Four psotcardiotomy patients ranging in age from 46 to 64 years, who did not demonstrate a previous history of mental illness or brain damage, and whose surgery had necessitated use of the cardiopulmonary bypass, were studied to ascertain if sleep deprivation does occur in selected postoperative cardiotomy patients, the amount of uninterrupted rest apparently available to the patients, if behavioral manifestations of sleep deprivation occur, and the nature of nursing activities which may interfere with the patients' sleep during the hours of 11 p.m. to 7a.m. Descriptive data were obtained by means of nonparticipant observation with subject awareness. Presence of sleep as well as potential interruptions of sleep were recorded at ten-minute intervals. None of the subjects obtained more than four potential sleep cycles per night, the mean being less than one per night. The frequency of periods of uninterrupted rest of 60 or more minutes varied from zero to seven, the mean being three and three-tenths. Two of four subjects demonstrated paranoid behavior. The mean number of potential interruptions of sleep varied from 56 on the first postoperative night to five and one-half on the eighth postoperative night. The most frequent sources of potential interruptions were direct and indirect monitoring and measures to promote respiration.