Spacing effect
The spacing effect is the well-replicated finding that you remember information better when you spread your study across several sessions over time, rather than cramming it all into one sitting. It is the science behind spaced repetition.
Each time you let a little forgetting happen and then study again, your brain has to work harder to bring the material back. That extra effort is what strengthens the memory. Spread-out sessions force this again and again, so the same total study time produces far more lasting recall than one long block.
A common misconception is that cramming does not work at all. It does help you pass a test the next morning, but the memory fades fast. The spacing effect is about durable learning, so it matters most when you need to keep material for a cumulative final, a board exam, or the next course that builds on this one.
A biochemistry student reviews the glycolysis pathway for 20 minutes on Monday, again on Thursday, then the following week. By the exam she recalls each step cold. Her roommate studied the same pathway for one 90-minute block the night before and blanked on half the enzymes.