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Circasemidian 12 h cycle of slow wave sleep under constant darkness
Mitsuo Hayashi1, Toshio Morikawa, Tadao Hori
1Department of Behavioral Sciences, Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Japan. mhayasi@hiroshima-u.ac.jp
Objectives:
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.