Association of napping and all-cause mortality and incident cardiovascular diseases: a dose–response meta analysis of cohort studies

Napping is a habit prevalent worldwide and occurs from an early age. However, the association between napping and the risk of incident cardiovascular disease (CVD) and all-cause mortality remains unclear. We conducted a systematic search of Medline, Embase, and Cochrane databases from inception to December 2019 for cohort studies investigating the association between napping and the risk of incident CVD and/or all-cause mortality. Overall estimates were calculated using random-effect models with inverse variance weighting. Dose–response meta-analysis was performed using restricted cubic spline models. A total of 313,651 participants (57.8% female, 38.9% took naps) from 20 cohort studies were included in the analysis. All-cause mortality was associated with napping overall (HR 1.19, 95% CI 1.12–1.26). Pooled analysis detected no association between daytime nap and incident CVD. However, in subgroup analysis including only participants who were female (HR 1.31, 95% CI 1.09–1.58), older (HR 1.36, 95% CI 1.07–1.72), or took a long nap (HR 1.34, 95% CI 1.05–1.63), napping was significantly associated with a higher risk of CVD. Dose–response analysis showed a J-curve relation between nap time and incident CVD. The HR decreased from 0 to 25 min/day, followed by a sharp increase in the risk at longer times. A positive linear relationship between nap time and all-cause mortality was also observed. Long napping was associated with increased risks of incident CVD and all-cause mortality. Further, large-scale studies and genetic studies need to confirm our conclusion and investigate the underlying mechanisms driving these associations.