Local and Widespread Slow Waves in Stable NREM Sleep: Evidence for Distinct Regulation Mechanisms.

Previous work showed that two types of slow waves are temporally dissociated during the transition to sleep: widespread, large and steep slow waves predominate early in the falling asleep period (), while smaller, more circumscribed slow waves become more prevalent later (). Here, we studied the possible occurrence of these two types of slow waves in stable non-REM (NREM) sleep and explored potential differences in their regulation. A heuristic approach based on slow wave synchronization efficiency was developed and applied to high-density electroencephalographic (EEG) recordings collected during consolidated NREM sleep to identify the potential and slow waves. Slow waves with characteristics compatible with those previously described for and were identified in stable NREM sleep. Importantly, these slow waves underwent opposite changes across the night, with only slow waves displaying a clear homeostatic regulation. In addition, we showed that the occurrence of slow waves was often followed by larger , whereas the occurrence of slow waves was usually followed by smaller waves. Finally, slow waves were associated with a relative increase in spindle activity, while slow waves triggered periods of high-frequency activity. Our results provide evidence for the existence of two distinct slow wave synchronization processes that underlie two different types of slow waves. These slow waves may have different functional roles and mark partially distinct "micro-states" of the sleeping brain.