The electroencephalographic features of the sleep onset process and their experimental manipulation with sleep deprivation and transcranial electrical stimulation protocols

The sleep onset (SO) process is characterized by gradual electroencephalographic (EEG) changes. The interest for the possibility to manipulate the electrophysiological pattern of the wake-sleep transition is recently growing. This review aims to describe the EEG modifications of the SO process in healthy humans and the evidence about their experimental manipulation. We provide an overview of the electrophysiological changes during the wake-sleep transition, considering several methods to study the EEG signals. We then describe the impact of sleep deprivation (SD) on the electrophysiology of SO. Finally, we discuss the evidence about the possibility to modulate the local EEG activity through transcranial current stimulation protocols with the aim to promote, hinder, or manipulate the electrophysiological mechanisms of the wake-sleep transition. The reviewed findings highlight the local nature of the EEG processes during the SO and their intensification and speedup after SD. The evidence about the possibility to non-invasively affect the EEG pattern of the wake-sleep transition may have important implications for clinical conditions that would benefit from its prevention or promotion.