How Sleep Helps You Remember What You Studied

Sleep is often the first thing cut when work piles up. It gets treated as the reward at the end of a study day, so it is the obvious thing to give up when the day runs out of hours. But how much you keep from a day of studying depends partly on how you sleep afterwards. This post covers what the research says sleep does for memory, and how to protect it in an ordinary week.

What sleep does for memory

A new memory is fragile at first. Over the following hours, and especially during sleep, the brain reactivates what you recently learned and moves it into long-term storage, where it links up with what you already know. This is called memory consolidation. More than a century of research has found that people usually remember more after a period of sleep than after the same time spent awake (Rasch and Born, 2013).

The effect is real, but probably smaller than early studies suggested. A 2021 review of failed replications and null results concluded that sleep's benefit to memory is smaller, depends more on the task, and lasts for less time than researchers used to assume (Cordi and Rasch, 2021). So sleep helps, but it works alongside how you studied and how often you come back to the material. It does not replace either.

What sleep cannot do

Sleep does not teach you anything new. It will not fill in a chapter you skipped or fix an explanation you never read. It works on what your studying left behind, and a half-attentive skim of a page leaves less to work with than active study does.

Active study means producing the material yourself: answering questions without looking, explaining a process in your own words, getting something wrong and finding out why. There is more on this in the section on ending a study session below.

Why ordinary nights matter

The night before an exam may matter less than the weeks before it. In a study that tracked the sleep of 88 students in an introductory chemistry course with wearable trackers, students who slept longer, better and more regularly got better grades. Sleep on the single night before a test was not related to the score; sleep over the week and the month before it was (Okano et al., 2019). The study shows an association, not proof that sleep caused the grades.

Sleeping between study sessions also seems to help. In one experiment, 40 people learned foreign-language vocabulary until they knew it perfectly, then relearned it 12 hours later. Those who slept between the two sessions needed about half as much practice to relearn it, and remembered more a week later and six months later, than those who stayed awake in between (Mazza et al., 2016).

This makes the ordinary nights in the middle of term worth protecting: the night after a lecture that introduced a hard idea, or after your first proper pass through a chapter.

How a short night affects the next day

A short night costs you in two ways. Yesterday's material gets less of the sleep that helps you keep it. And today's material goes in less well: in one study, people who had been kept awake for a night remembered less of new material they studied the next day than people who had slept (Yoo et al., 2007).

Sleep loss also makes attention unreliable. You work normally for a while, then lapse for a moment where nothing registers. You can reach the bottom of a page with no idea what was on it. What an all-nighter costs you goes through the effects on attention and exams in more detail.

How to protect your sleep in a normal week

The aim is to stop sleep from being the thing that gives way every time the day overruns.

1. Decide when the study day ends before it starts. A fixed finish time turns the evening into a set amount of time to plan, instead of an open question you keep answering with "a bit more".
2. Do new and difficult material when you are most alert, and leave familiar review for when you are tired.
3. Keep your wake-up time roughly the same every day, including days with nothing early. Regular sleep went with better grades in the chemistry study above.
4. Spread each topic over several days instead of one long night. You get the benefit of spacing and the nights in between. The spaced repetition planner lays out the days for you from your exam date and topics.
5. Before bed, write a specific to-do list for tomorrow: the topics, questions or pages you did not reach. In a sleep-lab study, young adults who spent five minutes writing a specific to-do list at bedtime fell asleep faster than those who wrote about what they had already done (Scullin et al., 2018).

How to end a study session

Use the last few minutes of a session to test yourself instead of reading the material one more time.

1. Close your notes and write down, or say out loud, the main points of what you covered.
2. Answer a few questions on it without looking.
3. Check your answers and mark what you missed.
4. Put the points you missed at the top of tomorrow's to-do list.

Testing yourself leads to better recall days later than rereading does, even though rereading makes people feel more confident at the time (Roediger and Karpicke, 2006). It also tells you where to start tomorrow.

If your study material is in SymbioLearn, a Quick Quiz on the Concepts you just covered does steps 2 and 3: after each answer you see the right answer, why it is right, and the page it came from. Anything you get wrong is kept in Mistakes with its Source line, so tomorrow you know which page to go back to.

What to do next

Tonight, write tomorrow's to-do list and set a finish time for tomorrow's studying. Tomorrow, do the hardest new material when you are most alert, and end your last session by testing yourself. If you use SymbioLearn, Today will bring those Concepts back when their review is due, and sooner for the ones you got wrong.