Daytime napping and risk of type 2 diabetes: a meta-analysis of prospective studies

Purpose: Prospective studies reported inconsistent findings on the relationship between daytime napping and risk of type 2 diabetes (T2D). Categorized and dose-response meta-analyses were performed to quantify this relation.

Methods: Potentially eligible studies were identified by searching PubMed and Embase databases. Dose-response effects were assessed by the generalized least squares trend estimation and study-specific summary relative risks (RRs) with 95% confidence intervals (CIs) were computed with a random-effects model.

Results: Seven prospective studies including one US, four European, and two Chinese cohorts involving 249,077 participants and 13,237 cases of T2D were included. The overall analyses showed a 17% increased risk of T2D when comparing habitual nappers with non-nappers (RR = 1.17, 95% CI 1.08-1.27). By region, the summary RR was 1.21 (95% CI 1.17-1.26), 1.15 (95% CI 1.03-1.30) and 1.23 (95% CI 0.87-1.73) for the US, European, and Chinese studies, respectively. Limiting to five studies that excluded subjects with known major chronic disorders yielded a summary RR of 1.16 (95% CI 1.03-1.30). A dose-response analysis suggested an 11% (95% CI 7-16%) increased T2D risk for each increment in daytime napping of 30 min/day and, despite no evidence for nonlinearity (P _nonlinearity = 0.65), the increased risk of T2D for short nap (<50 min/day) was dominated by the US study.

Conclusions: This meta-analysis suggests that daytime napping is associated with an increased risk of T2D. Given the limited number of cohorts and inconsistency in terms of methodological and population characteristics across these cohorts, residual confounders and/or reverse causality cannot be fully addressed, and our findings should be interpreted with great caution. Future well-designed prospective studies are still warranted.