We studied 26 physicians in postgraduate medical training ("house staff") to objectively quantify their sleep, alertness, and psychomotor performance while working on call. This study provided precise data on the extent of sleep deprivation during a typical call night, the workload factors predictive of sleep loss, and the extent to which protected time for sleep within the call night can ameliorate sleep loss and consequent daytime sleepiness. We used ambulatory EEG recording equipment and a standardized computer-based performance test to monitor sleep and alertness over the course of a 36-hour call day. Comparisons were made between interns provided with 4 hours of protected time for sleep by a covering resident ("night-float") and interns without such coverage. As anticipated, we found evidence that hospital interns were severely sleep-deprived, to an extent even greater than prior behavioral observations have suggested. Interns in both conditions spent an average of less than 5 hours (295.4 minutes) in bed attempting to sleep and obtained an average of 3.67 hours (220.1 minutes) of sleep (range 37.4-358.4 minutes). Provision of the night-float for 4 hours did not significantly change total sleep time (TST) (212.8 minutes covered vs. 224.9 minutes uncovered), but sleep efficiency was significantly improved (86.5% vs. 70.3%; p = 0.001). Covered interns also obtained significantly more slow-wave sleep than the uncovered interns (65.4 minutes vs. 51.1 minutes; p = 0.05). However, measures of alertness and performance were not significantly different between the two groups and were only weakly related to TST. These data suggest that significant chronic sleep deprivation is relatively unaffected by sleep obtained in the hospital and that provision of protected time for sleep does not significantly improve TST.