The relationship between sleep and EEG interictal epileptiform abnormalities (IEA) has been studied from different perspectives. One of the most followed orientations regards the investigation of the effects of IEA on sleep organization, while another approach considers the modulation of IEA caused by sleep. Only the latter approach, for its practical diagnostic implications, is covered by the present review. In particular, on the basis of the literature and of personal studies, we review some relevant aspects of the relationship between the different stages of the sleep and the EEG epileptic abnormalities in partial epilepsy. In addition, the modulation of IEA by fluctuations of the level of arousal and by sleep microstructure is reviewed. Finally, the information obtained on localization of epileptic foci from recordings during wakefulness and different sleep stages is discussed.