Hermann Ebbinghaus documented it in 1885 using himself as the sole test subject: spaced study sessions produce better retention than massed study of the same total duration. The effect has been replicated across 140 years, thousands of studies, and every domain from motor skills to second-language vocabulary.
The mechanism involves sleep. Each interval in a spaced sequence interposes one or more sleep cycles, during which the hippocampus replays the day’s traces and begins transferring them to neocortical long-term storage.
Fluera’s spaced repetition scheduler is the most direct instantiation of this principle — widening intervals, returns timed to cross sleep boundaries, interleaved review paths. Cramming is the default failure mode it is built to prevent.