Sleep is a reversible, periodical state of quiescence that is more than the absence of being awake—it is an active state that is regulated. Typical changes in the pattern of the electroencephalogram (EEG) serve to discriminate sleep and waking. Modern sleep research is intimately linked to the ability of recording the EEG and to its quantitative analysis. The cyclic alternation of the two basic sleep states, non rapid-eye-movement (non REM) sleep and rapid-eye-movement (REM) sleep, is one of the hallmarks of sleep. Sleep homeostasis denotes a basic principle of sleep regulation. A sleep deficit elicits a compensatory increase in the intensity and duration of sleep, whereas excessive sleep reduces sleep propensity. EEG slow-wave activity (SWA), a correlate of sleep intensity, serves as an indicator of sleep homeostasis in nonREM sleep. The level of SWA is determined by the duration of prior sleep and waking. Recent evidence exists for a local use-dependent facet of sleep regulation. Selective unihemispheric regional cerebral activation during waking gives rise to a predominant increase of sleep intensity in the previously activated region as reflected by enhancement of SWA.