In a recent study, researchers found 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.
The study examined several hundred thousand women and aimed to find out whether the way people sleep can contribute to the development of breast cancer.
It found some evidence for a causal link between sleeping for longer and breast cancer.
Dr. Rebecca Richmond at the University of Bristol, UK and colleagues looked at data from 180,215 women enrolled with the UK Biobank project.
They also examined 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).
They tested people’s preference for morning or evening, sleep duration and insomnia.
These factors had previously been identified by three recent UK Biobank genome-wide association studies.
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 included data from BCAC of 122,977 cases of breast cancer and 105,974 women without the disease (the controls).
The researchers 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 randomization 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.
The findings of a protective effect of morning preference on breast cancer risk in this 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.
The researchers believe their findings have implications for policy-makers and employers.
The team planning to investigate the mechanisms underlying the effects of different sleep characteristics on the risk of developing breast cancer.
The finding was presented at the 2018 NCRI Cancer Conference.
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Source: 2018 NCRI Cancer Conference.