Spaced Repetition: How It Works and How to Use It

Spaced repetition is the practice of reviewing material at increasing intervals over time, rather than cramming repeated exposures close together. The idea has a specific, research-backed mechanism behind it, studied directly in a 2008 paper by Nicholas Cepeda and colleagues, titled "Spacing Effects in Learning: A Temporal Ridgeline of Optimal Retention," published in Psychological Science.

What the study examined

The study looked at how the gap between study sessions relates to how well information is retained after a delay, specifically examining what the researchers call the optimal gap, meaning the ideal spacing between study sessions, measured relative to how long after studying you need to still remember the material, called the test delay.

The key finding: the optimal gap shrinks as a proportion of the delay

One of the paper's central findings is that the ideal spacing gap, expressed as a percentage of the total test delay, is not constant. According to the study, when measured as a proportion of test delay, the optimal gap declined from about 20 to 40 percent of a 1-week test delay to about 5 to 10 percent of a longer test delay.

In plain terms, this means the proportion of time you should wait between study sessions, relative to how far away your test is, gets smaller as the total time until your test grows longer. For a test one week away, spacing your two study sessions by roughly 20 to 40 percent of that week, meaning somewhere between about 1 and a half to around 3 days apart, falls within the range the study associated with optimal retention. For a test much further away, the ideal gap as a percentage of that longer time window shrinks.

Why this is different from simply "spacing things out more"

A common oversimplification of the spacing effect is "wait longer between study sessions and you'll remember better." The Cepeda et al. findings complicate that a bit: there's an actual peak, described in the paper's title as a "temporal ridgeline," meaning retention improves as you increase the gap up to a point, but doesn't keep improving indefinitely as you keep spacing sessions further and further apart. The relationship between gap and retention has a measurable shape to it, not a straight line that keeps going up forever.

Applying this to real studying without precise percentages

Because the exact optimal gap depends on how far away your test is, practical application comes down to a general principle rather than a fixed day count that works for every situation:

• For a test about a week away: space your review sessions across multiple days rather than reviewing the same material twice in one sitting or on consecutive days.
• For a test a month or more away: the ideal gap as a percentage of that total time shrinks, which in practice means you don't need to wait nearly as long, proportionally, between reviews as you would for a one-week timeline, even though the absolute number of days between reviews might still be larger than a one-week study plan.

The core actionable idea from this research isn't a specific day count to memorize, but the general practice of deliberately spacing your review sessions apart rather than clustering them close together, and recognizing that the right amount of spacing depends on how far out your test actually is.

Why cramming conflicts with this mechanism

Cramming, by definition, compresses all your study sessions into a very short window right before a test, which is close to the opposite of what this research describes as producing strong retention over any meaningful delay. The spacing effect research specifically measures retention after some test delay, not immediately after studying, which is the scenario cramming is weakest at, since cramming can produce short-term familiarity that fades quickly once the delay between study and test grows.

A simple way to build spacing into an exam prep schedule

If you know how far away your exam is, you can roughly apply the pattern described by Cepeda et al.: for a test about a week out, plan at least two separate review sessions spaced a day or more apart rather than one long session. For a test weeks away, extend the overall study timeline but don't feel obligated to extend the individual gaps between sessions by the same proportion, since the research found that proportion shrinks as the total timeline grows.

A broader review reached a similar conclusion

A 2013 review by John Dunlosky and colleagues, published in Psychological Science in the Public Interest, evaluated a wide range of study techniques reported by students, including rereading and highlighting, against techniques like distributed practice, which is the review's term for spacing study sessions apart over time. The review concluded that distributed practice received a high utility rating because it benefits learners of different ages and abilities and has been shown to boost performance across many tasks, including in real educational contexts, while rereading and highlighting, despite being the most commonly reported study methods among students, do not consistently boost performance.

Key takeaways

• A 2008 study by Cepeda and colleagues, published in Psychological Science, found that the ideal spacing gap between study sessions is not a fixed percentage of your time until the test.
• For a roughly 1-week test delay, the optimal gap was around 20 to 40 percent of that week; for longer delays, the optimal gap shrinks to around 5 to 10 percent of the total delay.
• Retention improves with more spacing only up to a point, described by the researchers as a "temporal ridgeline," not indefinitely as you keep spreading sessions further apart.
• Cramming compresses study sessions into a short window, which works against the mechanism this research identifies for retention after a delay.
• Practical application means deliberately separating review sessions across multiple days, with the appropriate gap depending on how far away your test actually is.