The Science of
Spaced Repetition

Imagine you're trying to remember how to spell rhythm. You practice it five times in a row, confident you've got it. But a week later? It's gone. This is the heart of a problem that educators have wrestled with for over a century.

The solution turns out to be beautifully simple: instead of cramming practice together, spread it out over time. This technique—called spaced repetition—is one of the most robust findings in all of learning science, backed by hundreds of studies spanning nearly a century.

And here's what makes it so remarkable: it works for almost everything, from spelling words to learning languages to memorizing medical terminology. Let's dive into why.

The story begins in 1939 with a researcher named H.F. Spitzer, who conducted an ambitious study with over 3,600 sixth-grade students learning science facts. Spitzer discovered something remarkable: students who reviewed material at spaced intervals remembered far more than those who crammed everything together.

But here's the fascinating part: his findings were largely ignored. It wasn't until the late 1960s that cognitive psychologists rediscovered this effect and began to understand why it works so well.

The magic of spaced repetition isn't in the total time you spend practicing—it's in how you distribute that time. When you space out practice sessions, several powerful learning mechanisms kick in simultaneously:

What's particularly fascinating is that research suggests reviewing material within the first 24 hours after initial learning helps "reset" the forgetting curve, setting up optimal conditions for long-term retention.

Recent research specifically tested spaced repetition for spelling acquisition. In a 2025 study with 108 German third-graders, researchers found that combining spaced repetition with interleaved practice (mixing different spelling rules rather than blocking them together) produced significantly better learning outcomes.

But here's the twist: even though students learned more with this approach, they perceived it as more difficult and rated their own learning as less successful. Researchers call this the "metacognitive illusion"—the feeling that harder equals less effective, when actually the opposite is true.

The good news is that spaced repetition has evolved from a laboratory finding to a practical learning tool. Multiple algorithms now exist to automate optimal spacing:

• The Leitner System (1970s): Uses physical flashcards with progressively longer intervals for mastered items.
• SuperMemo SM-2: A digital algorithm that adjusts intervals based on how easily you recall each item.
• FSRS & Half-Life Regression: Modern algorithms that use sophisticated models to predict optimal review timing.

Companies like Duolingo have even developed proprietary algorithms using data from over 40 million users to fine-tune spacing intervals. The science is solid, and the tools to implement it are more accessible than ever.

The Bottom Line

Spaced repetition isn't a hack or a shortcut—it's simply how human memory works best. Nearly a century of research consistently shows that distributing practice over time produces dramatically better long-term retention than massed practice.

For spelling specifically, the evidence is clear: combining spaced repetition with interleaved practice (mixing different spelling patterns) helps students build flexible, lasting spelling knowledge. Yes, it might feel harder at first. But that difficulty is the very thing that makes the learning stick.

The science has spoken. The only question left is: are you ready to put it into practice?