By means of combined surface and depth EEG recordings (32 channels) and split-screen video-monitoring (infra red light), complete all-night sleep studies were carried out in drug-resistant epileptics suffering from complex partial seizures. These data were supplemented by (1) all-night sleep recordings from three patients who had chronically implanted electrodes in the periaqueductal grey matter because of severe pain problems, and (2) incomplete sleep recordings (day-time or only partial night) from other drug-resistant epileptics, who underwent chronic stereo-electroencephalographic (SEEG) examination in view of the surgical therapy. The following questions of importance were studied and the results were compared between different patient groups. 1. Relationship between overt clinical seizures or subclinical seizure discharges and the time course of night sleep. 2. Topic behavior of the epileptogenic area and different propagation phenomena of interictal spikes and ictal seizure discharges in different sleep stages. 3. Influence of arousal on the average spike incidence of any location with special attention to the different (slow and fast) components of the K-complex and their association with spiky elements, thus testing the hypothesis that the K-complex might serve as a vehicle for the seizure discharge. 4. Incidence and time-relationship of seizure discharges with clinical overt awakenings.