Memory consolidation — the process of stabilizing a newly formed memory and transferring relevant parts of it from the hippocampus toward more permanent cortical storage — happens substantially during sleep, particularly during slow-wave sleep and REM. Studies comparing groups that learn material and then sleep versus stay awake for an equivalent period consistently show better retention in the sleep group, even when neither group gets additional study time.
This has a very concrete practical implication that's easy to underweight: pulling an all-nighter to cram right before a test doesn't just cost you alertness the next day — it actively undermines the consolidation of what you crammed, because the sleep window where that information would have been stabilized never happened. Sleep isn't a passive break from learning; it's an active part of the learning process itself.