title: Daytime Napping and the Risk of Cardiovascular Disease and All-Cause Mortality: A Prospective Study and Dose-Response Meta-Analysis.
abstract: OBJECTIVE
  To summarize evidence about the association between daytime napping and the risk of cardiovascular disease and all-cause mortality, and to quantify the potential dose-response relation.
  
  
  METHODS
  Meta-analysis of prospective cohort studies.
  
  
  RESULTS
  Electronic databases were searched for articles published up to December 2014 using the terms nap, cardiovascular disease, and all-cause mortality. We selected well-adjusted prospective cohort studies reporting risk estimates for cardiovascular disease and all-cause mortality related to napping. Eleven prospective cohort studies were identified with 151,588 participants (1,625,012 person-years) and a mean follow-up period of 11 years (60% women, 5,276 cardiovascular events, and 18,966 all-cause deaths). Pooled analysis showed that a long daytime nap (≥ 60 min/day) was associated with a higher risk of cardiovascular disease (rate ratio [RR]: 1.82 [1.22-2.71], P = 0.003, I(2) = 37%) compared with not napping. All-cause mortality was associated with napping for ≥ 60 min/day (RR: 1.27 [1.11-1.45], P < 0.001, I(2) = 0%) compared with not napping. In contrast, napping for < 60 min/day was not associated with cardiovascular disease (P = 0.98) or all-cause mortality (P = 0.08). Meta-analysis demonstrated a significant J-curve dose-response relation between nap time and cardiovascular disease (P for nonlinearity = 0.01). The RR initially decreased from 0 to 30 min/day. Then it increased slightly until about 45 min/day, followed by a sharp increase at longer nap times. There was also a positive linear relation between nap time and all-cause mortality (P for non-linearity = 0.97).
  
  
  CONCLUSIONS
  Nap time and cardiovascular disease may be associated via a J-curve relation. Further studies are needed to confirm the efficacy of a short nap.
authors.affiliation: Department of Diabetes and Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Japan.
authors.forename: Tomohide
authors.initials: T
authors.lastname: Yamada
authors.affiliation: Department of Diabetes and Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Japan.
authors.forename: Kazuo
authors.initials: K
authors.lastname: Hara
authors.affiliation: Department of Diabetes and Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Japan.
authors.forename: Nobuhiro
authors.initials: N
authors.lastname: Shojima
authors.affiliation: Department of Diabetes and Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Japan.
authors.forename: Toshimasa
authors.initials: T
authors.lastname: Yamauchi
authors.affiliation: Department of Diabetes and Metabolic Diseases, Graduate School of Medicine, University of Tokyo, Japan.
authors.forename: Takashi
authors.initials: T
authors.lastname: Kadowaki
mesh_terms: D000368:Aged; D002318:Cardiovascular Diseases / Q000453:epidemiology* / Q000209:etiology*; D002423:Cause of Death; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D011446:Prospective Studies; D012307:Risk Factors; D012890:Sleep / Q000502:physiology*; D013997:Time Factors
keywords: all-cause mortality; cardiovascular disease; meta-analysis; napping; siesta