Scientists have long understood that, when it comes to learning, repeated practice of a task and getting a good night's sleep are both beneficial to reinforcing memories. However, little research has been conducted into how repetition and sleep in combination affect memory.
According to a new study published in the journal Psychological Science, sleeping in between study sessions may enhance the ability to recall what was learnt in the first session and relearn anything that was forgotten.
"Our results suggest that interleaving sleep between practice sessions leads to a twofold advantage, reducing the time spent relearning and ensuring a much better long-term retention than practice alone," explains psychological scientist Stephanie Mazza of the University of Lyon, and author of the study. "Previous research suggested that sleeping after learning is definitely a good strategy, but now we show that sleeping between two learning sessions greatly improves such a strategy."
The study suggests that separating two study sessions that cover the same material with sleep makes the learning process more efficient, reducing the effort needed to commit information to memory.
To test this, French 40 adults were randomly assigned to either a "sleep" group or a "wake" group and were then presented with 16 French-Swahili word pairs to learn in random order. After being given seven seconds to study each pair, the Swahili word appeared on its own and participants were asked to type in the French translation. If correct, then the word pair was shown for four seconds, while any words that weren't translated correctly were presented again, until they were.
Twelve hours after the initial session, the participants were asked to complete the same recall task. The "wake" group completed the first session in the morning and the second in the evening of the same day, while the "sleep" group had one session in the evening, before going to sleep and completing the second session the next morning.
The data from the first session shows there was no difference between the two groups when comparing how many words they were able to initially recall correctly or the number of retrials needed to remember all 16 pairs.
For the second session however, the "sleep" group remembered about 10 of the words the first time round, while the "wake" group recalled on average only about 7.5 words. Relearning was also more efficient for the "sleep" group, who only needed three retrials on average to recall all 15 words, as opposed to six for the "wake" group. So both groups were able recall all 16 pairs eventually but sleeping in between sessions appeared to enable participants to learn them in less time and with less effort.
"Memories that were not explicitly accessible at the beginning of relearning appeared to have been transformed by sleep in some way," says Mazza. "Such transformation allowed subjects to re-encode information faster and to save time during the relearning session."
In addition, the data suggested that the "sleep" group displayed lasting benefits from their study strategy. When the participants were tested again one week later. The sleep group performed very well, remembering on average 15 words, compared to the wake group who recalled about 11. Six months later this improved recall ability was still present in the sleep group.
The two groups showed no differences between sleep quality or memory capacity, so the benefits displayed by the sleep group could not be down to those factors.
The team came to the conclusion that interspersing sleep and study sessions could be an easy and effective way to remember information over long periods of time with less study.