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.