To review the numerous works concerning sleep and epilepsy, this review considers the effects of sleep, firstly on seizures and secondly on paroxysmal interictal EEG activity (PA), in the different types of epilepsy according to the International League against Epilepsy classification. Apart from the exceptions of the definite nocturnal preponderance of seizures in idiopathic rolandic epilepsy and of the mostly nocturnal occurrence of seizures in some types of familial or sporadic frontal-lobe epilepsy, assessing a seizure according to the time of day it occurs is of no diagnostic or predictive value. In generalised idiopathic epilepsy, as in partial symptomatic or cryptogenic epilepsy, only about 20% of the patients had a sleep increase in PA. This percentage is higher (75%) in idiopathic partial epilepsy. Stereoelectroencephalography demonstrates a relative stability of spiking within the focus across the states of vigilance and an increase in transmitted discharges during stages 3 and 4. In the Landau and Kleffner syndrome, as in the syndromes of continuous spike-waves during sleep, there is a huge, unexplained increase in PA during sleep. The neuropsychological consequences of this PA have some relationship with their localisation and the patient's age at the time of occurrence. Sleep PA has also been reported in several groups of non-epileptic subjects. As regards the effect of epilepsy on sleep, sleep may be lighter and abnormally discontinuous in the absence of seizures, particularly in temporal-lobe epilepsy.