title: Statistical properties of sleep-wake behavior in the rat and their relation to circadian and ultradian phases.
abstract: OBJECTIVE
  To examine the statistical characteristics of short-term sleep-wake architecture and to evaluate their dependence on ultradian and circadian phase.
  
  
  METHODS
  Observational, time series.
  
  
  METHODS
  Laboratory.
  
  
  METHODS
  Ten male adult Sprague-Dawley rats.
  
  
  METHODS
  N/A.
  
  
  RESULTS
  States of wakefulness (WAKE), rapid eye movement sleep (REM) and nonrapid eye movement sleep (NREM) were recorded in 5-sec epochs over 7 consecutive days. State bout durations were analyzed using parametric regression of survival curves, comparing exponential, biexponential, and power law models. WAKE survival curves were best fit by biexponential models, suggesting that there are two statistically distinct stochastic mechanisms generating two types of WAKE--"brief" WAKE and "long" WAKE. Exponential time constants varied as a function of circadian and ultradian phase, with "long" WAKE showing the largest effect. NREM survival curves exhibited biexponential and monoexponential distributions in light and dark, respectively, with weak effects of ultradian phase. REM survival curves approximated a monoexponential distribution that varied with circadian but not ultradian phase. χ(2) analysis was used in a three-state Markov model to evaluate whether conditional state transition probabilities exhibit the property of first-order dependence. This was partially confirmed, but only after accounting for heterogeneity associated with circadian and ultradian phase. However, there was evidence of residual second-order dependence indicating that additional sources of statistical heterogeneity may remain to be identified.
  
  
  CONCLUSIONS
  Sleep-wake state is regulated over short timescales by stochastic mechanisms. When the major sources of heterogeneity are taken into account, including two-component WAKE and NREM states, the sleep-wake system of the rat behaves, to a reasonable approximation, as a Markovian system that is modulated over ultradian and circadian timescales.
authors.affiliation: Department of Cell and Systems Biology, University of Toronto, Toronto, Ontario, Canada. richard.stephenson@utoronto.ca
authors.forename: Richard
authors.initials: R
authors.lastname: Stephenson
authors.forename: Svetlana
authors.initials: S
authors.lastname: Famina
authors.forename: Aimee M
authors.initials: AM
authors.lastname: Caron
authors.forename: Joonbum
authors.initials: J
authors.lastname: Lim
mesh_terms: D000204:Activity Cycles / Q000502:physiology*; D000818:Animals; D002940:Circadian Rhythm / Q000502:physiology*; D053208:Kaplan-Meier Estimate; D008297:Male; D008390:Markov Chains; D051381:Rats; D017207:Rats, Sprague-Dawley; D012890:Sleep / Q000502:physiology*; D012895:Sleep, REM / Q000502:physiology; D013997:Time Factors; D014851:Wakefulness / Q000502:physiology*
keywords: Markov analysis; sleep architecture; state transition probability; survival curve; ultradian rhythm