The frequency of epileptiform discharges shows a wide intraindividual variability; it is influenced by the sleep-waking cycle and by the different stages of sleep: generalized seizures (tonic-clonic, tonic and myoclonic) are activated in nonrapid eye movement sleep, partial seizures tend to have more complex relationships with sleep states. Generally, sleep stages I and II and transit stages have an activating effect on discharges. Rapid eye movement sleep has an "anticonvulsive" effect and focalizes paroxysmal activity. Sleep recordings are useful for localizing a focus in temporal lobe seizures and for differentiation of epileptic seizures from non-epileptic seizure-like sleep disturbances. EEG after sleep deprivation should be used in patients suspected of suffering from epilepsy whose prior EEGs (including hyperventilation, photic stimulation and short sleep) were normal or yielded borderline abnormalities. Epileptiform discharges can--without clinical seizures--produce arousal reactions and sleep disturbances.