Frequent Daytime Napping is Detrimental to Human Health: A phenotype-wide Mendelian Randomization Study

ABSTRACT Background It remains controversial whether daytime napping is beneficial for human health. Objective To examine the causal relationship between daytime napping and the risk for various human diseases. Design Phenotype-wide Mendelian randomization study. Setting Non-UK Biobank cohorts reported in published genome-wide association studies (GWAS) provided the outcome phenotypes in the discovery stage. The UK Biobank cohort provided the outcome phenotypes in the validation stage. Participants The UK Biobank GWAS included 361,194 European-ancestry residents in the UK. Non-UKBB GWAS included various numbers of participants. Exposure Self-reported daytime napping frequency. Main outcome measure A wide-spectrum of human health outcomes including obesity, major depressive disorder, and high cholesterol. Methods We examined the causal relationship between daytime napping frequency in the UK Biobank as exposure and a panel of 1,146 health outcomes reported in genome-wide association studies (GWAS), using a two-sample Mendelian randomization analysis. The significant findings were further validated in the UK Biobank health outcomes of 4,203 human traits and diseases. The causal effects were estimated using a fixed-effect inverse variance weighted model. MR-Egger intercept test was applied to detect horizontal pleiotropy, along with Cochran’s Q test to assess heterogeneity among the causal effects of IVs. Findings There were significant causal relationships between daytime napping frequency and a wide spectrum of human health outcomes. In particular, we validated that frequent daytime napping increased the risks of major depressive disorder, obesity and abnormal lipid profile. Interpretation The current study showed that frequent daytime napping mainly had adverse impacts on physical and mental health. Cautions should be taken for health recommendations on daytime napping. Further studies are necessary to precisely define the best daytime napping strategies.