Spaced repetition is a learning technique where you review information at deliberately increasing time intervals — not all at once, not the night before the exam, but at the exact moments when your memory is about to forget. This precision is why it works.
HOW THE FORGETTING CURVE WORKS
Every time you learn something new, your brain begins to forget it almost immediately. Ebbinghaus found that without reinforcement, people forget roughly 70% of new information within 24 hours, and about 80–90% within a week.
But each time you retrieve a memory just before it fades, the memory trace becomes stronger and the next forgetting curve decays more slowly — compounding across every review.
The psychological name for this is the spacing effect — the well-established finding that distributed practice is superior to massed practice for long-term retention. It is one of the most replicated findings in all of cognitive psychology.
THE LEITNER SYSTEM — SPACED REPETITION IN PRACTICE
The most practical implementation of spaced repetition for students is the Leitner System, developed by German science journalist Sebastian Leitner in 1972. It uses physical or digital flashcards sorted into boxes by retention level:
Cards you answer correctly advance to the next box — reviewed less frequently. Cards you get wrong return to Box 1 — reviewed more frequently. Over time, cards you genuinely know drift towards Box 5 where they barely need attention. Cards you struggle with stay in Box 1 where they get daily practice.
The system is elegant because it is self-correcting: your struggle becomes the algorithm.
Spaced repetition does not require special software — but software makes it dramatically easier by calculating the optimal review moment for hundreds of cards simultaneously. ShiftGlitch implements the full Leitner 5-box system free of charge. No credit card required.