Being a morning person is associated with a lower risk of developing breast cancer than being an evening person, suggests new research in the BMJ. Meanwhile, sleeping longer than the recommended seven to eight hours a night could also carry an increased breast cancer risk, the results suggest.
Using Mendelian randomisation, an international team of researchers analysed genetic variants associated with three particular sleep traits: chronotype, sleep duration and insomnia for 180,216 women in the UK Biobank study and 228,951 women in the Breast Cancer Association Consortium (BCAC) study.
An observational analysis of data from the UK Biobank study showed morning preference was inversely associated with breast cancer, compared with evening preference. Mendelian analysis of UK Biobank data provided some supportive evidence for a protective effect of morning preference on breast cancer risk. Mendelian analysis of data from the BCAC study supported findings for a protective effect of morning preference and showed an adverse effect of increased sleep duration on breast cancer risk.
The authors said the findings "provide strong evidence for a causal effect of chronotype on breast cancer risk” and “have potential implications for influencing sleep habits of the general population to improve health.”