ABSTRACT
Theories on the broader function of sleep centre on the idea that sleep states are important for the restoration of neural processes exerted during wakefulness. Traditionally, quantitative measurement of sleep has been conducted using the electroencephalogram (EEG). In young healthy adults, alert wakefulness is characterised by high-frequency low-amplitude asynchronous EEG activity, while in most people relaxed or quiet wakefulness is characterised by 8–12 Hz activity. All non-rapid eye movement stages are associated with low levels of tonic muscle activity and slow or non-existent eye movements. Rapid eye movement (REM) sleep is characterised by mixed frequency-low voltage EEG, phasic REMs and skeletal muscle atonia. The principle of wave summation underlies the notion of synchronous and asynchronous EEG activity that is commonly used to describe different levels of cortical arousal. The main modulators of cortical activity come from ascending input from the brainstem reticular activating system, basal forebrain and hypothalamus.
