Circasemidian 12 h cycle of slow wave sleep under constant darkness

Objectives: Afternoon sleepiness is a widespread phenomenon. The present study aimed to test Broughtonʹs 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 12 h 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 41.6 h (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 (2 cycles/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 9–10 h 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.