Both repeated practice and sleep improve long-term retention of information. The assumed common mechanism underlying these effects is memory reactivation, either on-line and effortful or off-line and effortless.
In the study reported here, we investigated whether sleep-dependent memory consolidation could help to save practice time during relearning. During two sessions occurring 12 hr apart, 40 participants practiced foreign vocabulary until they reached a perfect level of performance. Half of them learned in the morning and relearned in the evening of a single day. The other half learned in the evening of one day, slept, and then relearned in the morning of the next day. Their retention was assessed 1 week later and 6 months later. We found that interleaving sleep between learning sessions not only reduced the amount of practice needed by half but also ensured much better long-term retention.
Sleeping after learning is definitely a good strategy, but sleeping between two learning sessions is a better strategy.
Figure 1: Overall results. The graph in (a) shows the mean number of correct translations (out of 16 possible) during the first and the last practice trials in the learning session (pair trials) and relearning session (list trials) and during the cued-recall task after 1 week and 6 months. Results are presented separately for the wake, sleep, and control groups. The relearning session in the control experiment consisted of only the first list trial. Error bars represent 95% confidence intervals. The box-and-whiskers plots in (b) indicate the number of pair trials necessary for the wake group and the sleep group to attain the performance criterion in the learning session and the number of list trials necessary for them to attain the performance criterion in the relearning session. The left and right edges of the boxes represent the boundaries of the first and third quartiles, respectively, and the lines down the center of the boxes represent the medians. The left and right ends of the whiskers represent the minimum and maximum scores, respectively. Asterisks indicate statistically-significant differences between groups (✱ p < 0.01).
Figure 2: Individual list-trial scores. The left and middle graphs show, respectively, the individual scores of members of the sleep and wake groups for each list trial in the relearning session. The maximum score was 16. The symbols enclosed in the dashed box indicate the successive scores for those participants in the wake group who still needed to continue after all of the participants in the sleep group had reached the criterion. The graph on the right shows individual scores of members of the sleep and wake subgroups for each list trial; the subgroups were matched on their performance in the first list trial. The arrows indicate the point at which all the participants in a given group reached the criterion.
Figure 3: Change in individual scores. Individual participants’ number of correct translations on the first list trial of the relearning session and at the delayed testing at 1 week is graphed separately for the wake and the sleep groups. The gray shaded area in each graph represents the remaining list trials in the relearning session. The dashed lines connect the two scores for each participant.
