CONCEPT
Spacing Effect
The finding—replicated over a thousand times since Ebbinghaus—that distributing practice across time produces vastly better retention than massing it into a single session, because gaps allow partial forgetting that makes re-learning effortful and deep.
The spacing effect is the most robust phenomenon in experimental psychology: when practice of the same material is distributed across multiple sessions separated by temporal gaps, long-term retention dramatically exceeds what massed practice (the same total time concentrated into one session) produces. A student studying vocabulary for ten minutes on five separate days will remember far more a month later than a student studying for fifty minutes straight—even though the massed student outperforms immediately after practice. The mechanism: gaps between sessions allow retrieval strength to decay, so that re-engagement requires effortful retrieval from a partially degraded trace. This effortful re-learning builds storage strength in ways that maintaining a fresh trace through continuous practice does not. The effect's magnitude increases with longer spacing intervals, up to an optimal gap that depends on the final test delay.
In The You On AI Field Guide
Hermann Ebbinghaus's 1885 self-experimentation established the basic finding, but for a century it was treated as a laboratory curiosity