What is Spaced Repetition?
Spaced repetition is a review schedule that spreads practice sessions out over increasing intervals of time instead of massing them together. Reviewing material just as it starts to fade produces far more durable memory than cramming, a finding replicated across more than a century of experiments on distributed practice.
The method rests on the spacing effect, first documented by Ebbinghaus in the 1880s and confirmed in a large quantitative synthesis by Cepeda and colleagues covering decades of verbal recall studies. Spreading the same amount of study across separate sessions reliably beats packing it into one, and longer gaps tend to support longer retention. The Dunlosky review rated distributed practice high utility, one of only two techniques to earn that rating.
Modern spaced repetition software turns this into an algorithm: each item you review is scheduled for its next appearance based on how well you knew it, so easy items drift far apart while shaky ones return quickly. Flashcard tools such as Anki and SuperMemo made this workflow mainstream.
For readers, the principle matters more than any specific app. Returning to your highlights days and weeks after saving them, rather than never or all at once, is spaced repetition in its simplest form, and it pairs naturally with active recall: test yourself on the material at each spaced visit.