The association between daytime napping and risk of diabetes: a systematic review and meta-analysis of observational studies

To investigate the association between daytime napping and prevalent/incident diabetes mellitus (DM) based on systematic review and meta-analytic data.The electronic databases of Embase, Medline, Pubmed and Web of Science were searched. Relevant studies were extracted by two reviewers independently. The associations between daytime napping (irrespective of duration), long nap (≥1 h/day) and short nap (<1 h/day), and risk of DM were assessed according to study types. Overall estimates were pooled using either fixed- or random-effect with inverse variance meta-analysis. Heterogeneity of included studies was assessed using the I2 test and possible cause of the heterogeneity was examined by meta-regression analyses.Ten studies (four cross-sectional and six longitudinal cohort) comprising a total of 304,885 individuals and 20,857 cases of DM were included in the systematic review, with an average napping prevalence of 47%. Nappers were found to have increased risk of DM in both cross-sectional and cohort studies. However, significant heterogeneity was present. Long nap (≥1 h/day) was associated with both prevalent and incident DM; in particular, those with a daily nap over 1 h had a 31% increased risk of developing DM during follow-up (95% confidence interval: 2-67%). Conversely, no such association was found in individuals with short naps (<1 h/day) in cohort studies.Long daytime napping over 1 h per day was associated with increased risk of both prevalent and incident DM. Further studies are needed to confirm the findings.