For faster learning and longer retention, interleave study sessions with sleep

Some basic rules of effective learning, informed by psychology, are already well established. Testing yourself and relearning any forgotten items is beneficial,  especially so when this is done after a sufficient delay, rather than "cramming". Sleep too is known to be incredibly helpful for consolidating new memories. Now a study  in Psychological Science has built upon these insights, showing how interleaving two study periods with sleep leads to particularly efficient and long-lasting learning. 

Stéphanie Mazza and her colleagues assigned 60 French participants to three different study conditions. All three groups faced the same initial challenge – they had to learn the French translation of 16 Swahili words. During this initial session, the word pairs were presented one at a time, then participants were tested at recalling the correct French word for each Swahili word. In cases where they were wrong or couldn't remember, they were given the correct answer. This process was repeated until the participant had successfully recalled all the word pairs.

One group completed this initial study session at 9am and then they returned for a second, "relearning" session twelve hours later. The relearning session involved further testing on the word pairs – again participants were told the correct answer for any they'd forgotten, and the process continued until they had successfully recalled all the word pairs.

Another group conducted the initial study session at 9pm, went to bed, then completed their relearning session at 9am the next morning. A final control group also did the initial study session in the evening, slept, but then just had a re-test in the morning, without a chance for re-learning.

To measure the effectiveness of the different study conditions, the researchers gave all participants a further test on the word pairs one week later, and then again after another six months. Several important group differences emerged.

The group who studied, slept, then restudied showed better initial test performance in the relearning session, and their relearning of forgotten items at this session was quicker, as compared with the group who'd studied in the morning then had their relearning session in the evening. Moreover, the study/sleep/relearn group outperformed the study/awake/relearn group both at the one-week retest and the six-month re-test.

Whereas the study/sleep/relearn group showed virtually no forgetting over one week, the study/awake/relearn group showed significant forgetting (roughly four to five items), equivalent to the forgetting also shown by the control group who had studied, slept, then simply had another test with no further relearning. The study/sleep/relearn group also showed superior performance to the other groups at the six-month test.

The researchers ruled out other explanations for the findings, such as possible group differences in feelings of sleepiness during the testing sessions (there weren't any). The outcomes suggest that completing two sessions of learning interleaved with sleep is an especially effective way to learn, improving long-term retention and boosting the efficiency of relearning at the second study session. We need more research to explore the precise mechanisms at play, but one possibility is that sleep ensures the initial memories are stored more strongly, and the subsequent relearning process is then quicker and more effective because it is acting on these sleep-consolidated memories. It will be interesting to see if the same principles apply for procedural memories, such as skill-learning.

Further reading

—Relearn Faster and Retain Longer: Along With Practice, Sleep Makes Perfect