Because we cannot possibly remember all of the information that we encounter, it is often necessary to go back and learn that information again. Thus, despite the fact that both groups had practiced to a level of perfect recall during the relearning session, only those that had slept prior to relearning accrued any long-term memory benefits. At present, these new results suggest that sleeping between study sessions can be a particularly potent learning strategy, and especially when learners take the time to repeatedly revisit and relearn in order to achieve a high level of performance.
– It's well established through previous research that sleep after learning is best for many memory-related tasks, including word lists, mazes, auditory tones, and so on. Sleep seems so vital to recall that some speculate it is directly responsible for, not just supportive of, learning, reports Scientific American. So researchers out of the University of Lyon chose to investigate another aspect of learning—not recall but relearning, where something previously learned has been forgotten and must be re-acquired. Reporting in the journal Psychological Science in August, they wroite that, coupled with practice, "Sleep makes perfect," and that "sleeping after learning is definitely a good strategy, but sleeping between two learning sessions is a better strategy." The team tested 40 French-speaking adults tasked with learning 16 Swahili words. Some learned them at night, slept, and relearned them in the morning, while others learned the words in the morning, didn't sleep, and relearned them at night. The group that slept between sessions performed so much better that even those who forgot the most words relearned them faster during their morning session than the least forgetful members of the group relearning the words at night. The results don't clarify which is at play—sleep boosting learning, which helps improve later relearning, or sleep simply allowing for uninterrupted learning processes to speed up relearning—but either way, sleeping between studying seems worth a shot. (This woman only sleeps three hours a night.)