In order to study the influence of sleep on the electroencephalograms of epileptic patients the tracings obtained of 1.868 such patients were analised (572 generalized seizures; 121 non-psychomotor temporal; 118 psychomotor; 410 non-temporal focal; 314 noctural seizures; 165 febrile seizures; 168 convulsive seizures associated with psychomotor retard). The electroencephalograms were made with the patients awake (rest and hyperpnea) and sleeping (slow phase), the findings being compared. There was not found any significant difference between the EEG tracings obtained during spontaneous sleep and medically induced sleep. In the majority of the cases the records obtained while the patients were asleep merely confirmed the ones made while the patients were awake. The activator action of sleep appeared only in small number of the cases, having it's peak on the group with psychomotor seizures (26%). In the generalized non-psychomotor temporal, non-temporal focal and noctural seizures the sleep acted as an antiactivator instead of an activator of the abnormalities recorded while the patients were awake.