What is spaced repetition and why does it work?

Spaced repetition means reviewing material at increasing intervals rather than repeatedly in one sitting. Instead of studying something five times today, you study it today, in two days, in a week, in three weeks. It is among the most robustly supported findings in learning research, replicated across a century and across subjects, ages and materials.

The mechanism relates to how forgetting works. Memory decays over time, and the useful part is that retrieving something just as it starts to fade strengthens it more than retrieving it while it is still fresh. Reviewing immediately feels easy and does little; reviewing at the edge of forgetting feels harder and does much more.

That produces a counterintuitive consequence worth naming: the study session that feels least productive is often the most effective. Difficulty during retrieval is the signal that learning is happening, which is why massed practice feels better and works worse.

The intervals do not need to be precise. Doubling roughly each time is fine, and the benefit comes from spacing at all rather than from any particular schedule. Software exists that calculates intervals per item based on how easily you recalled it, which is useful for large volumes such as vocabulary or medical terminology, but a handwritten schedule captures most of the gain.

The practical requirement is starting early. Spacing cannot be compressed into the final week, which is the single most common reason people conclude it does not work for them.

It combines naturally with retrieval practice, since the review should be an attempt to recall rather than a re-reading.