Consolidation
The brain turning a fresh, fragile memory into a lasting one. Much of it happens while you sleep.
A new memory starts out fragile. Over the hours and days after you learn something, the brain firms it up. That process is called consolidation. Evidence: solid It also seems to tie the new memory into what you already know, though how that works is still argued over. Evidence: decent
A good part of it seems to happen in sleep. Evidence: decent In rats, cells in the hippocampus, the brain's memory centre, fire in roughly the same order during sleep as they did while the animal learned a route, only faster. Evidence: decent In people, sleeping soon after learning protects more of it than staying awake for the same hours. Evidence: decent
Remembering is not opening a stored file. The brain fires up roughly the same pattern of activity it used when you learned the thing. Evidence: decent
Why it matters
Spreading study over days beats cramming it into one night. Evidence: solid Consolidation between sessions is probably part of why, though not the whole story. Evidence: decent Cut sleep short and you cut into the time when much of the day gets written down. Evidence: decent