What Is the Forgetting Curve?
The forgetting curve is a graph showing how quickly newly learned information becomes hard to recall over time. Described by the German psychologist Hermann Ebbinghaus in his 1885 work Über das Gedächtnis (On Memory), the curve shows that forgetting is not random but follows a predictable pattern. The steepest drop happens right after learning: within the first hours and the first day, much of the material becomes inaccessible, after which the loss slows down.
This is not a flaw of memory but the way the brain works. Out of the countless inputs it meets every day, the brain moves into long-term storage only what proves useful again and again. If you never return to a piece of information, the brain treats it as unimportant and lets it go. The real lesson of the forgetting curve is that learning something once is not enough; durability is built through well-timed review.
Ebbinghaus’s Experiment and the "Savings" Method
To rule out the help that meaning gives to memory, Ebbinghaus created about 2,300 nonsense syllables such as "wid" or "zof" and memorised them on himself. After set intervals he tried to relearn the same list and measured how much less repetition the second learning required. He called this measure "savings": if relearning a list took noticeably less time than the first time, part of the information was still in memory.
The savings method matters because, instead of a yes/no measure of recall, it also quantifies partial memory. Ebbinghaus’s results were consistent: about half the material was inaccessible after an hour, and even more after a day. But every review pulled the curve back up and slowed the next decline. Modern research has largely confirmed Ebbinghaus’s core finding.
Why Does Spaced Repetition Work?
Spreading the same total study time across days or weeks, rather than cramming it into a single session, markedly improves long-term recall. Known in cognitive psychology as the "spacing effect," this finding has been replicated in hundreds of studies from the lab to the classroom. The meta-analysis by Cepeda and colleagues, covering 184 articles and 317 experiments, showed that spaced study beats massed (blocked) study under almost every condition.
One reason is that when each review happens after a little forgetting has set in, the brain works harder to retrieve the item, and that effort strengthens the memory trace. A second reason is that information encoded at different times and in different contexts gains more varied "access routes" in memory. The same meta-analysis also shows that the ideal gap is not fixed: the longer you want to keep the information, the longer the interval between reviews should be.
When Should You Review?
There is no single "magic" interval, but a practical starting schedule looks like this: do the first review on the day you learn the material or the next day; the second about a week later; the third about a month later; then keep widening the gaps. After each successful recall you can push the next review a little further out.
Scale the interval to when you will need the information. If the exam is a week away, daily reviews make sense; if you will still need the knowledge months later, reviews should be weeks apart. Research shows that for long retention, gaps of several weeks work better than gaps of a few days. If you forgot a lot during a review, shorten the interval; if you recalled everything easily, lengthen it.
Combining It with Active Recall
Spaced repetition is at its strongest when paired not with passive rereading but with active recall. Rereading a text creates a feeling of familiarity, but that feeling is not the same as the "produce it from a blank page" skill an exam demands. Instead, close the book, ask yourself the question, try to recall the answer, and then check. This "testing effect" rests on the fact that every retrieval attempt rebuilds and strengthens the memory.
Studies in real classroom settings support this combination. In research by Donker and colleagues with medical students, those who did spaced recall exercises retained their knowledge markedly better at the end of term than peers who only attended lectures or reread their notes. So the answer to "how" matters as much as the answer to "when": in spaced sessions, testing yourself.
Practical Tips for Everyday Use
- Use flashcard systems. Software such as Anki or a physical Leitner box brings each card back at longer intervals as you answer it correctly, doing the review scheduling for you.
- Put review blocks in your calendar. On the day you learn a new topic, schedule short (10–20 minute) recall sessions for the next day and one week later.
- Interleave topics. Rather than studying one topic for hours, alternating between several (interleaving) sharpens your ability to tell them apart, on top of the spacing effect.
- Explain it to someone. Saying a topic out loud without looking at your notes is a powerful recall exercise.
- Don’t neglect sleep. Much of memory consolidation happens during sleep; leaving enough time between review and sleep supports learning.
Common Misconceptions
The most common mistake is studying everything in one sitting the night before an exam. This seems to work in the short term, because the information is still fresh the next morning; but within a few days almost all of it is gone. A second misconception is thinking that recognition is the same as recall: saying "I knew that" when you see an answer does not mean you could have produced it yourself.
A third misconception is that forgetting something between reviews is bad. On the contrary, mild forgetting is healthy: when the brain has to strain a little, learning sticks better. The goal is not to never forget, but to return at the right moment before forgetting deepens. Treat the forgetting curve not as an enemy but as a guide for planning your review schedule.
Sources
- Britannica, "Hermann Ebbinghaus" — https://www.britannica.com/biography/Hermann-Ebbinghaus
- Cepeda, N. J., Pashler, H., Vul, E., Wixted, J. T. & Rohrer, D. (2006). "Distributed Practice in Verbal Recall Tasks: A Review and Quantitative Synthesis." Psychological Bulletin, 132(3), 354–380 — https://digitalcommons.usf.edu/psy_facpub/1771/
- Donker, S. C. M., Vorstenbosch, M. A. T. M., Gerhardus, M. J. T. & Thijssen, D. H. J. (2022). "Retrieval practice and spaced learning: preventing loss of knowledge in Dutch medical sciences students in an ecologically valid setting." BMC Medical Education, 22 — https://pmc.ncbi.nlm.nih.gov/articles/PMC8793259/