Hermann Ebbinghaus (1885) showed that memory decays roughly exponentially: a steep drop within the first day, then a slow flattening.

Ebbinghaus, a German psychologist, ran the first rigorous memory experiments on a single subject — himself. He memorized long lists of "nonsense syllables" (meaningless three-letter combinations like WID or ZOF, chosen so prior knowledge couldn't help) and then tested how much he could relearn after various delays. Plotting retention against time produced a curve that falls fast at first and then levels off. The practical lesson: most of what you forget, you forget quickly — so the timing of review matters enormously.