http://web.arizona.edu/~tigger/assets/documents/Gomez(Inbal-Clark%20Eds)11.pdf

There is a direct correlation between the number of hours that a person sleeps and the quality of their memory

http://healthysleep.med.harvard.edu/healthy/matters/benefits-of-sleep/learning-memory

There are 3 stages of learning: acquisition, consolidation, and recollection. Acquisition and recollection occur when a person is conscious, however, consolidation only occurs during sleep.