P050 Process O (onset/offset) cause of the 90-min sleep cycle

The ubiquitous sleep cycles across a normal sleep of approximately 90 minute period length were originally suggested to be a reflection of a 90-minute basic rest-activity cycle (BRAC) operating across the full circadian cycle of wakefulness and sleep. The evidence for a BRAC during the wakeful period is sparce requiring other explanations for the prominent 90min cycles during sleep. Recently another process, Process O (onset/offset), has been suggested in addition to Processes S (homeostatic) and C (circadian). It was hypothesised in order to account for the recuperative effects of a brief power nap, inexplicable by the other two processes. Process O hypothesises a development of inhibition to any ongoing state (wake, NREM, REM). The inhibition develops rapidly over 1-2 hours to a plateaued limited level and then, following the cessation of that state, dissipates rapidly (5-15 minutes). Thus, a “power” nap brings improved alertness because of the short break from wakefulness allowing the dissipation of wakefulness Process O inhibition. It is suggested that Process O inhibition accumulates also to periods of NREM sleep and REM sleep. The ‘sleep switch’ accounting for bi-stable states of sleep or awake is hypothesised to include REM and NREM as separate states accounting for stability and mutual inhibition between 3 rather than just 2 states. Thus Process O inhibition developing to an ongoing state tips the balance to other states (REM or NREM) resulting in the alternation between them of the sleep cycles and allowing longer sleep epochs after brief awakenings.