title: Sleep duration and mortality in Japan: the Jichi Medical School Cohort Study.
abstract: BACKGROUND
  Although sleep is one of the most important health-related factors, relationship between sleep duration and mortality has not been fully discussed.
  
  
  METHODS
  Study subjects were 11,325 participants (4,419 males and 6,906 females) in the Jichi Medical School Cohort Study, a population-based prospective study. Baseline data were obtained by questionnaire and health checkups between April 1992 and July 1995 in 12 rural areas in Japan. Main outcome measures were all-cause and cause-specific mortality derived from death certificates up to December 31, 2001. Cox's proportional hazard models were applied to analyze the association of sleep duration with mortality.
  
  
  RESULTS
  A total of 495 deaths (289 males and 206 females) were observed during the average of 8.2-year follow-up period. After adjusting for age, systolic blood pressure, serum total cholesterol, body mass index, smoking habits, alcohol drinking habits, education, and marital status, the hazard ratios (95% confidence intervals) of all-cause mortality for individuals sleeping shorter than 6 hours and 9 hours or longer were 2.4 (1.3-4.2) and 1.1 (0.8-1.6) in males, and 0.7 (0.2-2.3) and 1.5 (1.0-2.4) in females, respectively, relative to those with 7-7.9 hours sleep.
  
  
  CONCLUSIONS
  Our data suggest that males with short sleep and females with long sleep were at an elevated risk of death.
authors.affiliation: Center for Community Medicine, Jichi Medical School, Minamikawachi, Tochigi 329-0498, Japan.
authors.forename: Yoko
authors.initials: Y
authors.lastname: Amagai
authors.forename: Shizukiyo
authors.initials: S
authors.lastname: Ishikawa
authors.forename: Tadao
authors.initials: T
authors.lastname: Gotoh
authors.forename: Yuriko
authors.initials: Y
authors.lastname: Doi
authors.forename: Kazunori
authors.initials: K
authors.lastname: Kayaba
authors.forename: Yosikazu
authors.initials: Y
authors.lastname: Nakamura
authors.forename: Eiji
authors.initials: E
authors.lastname: Kajii
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D007564:Japan / Q000453:epidemiology; D008297:Male; D008875:Middle Aged; D009026:Mortality / Q000639:trends*; D016016:Proportional Hazards Models; D011446:Prospective Studies; D012307:Risk Factors; D012577:Schools, Medical; D012890:Sleep / Q000502:physiology*; D011795:Surveys and Questionnaires; D013997:Time Factors