title: Daytime napping and mortality from all causes, cardiovascular disease, and cancer: a meta-analysis of prospective cohort studies.
abstract: OBJECTIVE
  The association between daytime napping and mortality remains controversial. We conducted a meta-analysis to examine the associations between daytime napping and the risks of death from all causes, cardiovascular disease (CVD), and cancer.
  
  
  METHODS
  PubMed and Embase databases were searched through 19 September 2014. Prospective cohort studies that provided risk estimates of daytime napping and mortality were eligible for our meta-analysis. Two investigators independently performed study screening and data extraction. A random-effects model was used to estimate the combined effect size. Subgroup analyses were conducted to identify potential effect modifiers.
  
  
  RESULTS
  Twelve studies, involving 130,068 subjects, 49,791 nappers, and 19,059 deaths, were included. Our meta-analysis showed that daytime napping was associated with an increased risk of death from all causes [n = 9 studies; hazard ratio (HR), 1.22; 95% confidence interval (CI), 1.14-1.31; I(2) = 42.5%]. No significant associations between daytime napping and the risks of death from CVD (n = 6 studies; HR, 1.20; 95% CI, 0.96-1.50; I(2) = 75.0%) and cancer (n = 4 studies; HR, 1.07; 95% CI, 0.99-1.15; I(2) = 8.9%) were found. There were no significant differences in risks of all-cause and CVD mortality between subgroups stratified by the prevalence of napping, follow-up duration, outcome assessment, age, and sex.
  
  
  CONCLUSIONS
  Daytime napping is a predictor of increased all-cause mortality but not of CVD and cancer mortality. However, our findings should be treated with caution because of limited numbers of included studies and potential biases.
authors.affiliation: Department of Hepatobiliary Surgery, The Second Affiliated Hospital of Chongqing Medical University, Chongqing, China.
authors.forename: Guochao
authors.initials: G
authors.lastname: Zhong
authors.affiliation: Department of Respiratory Medicine, The Second Affiliated Hospital of Chongqing Medical University, Chongqing, China.
authors.forename: Yi
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Department of Cardiothoracic Surgery, The First Affiliated Hospital of Chongqing Medical University, Chongqing, China.
authors.forename: TieHong
authors.initials: T
authors.lastname: Tao
authors.affiliation: Department of Environmental Health, University of Cincinnati College of Medicine, Cincinnati, OH, USA.
authors.forename: Jun
authors.initials: J
authors.lastname: Ying
authors.affiliation: School of Public Health and Management, Chongqing Medical University, Chongqing, China. Electronic address: gczhong1991@sina.cn.
authors.forename: Yong
authors.initials: Y
authors.lastname: Zhao
mesh_terms: D000368:Aged; D002318:Cardiovascular Diseases / Q000401:mortality*; D002423:Cause of Death; D002940:Circadian Rhythm; D015331:Cohort Studies; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009369:Neoplasms / Q000401:mortality*; D011446:Prospective Studies; D012307:Risk Factors; D012890:Sleep; D013223:Statistics as Topic; D011795:Surveys and Questionnaires
keywords: Cancer; Cardiovascular disease; Death; Meta-analysis; Mortality; Napping