Cyclical activity in non-rapid eye movement sleep: a proposed arousal inhibitory mechanism

The presence of an endogenous arousal cycle in stage 1 sleep with a dominant length of 15-18 sec has been previously demonstrated. This allows the division of the electroencephalogram (EEG) into naturally occurring epochs consisting of periods of higher arousal, associated with a shift towards wakefulness and an alpha burst, and periods of lower arousal with a shift towards sleep and a theta period. The present study seeks to determine the changes in arousal as sleep progresses to stage 4. The polygraphic traces of 63 subjects with no demonstrable EEG abnormality or sleep disturbance were examined. When vertex sharp waves appeared in late stage 1 sleep the dominant cycle length increased to 30-34 sec (P < 0.001 Kolmogorov-Smirnov). In stage 2 arousals showing a definite shift towards wakefulness (Definite Arousals) occurred at intervals predominantly 51-60 sec in length (P < 0.0005, chi square). Few Definite Arousals were seen during stage 3 and none in stage 4 apart from awakening or sleep stage change. These findings suggest a steady decline in arousal between waking and stage 4, parallelled by the EEG changes of stage 1-4 sleep. It is suggested that there may be an arousal inhibitory mechanism operating during stages 1, 2 and 3 of NREM sleep.