Last updated Jan 24, 2019
Analyses of genetic variants show the effect of sleep on breast cancer risk
Women who are “larks”, functioning better at the beginning of the day than the end of the day, have a lower of risk breast cancer, according to new research presented at the 2018 NCRI Cancer Conference on the 6th November.
The study also found some evidence for a causal link between sleeping for longer and breast cancer
Dr Rebecca Richmond, a research fellow in the CRUK Integrative Cancer Epidemiology Programme and the MRC Integrative Epidemiology Unit at the University of Bristol, UK, and colleagues looked at data from 180,215 women enrolled with the UK Biobank project, and 228,951 women who had been part of a genome-wide association study of breast cancer conducted by the international Breast Cancer Association Consortium (BCAC), which has the largest collection of genetic data on women with breast cancer obtained so far.
“Using genetic variants associated with people’s preference for morning or evening, sleep duration and insomnia, which had previously been identified by three recent UK Biobank genome-wide association studies, we investigated whether these sleep traits have a causal contribution to the risk of developing breast cancer,” she said
The team used a method called ‘Mendelian randomisation’, which uses genetic variants associated with possible risk factors, such as sleep characteristics, to investigate whether they are involved in causing diseases such as breast cancer.
The Mendelian randomisation analysis, which included data from BCAC of 122,977 cases of breast cancer and 105,974 women without the disease (the controls), found that a preference for mornings reduced the risk of breast cancer by 40% compared with being an evening type (an ‘owl’). It also found that women who slept longer than the recommended seven to eight hours had a 20% increased risk of the disease per additional hour slept. 
Analysis of data obtained from the UK 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%. Mendelian randomisation analysis of these data revealed that approximately one less person per 100 will develop breast cancer if they have a morning preference compared to people who have an evening preference. There was less evidence of an association with either insomnia or sleep duration on risk of breast cancer in this study.
Dr Richmond said: “We would like to do further work to investigate the mechanisms underpinning these results, as the estimates obtained are based on questions related to morning or evening preference rather than actually whether people get up earlier or later in the day. In other words, it may not be the case that changing your habits changes your risk of breast cancer; it may be more complex than that.
“However, the findings of a protective effect of morning preference on breast cancer risk in our study are consistent with previous research highlighting a role for night shift work and exposure to ‘light-at-night’ as risk factors for breast cancer.
“We also found some evidence for a causal effect of increased sleep duration and sleep fragmentation on breast cancer, assessed using objective measurements of sleep obtained from movement monitors worn by around 85,000 UK Biobank participants.
The researchers believe their findings have implications for policy-makers and employers. Dr Richmond said: “These findings have potential policy implications for influencing sleep habits of the general population in order to improve health and reduce risk of breast cancer among women.”
The study was funded by Cancer Research UK and the Medical Research Council.
Abstract no: Poster 1822, “Investigating causal relationships between sleep characteristics and risk of breast cancer: a Mendelian randomization study.” Silent theatre 2, Tuesday 6 November.
The full paper is also published on a pre-print server (not yet peer-reviewed): https://doi.org/10.1101/457572
 Some of the percentages and numbers of cases in this press release have been updated since the abstract was submitted.