Slow eye movements distribution during nocturnal sleep.

Objective: To assess the distribution across nocturnal sleep of slow eye movements (SEMs).

Methods: We evaluated SEMs distribution in the different sleep stages, and across sleep cycles in nocturnal recordings of 10 healthy women. Sleep was scored according to standard criteria, and the percentage of time occupied by the SEMs was automatically detected.

Results: SEMs were differently represented during sleep stages with the following order: wakefulness after sleep onset (WASO): 61%, NREM sleep stage 1: 54%, REM sleep: 43%, NREM sleep stage 2: 21%, NREM sleep stage 3: 7%, and NREM sleep stage 4: 3% (p<0.0001). There was no difference between phasic and tonic REM sleep. SEMs progressively decreased across the NREM sleep cycles (38%, 15%, 13% during NREM sleep stage 2 in the first three sleep cycles, p=0.006), whereas no significant difference was found for REM, NREM sleep stage 1, slow-wave sleep and WASO.

Conclusions: Our findings confirm that SEMs are a phenomenon typical of the sleep onset period, but are also found in REM sleep. The nocturnal evolution of SEMs during NREM sleep stage 2 parallels the homeostatic process underlying slow-wave sleep.

Significance: SEMs are a marker of sleepiness and, potentially, of sleep homeostasis.