Definition
The spacing effect is a psychological phenomenon in which you remember things much longer when you split your study or practice into several sessions with time in between. Think of painting a wall: one thick coat doesn't last as well as thin coats applied after each one dries completely. It is also called the "distributed practice effect."
Why Cramming Disappears After the Test
If you stay up all night stuffing facts into your head, you can often pull off the exam the next morning. But most of us have had this experience: a week later, your mind is a blank. A common explanation is that your brain never got the time to move the information into long-term memory (the storage room for things you keep for a long time) and lock it in.
The same goes for laying bricks. You spread the mortar, then wait until it firms up a bit before placing the next brick. If you keep stacking bricks on wet mortar, the wall can't hold the weight and it all comes crashing down. Memory works the same way. After you take in information, your brain needs time to sort it out and set it.
Cramming is like a wall built on mortar that hasn't dried. It may look solid for now, but within a few days it scatters without a trace.
Review When You've Just Started to Forget
If you look at a vocab word again one minute after memorizing it, it's so easy that your brain barely tries. It figures, "I already know this, so I can coast." But if you wait a day or two, until the memory gets fuzzy, and then try to recall the word, you really have to work for it.
Many researchers think this hard work of pulling a memory back up is what makes it stronger. Plenty of experiments show that information you struggled to recall sticks longer than information you simply re-read. That said, there are several other explanations for why the spacing effect happens.
So even if you study for the same 3 hours, it works much better to study 1 hour a day over three days than to cram all 3 hours into one day. Just spreading out the time can stretch how long a memory lasts.
A Little More Precisely: The Forgetting Curve and Review Intervals
The German psychologist Hermann Ebbinghaus tested how human memory fades over time. In his experiments, where he memorized meaningless syllables, he forgot more than half within a single day. Material that has meaning, or that connects to what you already know, fades more slowly. This pattern is called the "forgetting curve."
But if you review at well-timed intervals, just before you forget something completely, the forgetting slows down noticeably. A widely used approach is to gradually widen the gaps between reviews: the first review the day after you learn, the second three days later, the third a week later. However, research is mixed on whether widening the gaps is always better than keeping them even. Researchers call this approach "spaced repetition."
The spacing effect isn't limited to memorizing words. Similar benefits have been reported when people learn skills like playing an instrument or practicing a physical movement. The size of the effect depends on the type and complexity of the task, so it's more accurate to say that a similar trend shows up across many fields.
Common misconceptions
The more times you repeat a review in one sitting, the more effective it is.
If you go over something again and again in one sitting, your brain thinks it already knows it and stops trying. Memory tends to last longer when you review after you've forgotten a little, while it's fuzzy and you have to rack your brain. The right gap does depend on how long you need to remember the material.
Where you meet it
Material you study for 1 hour a day over a week is remembered much longer after the test than material you cram in an 8-hour all-nighter the night before.
When you study flashcards, you remember missed words longer if you revisit them a day later, then three days later, than if you go over them many times on the same day.
In one sentence
Review after you've forgotten a little, with time in between, and the memory will stay in your brain the longest.