Women who are "larks", which work better earlier in the day than at the end of the day, have a lower risk of breast cancer, according to new research conducted by the University of Bristol today [Tuesday 6 November].
The study of several hundred thousand women, who was investigating whether the way people sleep can contribute to the development of breast cancer, also found some evidence of a causal link between longer sleep and breast cancer.
Researcher Rebecca Richmond, a researcher with the Cancer Integrative Epidemiology Program at Cancer Research and the MRC Unit of Integrative Epidemiology at the University of Bristol, analyzed data from 180,215 women enrolled in the UK Biobank project and 228,951 women who participated in the study. a broad genomic association study of breast cancer conducted by the International Consortium of Breast Cancer Association (BCAC), which has the largest collection of genetic data on women with breast cancer obtained so far.
"Using genetic variants associated with morning or evening preference, sleep duration and insomnia, which had been previously identified by three recent UK genomic association studies, we investigated whether these sleep characteristics have a causal contribution to the risk to develop breast cancer. " cancer, "she said.
The team used a method called "mendelian randomization," which uses genetic variants associated with possible risk factors, such as sleep characteristics, to investigate whether they are involved in the cause of diseases such as breast cancer.
The Mendelian randomization analysis, which included BCAC data from 122,977 breast cancer cases and 105,974 women without the disease (the controls), found that morning preference reduced the risk of breast cancer by 40% compared to a night type (an owl). She also found that women who slept longer than the recommended seven to eight hours had a 20% increase in disease risk for each additional sleeping hour. 
The analysis of the data obtained from the UK's Biobank women (2,740 new cases of breast cancer and 149,064 controls) found similar results; morning preference reduced the risk of breast cancer by 48 percent. Mendelian randomization analysis of these data has revealed that approximately one person less than 100 will develop breast cancer if they prefer it in the morning compared to people who have a nightly preference. There was less evidence of an association with insomnia or sleep duration on the risk of breast cancer in this study.
The Doctor. Richmond said: "We would like to do more work to investigate the mechanisms behind these results, as estimates are based on morning or evening preference issues rather than whether people wake up sooner or later. words, it may not be the case that changing your habits alters the risk of breast cancer, which can be more complex than that.
"However, the results of a protective effect of morning preference on breast cancer risk in our study are consistent with previous research highlighting the role of night work and exposure to night light as risk factors for breast cancer, .
"We also found some evidence of a causal effect of increased sleep duration and sleep fragmentation on breast cancer, assessed using objective sleep measurements obtained from motion monitors used by about 85,000 participants of the UK Biobank.
"The method of Mendelian randomization applied in this research is particularly useful in identifying causal risk factors for diseases, since the genetic variants identified in relation to the characteristics of sleep are probably not influenced by any external or environmental factors nor by the development of cancer and therefore can be used to determine cause and effect relationships. "
Researchers believe their findings have implications for policymakers and employers. Richmond said, "These findings have potential policy implications for influencing sleep habits in the general population in order to improve health and reduce the risk of breast cancer among women."
Richmond and her colleagues plan to investigate the mechanisms underlying the effects of different sleep characteristics on the risk of developing breast cancer. "We would like to use genetic data from large populations to better understand how disrupting the body's natural biological clock may contribute to the risk of breast cancer," she said.
Cliona Clare Kirwan, of the University of Manchester, who is a member of the NCRI Breast Clinical Trials Group and who was not involved in this research, said: "These are interesting findings that provide further evidence of how our biological clock and our sleep is implicated in the onset of breast cancer.
"We already know that night work is associated with worse mental and physical health." This study provides further evidence to suggest that interrupted sleep patterns may play a role in cancer development. "The use of Mendelian randomization in this study allows researchers to examine Effect on breast cancer of different sleep patterns by looking at variations in certain genes already known to be associated with sleep characteristics. This helps avoid misleading conclusions that could have been affected by confounding factors. "
Investigating causal relationships between sleep characteristics and risk of breast cancer: a Mendelian randomization study & # 39; by Dr. Richmond et al. Presented at the 2018 NCRI Cancer Conference
 Some of the percentages and case numbers in this press release have been updated since the presentation of the abstract.
The study was funded by Cancer Research UK and the Medical Research Council.
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