Richmond RC, et al. - Via performing a Mendelian randomization study in a cohort of 156,848 women, researchers investigated whether sleep traits have a causal effect on the risk of breast cancer. There were 122,977 breast cancer cases and 105,974 controls from the Breast Cancer Association Consortium (BCAC) in the two sample mendelian randomization (MR) analysis. Although breast cancer and morning sleep were inversely related to each other, little data for an association between sleep term and insomnia symptoms were observed. Through 341 single nucleotide polymorphisms (SNPs) correlated with chronotype (91 SNPs associated with sleep duration and 57 SNPs associated with insomnia symptoms), one sample MR analysis in UK Biobank contributed some supportive proof for a protective impact of morning preference on breast cancer risk, but imprecise estimations for sleep duration and insomnia symptoms. A shielding effect of morning preference and unfavorable effect of increased sleep duration on breast cancer risk were observed through two sample MR using data from BCAC, whereas inconsistent data for insomnia symptoms were noted. Findings thus demonstrated consistent evidence for a protective effect of morning preference and suggestive evidence for an adverse effect of increased sleep duration on breast cancer risk.