The architecture of nocturnal sleep in twenty subjects (9 males, 11 females, mean age 27 years) affected by partial cryptogenic epilepsy was investigated by means of ambulatory EEG (A-EEG) recording performed at home. The study aimed in particular to ascertain the immediate effects of nocturnal partial epileptic seizures on sleep stability and continuity. Data for a total of 49 recorded seizures indicate that 72% of the partial seizures which occurred during sleep were followed by arousal and awakening, and that sleep interruption lasted significantly longer when the seizure occurred between 4 and 7 a.m.