Electroencephalographic sleep inertia of the awakening brain.

Objectives: Sleep Inertia (SI) is part of a complex transition from sleep to wakefulness, and denotes a period of hypovigilance, confusion and impaired cognitive and behavioral performance that immediately follows awakening. Based on the observation that the reactivation of some cortical areas is faster than other upon awakening (e.g., Ferrara et al., 2006), here we further examined regional differences between presleep and postsleep waking period. Moreover, we also compared REM and stage 2 NREM awakenings in a within-subject design. Methods: Presleep and postsleep waking EEG (5 min with eyes-closed and 5 min. with eyes-open) of 18 healthy subjects (12 males, mean age=23.8 ± 2.3 yrs) were recorded from 19 derivations. Participants slept for 2 consecutive nights in the laboratory. In one night they were awakened from stage 2 NREM, while in the other from REM sleep. EEG power spectra were calculated across the following bands: delta (1–4 Hz), theta (5–7 Hz), alpha (8–12 Hz), beta1 (13–16 Hz), and beta2 (17–24 Hz). Results: Low-frequency bands (i.e., delta-theta-alpha) exhibited a prevalence upon awakening compared to presleep wakefulness, while beta activity was higher in presleep compared to postsleep wakefulness. As regards topographic differences, the increase of EEG delta power upon awakening showed a posterior prevalence, whereas presleep exhibited a significant predominance of desyncronized activity over almost all scalp locations for the beta2 range, and in correspondence of frontal and temporal areas for beta1. The difference between postsleep and presleep waking was more prominent in the eyes-closed than in the eyes-open condition after REM awakenings. Moreover, NREM awakenings were characterized by a higher beta-1 and beta-2 EEG power than REM awakenings. Conclusions: Altogether these findings support the hypothesis that a generalized reduction in beta activity and increased delta activity in more posterior areas upon awakening may represent the EEG substratum of the sleep inertia phenomenon. An enhanced posterior EEG synchronization may represent an electrophysiological basis of the reduced sensory-motor performance at sleep offset. On the other hand, the decreased EEG synchronization on anterior brain locations may be associated to better performances in processes involving associative prefrontal areas.