certain stages of sleep : improved relearning .
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 .
because we can not possibly remember all of the information that we encounter , it is often necessary to go back and learn that information again .
given all of these benefits , the fact that a third of the human lifespan is spent sleeping makes evolutionary sense .
during two sessions occurring 12 hr apart , 40 participants practiced foreign vocabulary until they reached a perfect level of performance .
however , sleep appears to have another important function : helping us learn .
across a plethora of memory tasks — involving word lists , maze locations , auditory tones , and more — going to sleep after training yields better performance than remaining awake .
this has prompted many sleep researchers to reach a provocative conclusion : beyond merely supporting learning , sleep is vital , and perhaps even directly responsible , for learning itself .
the other half learned in the evening of one day , slept , and then relearned in the morning of the next day .