Study also found women who sleep more than 8 hours are at higher risk
Women who start the day early are at lower risk of breast cancer.
New research presented at the 2018 NCRI Cancer Conference claims that 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. Breast cancer is the most common cancer among women in India.
The study of several hundred thousand women, which was investigating whether the way people sleep can contribute to the development of breast cancer, also found some evidence for a causal link between sleeping for longer and breast cancer.
Preference for mornings reduced the risk of breast cancer by 40% … women who slept longer than the recommended seven to eight hours had a 20% increased risk
Dr Rebecca Richmond, a research fellow in the Cancer Research UK 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).
“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.
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,” he added, “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.”