Recent evidence suggests that sleep traits might have an impact on the risk and incidence of breast cancer in women, but there are no concrete studies to back the evidence. The objective of this study is to examine the causal effect of sleep traits on the risk of breast cancer.

This is a mendelian randomization study that included the participants of the UK Biobank prospective cohort study and Breast Cancer Association Consortium (BCAC) case-control genome-wide association study. The participants included a total of 156,848 women in the multivariable regression, 122,977 breast cancer cases, and 105,974 controls. The participants were assessed based on self-reported chronotype (morning or evening preference), insomnia, and sleep duration, and the primary outcome of the study was the diagnosis of breast cancer. 

Morning preference was inversely associated with the risk of breast cancer. Some associations were identified between sleep duration and insomnia. The researchers found an inverse association between morning preference and breast cancer and a positive association with increased sleep duration. The associations between insomnia and breast cancer were consistent.

The research concluded that morning preference had a protective effect on the risk of breast cancer in women, whereas increased sleep duration was linked with adverse effects.