A Mendelian randomization (MR) study suggests a protective effect of morning preference on breast cancer risk add to other evidence of decreased risk of schizophrenia and depression. The study included 156 848 women in the multivariable regression and one sample MR analysis (7784 with a breast cancer diagnosis) and 122 977 breast cancer cases and 105 974 controls in the two sample MR analysis.In multivariable regression analysis, morning preference was inversely associated with breast cancer (hazard ratio 0.95). Using 341 single nucleotide polymorphisms (SNPs) associated with chronotype, 91 SNPs associated with sleep duration, and 57 SNPs associated with insomnia symptoms, one sample MR analysis provided some supportive evidence for a protective effect of morning preference on breast cancer risk (0.85). Two sample MR supported findings for a protective effect of morning preference (inverse variance weighted odds ratio 0.88) and adverse effect of increased sleep duration (1.19) on breast cancer risk. Results were largely robust to sensitivity analyses accounting for horizontal pleiotropy. The findings provide strong evidence for a causal effect of chronotype on breast cancer risk. Source: https://www.bmj.com/
