Sleep deprivation suppresses the increase of rapid eye movements across sleep cycles.

Objectives: Rapid eye movement density (REMd) usually increases progressively across the REM sleep periods. To date, contrasting findings do not permit to unequivocally discern if REMd is mainly influenced by circadian or sleep-dependent factors. However, the decreased REMd after sleep deprivation (SD) and the inverse relation between REMd and sleep depth suggest the involvement of a sleepdependent mechanism. To date, a direct evaluation of the relationship between changes in REMd and slow-wave activity (SWA) in conditions of increased homeostatic pressure still lacks. Therefore, the aim of the present study is to investigate the association between REMd and EEG power during NREM sleep. A further aim of the study is to evaluate whether and how the progressive REMd increase across sleep cycles is affected by SD. Methods: Fifty normal subjects (29 males; mean age = 24.3 ± 2.2 years) were studied during four consecutive days and nights. Sleep recordings were scheduled in the first (adaptation), second (baseline) and fourth night (recovery, after a protocol of 40-hours SD). Power spectra were calculated for NREM sleep (stage 2 + 3 + 4) from Cz derivation. PSG measures, REM density and quantitative EEG measures during NREM sleep of baseline and recovery nights were compared. Results: We found a significant reduction of REMd in the recovery night compared to baseline, due to a lack of REMd changes across sleep cycles. Correlational analyses revealed that oculomotor changes were positively associated with a decreased power in a specific range of fast sigma activity (14.75–15.25 Hz), indicating that the decreased REMd after SD is related to the concomitant decreased fast spindle activity. Contrarily, changes of SWA did not correlate with changes in REMd.