Sleep drunkenness (SD) consists of difficulty in coming to complete wakefulness accompanied by confusion, disorientation, poor motor coordination, slowness, and repeated returns to sleep. Patients report that these symptoms occur at almost every awakening; nearly all report abnormally ``deep'' and prolonged sleep as well. Approximately one third of all hypersomniacs seen suffered from sleep drunkenness. Of 58 cases of hypersomnia with sleep drunkenness examined, 52 were apparently idiopathic; six cases were possibly symptomatic of organic brain disturbance. A familial history of the disorder was found in 36% of the idiopathic cases. Electroencephalographic abnormalities were not remarkably prevalent or distinctive. Except for relatively high heart and respiratory in nocturnal polygraphic records, and except for extended sleep, no anomalies of sleep patterns were observed.