Spaced Repetition Explained: The Science of Long-Term Memory

You've probably experienced this: you study something intensely, feel like you know it, and then a week later it's gone. Like it was never there.

This isn't a personal failing. It's how human memory works. And once you understand it, you can hack it.

The forgetting curve

In 1885, German psychologist Hermann Ebbinghaus discovered something important: forgetting follows a predictable pattern.

After learning something new:

• Within 20 minutes, you forget ~40%
• Within 1 hour, you forget ~55%
• Within 1 day, you forget ~75%
• Within 1 week, you forget ~90%

This is the forgetting curve. And it's brutal.

But Ebbinghaus also discovered something else: each time you review information, the forgetting curve flattens.

How spaced repetition works

Here's the key insight: there's an optimal time to review something. Too early, and you're wasting time on something you already remember. Too late, and you've already forgotten it.

The sweet spot is right before you're about to forget.

Spaced repetition systems (SRS) like Anki track this for you. They calculate when you're likely to forget each piece of information and schedule reviews at exactly the right time.

The result:

• First review: 1 day later
• Second review: 3 days later
• Third review: 1 week later
• Fourth review: 2 weeks later
• Fifth review: 1 month later
• And so on...

Each successful review doubles (or more) the interval. Eventually, you're reviewing something once a year—and still remembering it perfectly.

Why it's so effective

Spaced repetition works because of how memory consolidation works in the brain.

When you first learn something, it's stored in short-term memory (the hippocampus). To move it to long-term memory (the neocortex), you need to reactivate that memory multiple times.

But here's the trick: reactivation is most effective when the memory has partially faded. This is called desirable difficulty.

Easy reviews don't strengthen memory much. Hard reviews—where you have to work to recall something—create lasting changes in neural connections.

Spaced repetition automatically creates this desirable difficulty by timing reviews right at the edge of forgetting.

The numbers are ridiculous

Studies on spaced repetition show:

• Medical students using Anki score 10-15% higher on board exams
• Language learners retain vocabulary 2-3x longer compared to massed practice
• With proper spacing, you can remember 90%+ of material indefinitely

And the time investment decreases over time. As intervals grow, you spend less and less time on each card while maintaining near-perfect retention.

The bottleneck isn't the system

If spaced repetition is so effective, why doesn't everyone use it?

The answer: creating the cards takes forever.

Anki, the most popular SRS, is powerful but requires manual card creation. For every hour of material you want to learn, you might spend 2-3 hours making cards.

Most people give up before they see the benefits.

The solution

This is why we built Oboeru. We believe spaced repetition should be accessible to everyone—not just people with unlimited time for card creation.

Oboeru generates high-quality Anki cards from any content in seconds. Notes, PDFs, textbooks, code. You focus on learning; we handle the tedious work.

Because the science is clear: spaced repetition works. The only question is whether you'll actually use it.