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-effects 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.

Obesity indicators as mediators of association between daytime napping and type 2 diabetes mellitus: the Guangzhou Biobank Cohort Study

“…First, the causal pathway from napping to obesity and then to diabetes could not be confirmed in this study. However, previous prospective cohort studies and Mendelian 24] and obesity [21][22][23][24]27]. These results support the causal link being inferred.…”

“…Moreover, reverse causation (i.e., patients with diabetes tended to have daytime napping) cannot be ruled out, although daytime napping is a long-accepted culture in China, which might have been formed since young age. Notably, previous prospective cohort studies and Mendelian randomization have shown a positive association from daytime napping to obesity, but not vice versa [21][22][23][24], supporting the causal direction from napping to obesity. Second, assessment of daytime napping was self-reported and thus misclassification error was possible.…”

“…One of the possible explanations lies in obesity. Previous studies including Mendelian randomization have shown that daytime napping was likely causally associated with both general and central obesity [21][22][23][24]. Moreover, obesity has been suggested as a major causal factor of T2DM [25,26].…”