The present study examined slow eye movements (SEMs) during early sleep in twenty female subjects, aiming at the detection of hypnagogic state. SEMs were recorded as slow deflections on a pair of horizontal EOGs with the opposite polarity-a phase method (Rechtschaffen & Kales, 1968). SEMs were prominent at the beginning of EEG stage 1, and they terminated when stage 2 lasted about 5 min and kept stable. No recovery of SEMs was seen during slow wave sleep. Analyses of a rising angle revealed that SEM wave form became slower with the progress of sleep. All epochs of the first SEM and 70% of maximum SEMs fell into stage W, preceding the first stage 1. Sleepiness, which subjects signaled by pressing a button switch, induced significant increases in SEMs, whether or not stage 1 developed. These data suggest that SEMs can give a useful criterion to define the temporal range of hypnagogic state.
