How spaced repetition works

Where spaced repetition comes from, what Ebbinghaus's forgetting curve and later spacing research actually show, and what a scheduler does with it.

By Bleart Emini, founder · Published October 26, 2026 · Last reviewed October 26, 2026 · 5 min read

Spaced repetition is a simple idea with a long research history: review something just before you would forget it, and each review makes the memory last longer. Software turns that idea into a schedule for every card in a deck. This article covers where the idea comes from, what the main experiments found, what a scheduler actually does, and where the evidence stops.

The problem: forgetting is fast, then slow

In 1885 the German psychologist Hermann Ebbinghaus published Über das Gedächtnis ("On Memory"). He made up lists of nonsense syllables, so that earlier knowledge could not help, learned them to a set standard, and tested himself after different delays. He measured retention with a "savings" method: how much less time it took to relearn a list than it took to learn it the first time. He was the only subject.

Plotted over time, his results gave the famous forgetting curve: a steep drop soon after learning that flattens out later. An English translation appeared in 1913 as Memory: A Contribution to Experimental Psychology.

One caution matters here. The curve describes meaningless syllables learned by one man. It is a useful picture of how forgetting behaves, not a number you can read off for your own chemistry notes. Real material that you understand and connect to other things is forgotten at different speeds.

Does the curve replicate?

A 2015 study by Murre and Dros repeated the experiment. One subject spent about 70 hours learning word lists and relearning them after 20 minutes, 1 hour, 9 hours, 1 day, 2 days or 31 days, again measuring savings. The authors report that they replicated Ebbinghaus's forgetting curve and that their results closely resembled his original data. They also report that the curve was not perfectly smooth: it most likely shows a jump upwards starting at the 24 hour point. Their discussion links that bump to possible effects of sleep but says this still needs testing.

That is two single-subject studies, 130 years apart, pointing the same way. It is good support for the shape of the curve and weak support for any specific schedule.

The spacing effect: what happens when you review

The curve says memories fade. The spacing effect says when to review matters. In a 2006 review, Cepeda and colleagues pooled 839 assessments of distributed practice from 317 experiments in 184 articles. Their main pattern: the best gap between study sessions grows as the time until the final test grows. A test tomorrow calls for a short gap; a test months away benefits from a longer one.

A flashcard study by Kornell (2009) shows the same idea in a form students recognize. Participants learned GRE-style word pairs on a computer across three experiments. Studying one large stack of cards, which spaces each card's repetitions further apart, beat studying four smaller stacks separately. Spacing also beat cramming everything on the day before the test. Spacing was more effective for 90 percent of participants, yet after the first study session 72 percent believed massing had been more effective.

That last figure is the practical lesson. Spaced study often feels worse in the moment, because the card you meet again has partly faded and takes effort to recall. That effort is part of why it works, so the feeling is a poor guide to whether your plan is working.

Limits to keep in mind: most of this work uses word lists and short facts, with delays from minutes to weeks, mostly in lab sessions with adults. It tells you much less about long chains of reasoning, and it does not give an exact schedule for any course.

What a scheduler does

Reviewing every card on a calendar by hand does not scale past a few dozen cards. A spaced repetition scheduler automates three steps.

1. Estimate memory. After each review it updates an estimate of how well you know that card, using your answer and how long it has been.
2. Pick the next date. It sets the next review for when you are expected to be near a chosen level of recall. A card you know well waits longer. A card you missed comes back soon.
3. Show what is due. Each day you see the cards whose time has come, not the whole deck.

Different programs use different models. Some describe FSRS as a newer alternative to the older SM-2 algorithm, one that aims to estimate more accurately how much you are likely to forget. Cardvoo schedules cards with FSRS. No study found for this article ranks one scheduling algorithm against another for students, so choose a tool for how you will actually use it.

Using it well

• Answer from memory first. Look at the front of the card, try to produce the answer, then flip. Flipping first turns a test into rereading. See active recall versus rereading.
• Grade yourself honestly. The schedule is only as good as your answers. Marking a guess as "good" makes the next gap too long.
• Keep cards small. One idea per card gives the scheduler a clean signal. The guide on writing good flashcards covers this.
• Review a little every day. Many short sessions fit the way the schedule works and are easier to keep than rare long ones.
• Do not skip the due cards. A backlog is the usual way a schedule breaks. Clear the oldest first, in smaller sessions, rather than abandoning the deck.
• Add an end date. If you have an exam on a known day, work backward from it, as in the one-week finals plan.

Key takeaways

• Ebbinghaus's forgetting curve, from nonsense syllables and one subject, shows memories fading fast at first and then more slowly. A 2015 replication with one subject found a similar curve.
• Across hundreds of experiments, the best gap between study sessions grows with the time until the test.
• Spaced study usually feels harder and less effective than cramming, even when it works better.
• A scheduler estimates how well you know each card and brings it back before you are likely to have forgotten it.
• The research is mostly word lists and short delays, so treat any schedule as a sensible default, not a formula.

Sources

1. Ebbinghaus, H. (1885). Über das Gedächtnis: Untersuchungen zur experimentellen Psychologie. Leipzig: Duncker and Humblot. English translation by H. A. Ruger and C. E. Bussenius (1913), Memory: A Contribution to Experimental Psychology, New York. (Publication details and method confirmed from the Wikipedia and History of Information summaries of the work, not from the book itself.)
2. Murre, J. M. J., and Dros, J. (2015). Replication and analysis of Ebbinghaus' forgetting curve. PLOS ONE, 10(7), e0120644. https://doi.org/10.1371/journal.pone.0120644 (abstract and discussion read at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0120644)
3. Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T., and Rohrer, D. (2006). Distributed practice in verbal recall tasks: A review and quantitative synthesis. Psychological Bulletin, 132(3), 354-380. https://doi.org/10.1037/0033-2909.132.3.354 (full text read at https://www.evullab.org/pdf/CepedaPashlerVulWixtedRohrer-PB-2006.pdf)
4. Kornell, N. (2009). Optimising learning using flashcards: Spacing is more effective than cramming. Applied Cognitive Psychology, 23(9), 1297-1317. https://doi.org/10.1002/acp.1537 (abstract read via Crossref)

Keep reading

• Active recall versus rereading
• How to study for finals in one week
• Spaced repetition explained
• Cramming versus spacing
• Active recall and retrieval practice

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