It seems sleep improves our ability to learn words (and presumably other things). An article in Science Daily says:
Scientists at the University of Chicago have demonstrated that sleeping has an important and previously unrecognized impact on improving people’s ability to learn language.
Researchers find that ability of students to retain knowledge about words is improved by sleep, even when the students seemed to forget some of what they learned during the day before the next night’s sleep. This paper, “Consolidation During Sleep of Perceptual Learning of Spoken Language,” is being published in the Thursday, Oct. 9 issue of the journal Nature. The paper was prepared by researcher Kimberly Fenn, Howard Nusbaum, Professor of Psychology, and Daniel Margoliash, Professor in Organismal Biology and Anatomy.
“Sleep has at least two separate effects on learning,” the authors write. “Sleep consolidates memories, protecting them against subsequent interference or decay. Sleep also appears to ‘recover’ or restore memories.” […]
Although the study dealt specifically with word learning, the findings may be relevant to other learning, Nusbaum said. “We have known that people learn better if they learn smaller bits of information over a period of days rather than all at once. This research could show how sleep helps us retain what we learn.”