Daytime napping, daytime sleepiness and the risk of metabolic diseases: dose-response meta-analysis
Tomohide Yamada, Nobuhiro Shojima, Toshimasa Yamauchi & Takashi Kadowaki
Background: Adequate sleep is important for good health, but it is not always easy to achieve because of social factors. Daytime napping is widely prevalent around the world. We recently published a meta-analysis, in which a J-shaped relationship was identified between naptime and cardiovascular diseases.
Objective: In this research, we also performed a meta-analysis to investigate the association between daytime sleepiness or napping and the risk of metabolic diseases, and to quantify the potential dose-response relation.
Methods: We searched electronic databases for articles published up to October 2015. The adjusted relative risk and 95% confidence interval were calculated with the random effect model. Dose-response relations were also evaluated by using restricted cubic spline models.
Results: About 300.000 Asian and Western subjects were selected. Pooled analysis revealed that excessive daytime sleepiness and a longer nap (60 min/day) each significantly increased the risk of type 2 diabetes by about 50% compared with the absence of these factors. In contrast, a shorter nap (60 min/day) did not increase the risk of diabetes (P=0.07). Nap time was not associated with an increased risk of obesity.
A dose-response meta-analysis using the cubic spline model showed a J-shaped relationship between nap time and the risk of diabetes or metabolic syndrome, with no effect of napping up to about 40 minutes/day followed by a sharp increase in the risk at longer times.
Conclusion: Nap time and diabetes or metabolic syndrome may be associated via a J-curve relation. Further studies are needed to confirm the efficacy of a short nap.