title: Empirical evidence for "syndrome Z": a hierarchical 5-factor model of the metabolic syndrome incorporating sleep disturbance measures.
abstract: OBJECTIVE
  Sleep disturbances have been associated with individual components of the metabolic syndrome ("syndrome X"), and, although the concept has been proposed, it is not known whether sleep disturbances actually cluster with features of the metabolic syndrome to produce a unifying trait, "syndrome Z". Therefore, we evaluated a second-order factor model, whereby syndrome Z was described by 5 first-order factors--insulin resistance, obesity, hypertension, dyslipidemia, and sleep disturbance--with the sleep disturbance factor defined using the apnea-hypopnea index, arousal index, percentage of sleep time with oxygen saturation less than 90%, and percentage of slow wave sleep.
  
  
  METHODS
  Observational, cross-sectional study.
  
  
  METHODS
  Clinical research center.
  
  
  METHODS
  Five hundred thirty-three adults from the Cleveland Family Sleep Study who underwent polysomnography and were not treated by continuous positive airway pressure.
  
  
  RESULTS
  When modeling syndrome Z as a second-order factor unifying 5 first-order factors, we observed good overall model fit (chi2/df= 3.20; CFI = 0.96; RMSEA = 0.06; SRMR = 0.05) and found that obesity was the most important determining factor (standardized loading = 0.85 +/- standard error = 0.02; P < 0.01) followed by sleep disturbance (0.82 +/- 0.03; P < 0.01), insulin resistance (0.67 +/- 0.03; P < 0.01), hypertension (0.64 +/- 0.04; P < 0.01), and dyslipidemia (0.60 +/- 0.05; P < 0.01). Simultaneous multiple group analyses revealed that this model was essentially generalizable across age, race, and sex subgroups.
  
  
  CONCLUSIONS
  Our results demonstrate that sleep disturbance coaggregates with other metabolic features to represent a single unifying trait, syndrome Z. Although our model awaits validation in other populations, it provides a tool for better understanding the synergistic risk of syndrome Z, compared with syndrome X, on type 2 diabetes and cardiovascular disease in future studies.
authors.affiliation: Department of Epidemiology and Biostatistics, Case Western Reserve University, 2103 Cornell Road, Cleveland, OH 44106-7281, USA. nora.nock@case.edu
authors.forename: Nora L
authors.initials: NL
authors.lastname: Nock
authors.forename: Li
authors.initials: L
authors.lastname: Li
authors.forename: Emma K
authors.initials: EK
authors.lastname: Larkin
authors.forename: Sanjay R
authors.initials: SR
authors.lastname: Patel
authors.forename: Susan
authors.initials: S
authors.lastname: Redline
mesh_terms: D000328:Adult; D015897:Comorbidity; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008297:Male; D024821:Metabolic Syndrome / Q000175:diagnosis* / Q000453:epidemiology; D008875:Middle Aged; D017286:Polysomnography; D020181:Sleep Apnea, Obstructive / Q000175:diagnosis* / Q000453:epidemiology; D012892:Sleep Deprivation / Q000175:diagnosis / Q000453:epidemiology; D013223:Statistics as Topic