This experiment evaluated further our previous finding that substitution of waking for the terminal 3-4 hr of sleep produces little or no increase in either visually scored or computer measures of delta sleep. Eleven young adults (mean age 24.5 yr) were studied on a baseline night, a night with sleep limited to an average of 188 min, and a recovery night. Visually scored sleep stages, eye movement activity and computer measures of 0-3 Hz were analyzed by nonrapid eye movement periods (NREMPs) and for all recorded sleep in each condition. In addition, we measured the heights, durations and areas under the curve manifested by the cyclic waxing and waning of 0-3-Hz integrated amplitude across sleep. Acute loss of 3.9 hr of sleep did not increase either visual or computer measures of delta electroencephalograms (EEG) on the recovery night, essentially confirming our previous findings. We hypothesize that augmentation of delta EEG above baseline levels after acute (one night's) sleep loss requires that disruption or loss of sleep from the first two NREMPs (or delta cycles). Rapid eye movement (REM) sleep durations on the recovery night were unaffected by the marked loss of REM sleep caused by partial deprivation. Although eye movements as well as stage REM were lost in the deprivation condition, eye movement density was significantly reduced rather than increased on the recovery night. This reduction is consistent with the hypothesis that REM activity varies inversely with sleep depth (or directly with central arousal level). The observations here, taken in association with previous results, suggest that a threshold for eye movement suppression by sleep deprivation in young adults lies in the range of 3-4 hr of prior sleep loss.