title: Circasemidian 12 h cycle of slow wave sleep under constant darkness.
abstract: OBJECTIVE
  Afternoon sleepiness is a widespread phenomenon. The present study aimed to test Broughton's hypothesis (Sleep and alertness: chronobiological, behavioral, and medical aspects of napping. New York, NY: Raven Press, 1989. p. 71-98) that afternoon sleep propensity might reflect the circasemidian 12h cycle of slow wave sleep (SWS).
  
  
  METHODS
  Nine subjects (21-27 year) stayed alone under constant darkness (0 lux) without social contact for 72 h. They were allowed to sleep and eat freely. Their polysomnograms during 72 h of constant darkness were analyzed.
  
  
  RESULTS
  The total sleep time (TST) accounted for 41.6h (57.9%) of the 72 h and decreased progressively as a function of time. The reduction in TST was dependent on the decrease in sleep stage 2 and rapid eye movement (REM) sleep. The amount of SWS did not significantly change among the days. The circadian (1 cycle/day) and circasemidian (2cycles/day) cycles were observed in SWS. Those accounted for 13.9 and 11.1% of the total variance, respectively. SWS during the time corresponding to daytime occurred 9-10h before and 15-16 h after the nocturnal sleep gate. In addition, weak but significant correlations were observed between the amounts of SWS and the waking time before the sleep episodes (r=0.332) and prior REM sleep (r=-0.236).
  
  
  CONCLUSIONS
  The present findings suggest that SWS might occur not only always in a homeostatic manner as a function of prior wakefulness, but also as a circasemidian rhythmic function.
authors.affiliation: Department of Behavioral Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Japan. mhayasi@hiroshima-u.ac.jp
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Hayashi
authors.forename: Toshio
authors.initials: T
authors.lastname: Morikawa
authors.forename: Tadao
authors.initials: T
authors.lastname: Hori
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D002940:Circadian Rhythm / Q000502:physiology; D003624:Darkness; D004569:Electroencephalography; D004576:Electromyography; D004585:Electrooculography; D006801:Humans; D008297:Male; D010507:Periodicity; D017286:Polysomnography; D012016:Reference Values; D012683:Sensory Deprivation / Q000502:physiology*; D012890:Sleep / Q000502:physiology*; D012895:Sleep, REM / Q000502:physiology; D013997:Time Factors