Sleep

Abstract

The time spent asleep consumes about one third of our lifetimes. In contrast to anaesthesia or a comatose state, the perceptual disengagement from the environment and alteration (loss) of consciousness can be reversed during sleep upon intense stimulation. A large body of knowledge on sleep-related processes has accumulated in the last few decades, but the precise functions of sleep are yet to be disclosed. Most of our current understanding of cortical activity across the different stages of vigilance is derived from EEG recordings. Concomitant to falling asleep, the changes in brain activity on a cellular level - namely switching from tonic to burst mode firing with increased periods of hyperpolarisation - induce typical changes in EEG (Steriade 2003; Carskadon and Dement 2005). Altered surface EEG during sleep in humans was first documented as far back as 1929 by Hans Berger (Berger 1929). This was later followed by Loomis' description of specific characteristics in sleep recordings, like K-complexes (KCs) and sleep spindles (Loomis et al. 1938). The misconception of sleep as a cessation of brain activity as compared to the reticular activation during wakefulness was finally overthrown in 1953, when Eugene Aserinsky and Nathaniel Kleitman first described an active brain state accompanied by rapid eye movements (REM) that recurs at regular intervals during sleep (Aserinsky and Kleitman 1953). Awakenings from this state of high cortical activity correspond with higher incidences of vivid dream reports, which putatively accompany the activated cortical states. REM sleep shares many features with wakefulness, and is therefore also referred to as paradoxical sleep. A further characteristic of REM sleep is the loss of voluntary muscle control. Based on these distinct features, international standards for scoring sleep stages have been established that rely not only on EEG, but also include electrooculogram (EOG) and electromyogram (EMG) criteria (Rechtschaffen and Kales 1968). For clinical purposes, this electrophysiological triad is usually extended by acquiring data on cardiovascular activity (electrocardiogram, pulsoximetry), breathing parameters (thoracic and abdominal movements, effective breathing), snoring, and EMG recordings of particular muscle groups (e.g. anterior tibialis muscle). This multimodal recording approach (polysomnography, PSG) represents the standard way of registering sleep in humans. © Springer-Verlag Berlin Heidelberg 2010.