Spacing Effect states that we learn material more effectively and easily when we study it several times spaced out over a longer time span, rather than trying to learn it in a short period of time.
As you can guess, this means that cramming for an exam the night before is not as effective as studying material each night over a week or some period of time. There's one caveat - this holds true for material you want to store for a long time (i.e., really store it in memory), whereas cramming can work to store information for short periods of time.
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