Abstract
The mechanism responsible for the ubiquitous NREM/REM sleep cycles of approximately 90-min period length in humans remains largely unknown. It was originally said to be a product of the 90-minute Basic-Rest-Activity-Cycle (BRAC) assumed to pervade throughout sleep and wakefulness. However, the evidence for the existence of the BRAC during wakefulness is weak and questionable. It is important to consider other possible contributors to this fundamental sleep cycle process. Process O (state onset, offset) has been suggested to account for empirical data inexplicable by Processes S and C. The data in question is the significant increase in alertness following a brief nap (<10min sleep) and the longer than predicted sleep episodes (40-90 min) following a brief awakening towards the end of a normal sleep period. Process O is proposed to operate similarly to Process S but with a build-up of inhibition to an ongoing state (wake or NREM sleep or REM sleep) that is rapid but self-limiting after 1-2 hours. It also has a rapid (~10min) dissipation of inhibition to the previous state with the offset of that state such that resuming the previous state (onset) rejuvenates the functions of that previous state (wakefulness, NREM, or REM sleep). In operation during the sleep period Process O accumulates inhibition to NREM sleep over the first 80-100 minutes enough to flip the ‘sleep switch’ to another state (REM sleep). Only a brief period (5-10min) is needed to dissipate NREM inhibition such that NREM, still stronger than REM, can resume another cycle.