Why sleep is part of studying, not separate from it

Students tend to think of sleep and studying as competitors for the same limited hours before an exam. The research on memory consolidation says they're closer to being the same activity, done in two different states.

What happens while you sleep

Learning a fact or a skill creates an initial, fragile memory trace. Sleep researchers - Matthew Walker's lab work is some of the most cited here - have shown that during sleep, particularly certain stages, the brain replays and strengthens recently formed memories, effectively moving them from a short-term, easily lost state into a more stable, long-term one. This isn't a metaphor; it shows up in sleep-stage activity that mirrors the activity recorded while the memory was first being formed.

Skip the sleep, and you skip a step in the process that turns "learned it an hour ago" into "will still know it next week." No amount of additional daytime review fully substitutes for it.

Why the all-nighter feels like it works

Cramming through the night gives you a memory that's maximally fresh at the moment of the test - you saw it four hours ago, so it's still sitting in short-term storage. That can genuinely carry you through a recognition-heavy test the next morning. It does very little for retention past that, partly because you skipped consolidation and partly because sleep-deprived encoding is worse in the first place: tired brains form weaker initial traces even during the studying itself.

So the trade an all-nighter makes is real, but it's narrower than it looks: it can rescue tomorrow morning, at the cost of everything the material was supposed to be useful for afterward.

The practical version

The last study session before a test matters less than people think; the sleep after it matters more.
If you have to choose between one more hour of review and one more hour of sleep the night before an exam, the sleep is very often the better trade for anything you'll need to remember past that exam.
This is one more reason cramming and spacing aren't equally good options with different time costs (see cramming vs. spacing) - spacing gives you sleep cycles between study sessions for free. Cramming, by construction, doesn't.