The spacing effect is one of the most robustly replicated findings in learning science, first documented by Hermann Ebbinghaus in the 1880s using himself as the subject: he memorized nonsense syllables and tracked his own recall over time, discovering what's now called the "forgetting curve" — memory fades fastest right after learning, then more slowly — and that spaced review sessions dramatically outperformed massed study for the same total time invested, even though cramming often produces better performance on an immediate test. This happens because spaced repetition forces retrieval after some forgetting has already begun, and the act of successfully retrieving a partially-forgotten memory strengthens it more than reviewing something still fresh in mind.
The practical implication is specific: if a test is tomorrow, cramming tonight will likely outperform a spaced schedule on that specific test. But for anything meant to be retained past that test — which is most of what's actually worth learning — spaced repetition wins decisively, which is the whole rationale behind flashcard systems that deliberately increase the interval between reviews of items you already know.
With no research assistants, no funding, and no prior model to follow, Ebbinghaus made himself the sole subject of his own research: he memorized lists of meaningless three-letter syllables (to rule out any advantage from real-word meaning or association), then tested his own recall at carefully measured intervals afterward — sometimes 20 minutes later, sometimes a month later. Plotting the results produced the now-famous "forgetting curve": recall dropped sharply within the first hour, then leveled off into a much slower decline. When he tried relearning the same lists using spaced sessions instead of one long study block, the spaced version required far less total relearning effort for the same eventual retention.