UK population studies found night shift work has little or no effect on the incidences of breast cancer.
The study involved 522, 246 participants in the Million Women Study, 22,559 EPIC-Oxford participants, and 251,045 UK Biobank participants.
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They answered questions on shift work and were followed to see who got breast cancer.
No increase in breast cancer risk associated with night shift work, including long-term night shifts, was found in any of these studies.
The three UK studies were then compared with those from all seven previously published prospective studies – two in the USA, two in China, two in Sweden, and one in the Netherlands.
The 10 studies included a total of 1.4 million women among whom 4,660 breast cancers occurred in women who reported ever having done night shift work.
Associate professor Dr Ruth Travis said: ‘We found that women who had worked night shifts, including long-term night shifts, were not more likely to develop breast cancer, either in the three new UK studies or when we combined results from all 10 studies that had published relevant data.’
The study which looked at data from more than a million women found there was no increased risk of breast cancer for women who worked at night
Compared with women who had not reported doing night shift work, the combined relative risks taking all 10 studies together were 0.99 for any night shift work, 1.01 for 20 or more years of night shift work, and 1.00 for 30 or more years night shift work.
In other words, the incidence of breast cancer was essentially the same whether someone did no night shift work at all or did night shift work for several decades, she said.
Sarah Williams, Cancer Research UK’s health information manager, said: ‘This study is the largest of its kind and has found no link between breast cancer and working night shifts.
‘Research over the past years suggesting there was a link has made big headlines, and we hope that today’s news reassures women who work night shifts.
‘Breast cancer is the most common cancer in the UK and research to fully understand the different risk factors is vital so that we can give women clear health advice.’
She said women can reduce their risk of breast cancer by keeping a healthy weight, drinking less alcohol and being active.
The study was published in the Journal of the National Cancer Institute.
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