Retention of a paired‐associate list of common nouns was tested under two conditions: original learning at night prior to 8 hrs of sleep (Sleep condition), and original learning in the morning prior to a day of normal waking activity (Waking condition). Both conditions were subdivided so that retention was tested at intervals of 8, 16, and 24 hrs after original learning. For both paced and free recall measures of retention, the Sleep condition proved superior to the Waking condition at the 8 hr interval. At 24 hrs, when the amounts of sleep and waking were equated across both conditions thus normalizing for potential interference, the superiority of the Sleep condition over the Waking condition was also observed. There were no differences in retention between the Sleep and Waking conditions at 16 hrs after original learning. This finding was largely influenced by improved recall on the part of subjects in the Waking condition in the interval between 8 and 16 hrs. It was concluded that the consolidation during sleep of verbal materials learned shortly before sleep onset has a beneficial effect on their recall and temporal stability in the 8 to 24 hr period following original learning.

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.

Retention of nonsense syllables was tested in two groups of Ss: those having initial learning in the morning and those having initial learning at night immediately before sleep. These groups were subdivided so that recall was tested either 8 or 24 hrs after initial learning. As Jenkins and Dallenbach first demonstrated, retention was superior after 8 hrs for Ss when learned at night compared to those who learned in the morning. Retention with night learning was equal after 24 hrs to that observed after 8 hrs. Surprisingly, retention scores after morning learning were superior after 24 hrs to those observed after 8 hrs. Some possible interpretations of this result are advanced.

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