Timing and Topography of Sleep Onset: Asynchronies and Regional Changes of Brain Activity

During the transition from wake to sleep, the human brain exhibits progressive, regional, and frequency-specific electroencephalographic (EEG) changes, mainly represented by (a) an increase of the slowest frequencies (≤ 7 Hz) with an anteroposterior gradient; (b) a shift from a posterior to an anterior dominance of alpha activity (8–12 Hz); (c) an increase in the sigma frequency range (~ 12–15 Hz), denoting the emergence of sleep spindles; and (d) a generalized reduction of the highest frequency activity (16–40 Hz). Moreover, several subcortical structures show sleep-like rhythms before the cortically defined sleep onset. Also, the dynamics of the interactions between cortical areas are characterized by progressive modifications during the wake-sleep transition. According to these findings, sleep onset should not be considered a global and unitary phenomenon, but a complex local and progressive process, during which sleep-like and wake-like EEG patterns coexist. Several peculiar phenomena often observed at sleep onset (hypnagogic hallucinations, sleep misperception, and mesograde amnesia) may be explained by such spatiotemporal asynchronies.