Timing and topography of sleep onset. Asynchronies and eegional 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).