Why Forgetting a Little Helps You Remember More

Go over something three times in one evening and it feels nailed down. Spread the same three sessions across a week and it feels shakier. Yet the spread-out version tends to win when it counts. The secret ingredient is a little bit of forgetting.

Imagine two revision timetables for the same test. Same material. Same total number of hours.

The first person does it all in one go. Three sessions back to back, the night before. By the end, the material is flowing. They can rattle it off. It feels learned.

The second person does one session, waits a couple of days, does another, waits again, then does the third. Each time they come back, it's a bit of a struggle. Some of it has slipped away and has to be dragged back up. It feels, if anything, worse.

Here's the twist. When it comes to remembering the material later, the second person is likely to be the one who does better.

That's the spacing effect. Learning is more effective when study sessions are spaced out. And it's a robust finding, backed by studies across many different kinds of memory task.

A man and his nonsense syllables

The story starts in the nineteenth century, with a German researcher called Hermann Ebbinghaus and an experiment that sounds faintly maddening.

He wanted to measure memory cleanly, without the meaning of words getting in the way. So he invented nonsense syllables, little meaningless scraps like WID and ZOF, and memorised lists of them. Then he tested himself after various lengths of time and plotted how much he'd kept.

It was a small study. He was his only subject. But the picture it painted has held up well. An attempt in 2015 to repeat his work, again with a single volunteer, came out with results similar to his originals.

What he found has become known as the forgetting curve. The classic forgetting curve suggests that, without any review, people tend to lose about half of newly learned material within a matter of days or weeks. That's the bad news.

The good news was in the second part. Every time the material was gone over again, the curve flattened. Forgetting slowed down. Ebbinghaus also found the spacing effect itself, publishing his detailed study of it in 1885.

Why the gap helps

So why should a gap help? You'd think the obvious thing is to keep the material fresh, not let it fade.

One explanation goes like this. When you come back to something after a gap, your brain has to go and fetch the earlier memory to connect it with what you're seeing now. The first encounter gets retrieved at the time of the second.

That retrieval is the important part. If you go over something again immediately, while it's still sitting right there, there's nothing to fetch. Your brain barely has to do anything. Psychologists have a phrase for this: the second go gets "deficient processing". It's a bit like being shown a phone number you're already looking at.

Leave a gap, and some of it slips. Now coming back takes real effort. And that effort of pulling a half-forgotten memory back up is what strengthens it. In effect, spacing gives you time to forget a little, so that relearning can strengthen what's stored for the long term.

It's a bit like a path through long grass. Walk the same route ten times in an afternoon and you barely flatten anything new. Walk it once a week, after the grass has started to spring back, and each trip does real work in wearing the path in.

Why cramming feels so good

Here's the counterintuitive part, and it's the reason people still cram.

Cramming feels better. That isn't an illusion about how you feel. It really does feel smoother and more confident. The trouble is that the feeling is a poor guide to how much you've actually learned.

Psychologists talk about fluency. One kind is retrieval fluency: the feeling that information is easy to recall, springing to mind without much effort. Another is perceptual fluency: the sense that something is familiar and easy to process. Both feel like signs of learning. Both can mislead you.

Rereading is a good example. It takes little effort, it makes the material look familiar, and so it feels good. But it's a passive, ineffective way to study. Meanwhile spacing, which feels harder, is the thing that's actually working. The struggle you feel when you come back after a gap isn't a sign the method is failing. It's the method.

Testing yourself

There's a close cousin to the spacing effect, and the two work well together.

It's called the testing effect, also known as retrieval practice. The idea is that long-term memory improves when part of the learning time is spent pulling information out of your memory, rather than just putting it in. Quizzing yourself beats rereading. Repeated testing does better than repeated study.

Combine the two and the effect gets bigger: spacing out your retrieval practice has been shown to boost the testing effect, especially when the testing is delayed.

It's the thinking behind spaced-repetition flashcard apps, which spread your reviews out over time. It's also why, with flashcards, dropping cards from the pile too early can lead to worse long-term memory. You stop the retrieval before it has done its work.

And, rather fittingly, students don't see retrieval practice as a useful learning tool. It doesn't feel as productive as it is.

How big a gap?

So how long should you wait between sessions? That's where it gets less tidy.

The best gap depends on how long you need to remember the material. Something you need for a test next week calls for a different rhythm from something you want to keep for years. For a single review, the evidence suggests one rule of thumb: wait around 10 to 20 per cent of the time until you'll need the information.

The overall pattern is strong. In one large review of studies, spaced practice beat massed (crammed) practice in 259 out of 271 comparisons. But that's a statement about averages across lots of experiments. How much it helps can vary with the material, and it isn't a promise about any one person's results.

Built in the returning

The deep lesson of all this is a slightly strange one. Forgetting isn't the opposite of learning. A bit of it, in the right place, is part of how learning works.

When you cram, you're repeating. When you space, you're returning. And it's the returning, the effort of going back and fetching something that has started to slip, that builds a memory to last.

SOURCES

1. Wikipedia, "Spacing effect." https://en.wikipedia.org/wiki/Spacing_effect (accessed 25 Oct 2026)

2. Wikipedia, "Forgetting curve." https://en.wikipedia.org/wiki/Forgetting_curve (accessed 25 Oct 2026)

3. Psychology in Action, "The Dangers of Fluency" (22 Oct 2018). https://www.psychologyinaction.org/2018-10-22-the-dangers-of-fluency/ (accessed 25 Oct 2026)

4. Wikipedia, "Testing effect." https://en.wikipedia.org/wiki/Testing_effect (accessed 25 Oct 2026)