Spaced Repetition: How Memory Really Works

The forgetting curve is not your fault

In 1885, Hermann Ebbinghaus measured how fast he forgot lists of nonsense syllables after learning them. His data produced the forgetting curve: within 24 hours, roughly 70% of new information is gone unless reviewed.

This is not a bug of human memory; it is the design. Your brain actively discards information that does not prove useful through repetition. Forgetting is how memory stays efficient.

Spaced repetition exploits the curve

The key insight: reviewing information right before you would otherwise forget it produces the strongest memory consolidation. Too soon and the review is wasted; too late and you relearn from scratch.

Spaced repetition schedules reviews at expanding intervals. A typical sequence: review after 1 day, then 3 days, then 7, then 21, then 60. Each successful recall pushes the next review further out. Each failure resets the schedule.

Why it outperforms rereading

Classic research by Karpicke and Roediger (2008) compared students who reread material versus students who practiced retrieval. On a test one week later, retrieval practitioners scored 50% higher for the same amount of study time.

The reason is counterintuitive: the effort of retrieval is what strengthens memory, not the exposure to information. Rereading feels productive because it is easy. Retrieval feels harder, which is precisely the signal that learning is happening.

The SM-2 algorithm in plain English

Most spaced repetition apps (Anki, FocusAI Coach) use a variant of the SM-2 algorithm, developed by Piotr Wozniak in 1990. It adapts the review schedule based on how easily you recall each item.

• Recall was trivial → double the next interval
• Recall was correct but effortful → mild increase
• Recall failed → reset the interval, review tomorrow

The practical effect: easy items appear rarely, hard items appear often. Your study time concentrates on exactly the material that needs it.

When spaced repetition is most useful

Spaced repetition excels for discrete facts: vocabulary, formulas, dates, definitions, proper names. It is weaker for concepts that require understanding — for those, combine spaced recall with active problem-solving and teaching others (the Feynman technique).

A realistic daily commitment is 10-15 minutes. That is enough to sustain a vocabulary of 5,000-10,000 items across languages, medical terminology or technical concepts indefinitely.

Getting started

Pick one subject you want to remember long term. Create 30-50 flashcards this week. Review them daily with a spaced repetition app. In one month, you will have built something that compounds — not a temporary cram that fades.