title: Inter-individual differences in the dynamics of sleep homeostasis.
abstract: OBJECTIVE
  The two-process model posits that sleep is regulated by 2 independent processes, a circadian Process C and a homeostatic Process S. EEG slow-wave activity (SWA) is a marker of NREM sleep intensity and is used as an indicator of sleep homeostasis. So far, parameters of the two-process model have been derived mainly from average data. Our aim was to quantify inter-individual differences.
  
  
  METHODS
  Polysomnographic recordings (analysis of existing data).
  
  
  METHODS
  Sound attenuated sleep laboratory.
  
  
  METHODS
  Eight healthy young males.
  
  
  METHODS
  40-h sustained wakefulness.
  
  
  RESULTS
  Process S was modeled by a saturating exponential function during wakefulness and an exponential decline during sleep. Empirical mean SWA (derivation C3A2) per NREM sleep episode at episode midpoint were used for parameter estimation. Parameters were estimated simultaneously by minimizing the mean square error between data and simulations of Process S. This approach was satisfactory for average data and most individual data. We further improved our methodological approach by limiting the time constants to a physiologically meaningful range. This allowed a satisfactory fit also for the one individual whose parameters were beyond a physiological range. The time constants of the buildup of Process S ranged from 14.1 h to 26.4 h and those of the decline from 1.2 h to 2.9 h with similar inter-individual variability of the buildup and decline of Process S.
  
  
  CONCLUSIONS
  We established a robust method for parameter estimation of Process S on an individual basis.
authors.affiliation: Institute of Pharmacology and Toxicology, University of Zurich, Zurich, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Rusterholz
authors.forename: Roland
authors.initials: R
authors.lastname: Dürr
authors.forename: Peter
authors.initials: P
authors.lastname: Achermann
mesh_terms: D000328:Adult; D006706:Homeostasis / Q000502:physiology*; D006801:Humans; D008297:Male; D017286:Polysomnography / Q000379:methods / Q000706:statistics & numerical data; D012016:Reference Values; D012815:Signal Processing, Computer-Assisted; D012890:Sleep / Q000502:physiology*; D012894:Sleep Stages / Q000502:physiology; D013997:Time Factors; D014851:Wakefulness / Q000502:physiology; D055815:Young Adult
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