Sleep and memory: what happens in your brain at night

People often think the brain rests while we sleep. It does not: at night, it sorts through the day, keeps what matters and files it away for the long term. Here is what research has found about sleep and memory, and how to use it to remember more.

Your brain does not switch off at night

A night's sleep is not one long, uniform block. It is made of cycles of about 90 minutes, moving through light sleep, deep slow-wave sleep and REM sleep, when the most vivid dreams happen. Deep sleep dominates the first half of the night; REM sleep gets longer towards morning.

Throughout, the brain stays very active. Two German researchers, Björn Rasch and Jan Born, brought together hundreds of studies on the subject in a major review published in 2013. Their conclusion: sleep does not just protect memories, it actively helps to fix them.

How sleep stores your memories

During the day, new memories are first recorded by the hippocampus, a small structure deep inside the brain. It works like a buffer: fast, but with limited capacity.

During deep slow-wave sleep, the hippocampus "replays" the day's experiences. These reactivations, paced by slow electrical waves, gradually transfer memories to the cortex, where they are stored for the long term and linked to what you already know. Researchers call this memory consolidation. The brain does not keep everything: it seems to favour what will matter later.

The idea is not new. As early as 1924, two American psychologists, John Jenkins and Karl Dallenbach, showed that people forget less of a list of syllables after sleeping than after staying awake for the same number of hours.

Sleep after learning, and between study sessions

A team in Lyon, France (Stéphanie Mazza and colleagues) ran a striking experiment in 2016. Forty people had to learn foreign vocabulary until they knew it perfectly, then relearn it 12 hours later. Half of them learned in the morning and relearned in the evening of the same day. The other half learned in the evening, slept, then relearned the next morning.

The result: those who slept between the two sessions needed half as much practice to get back to a perfect score. And they remembered the words much better a week later, and again six months later. As the authors put it, sleeping after learning is a good strategy; sleeping between two learning sessions is an even better one.

An all-nighter also hurts the next day's learning

Sleep does not only consolidate. It also gets the brain ready to learn. In 2007, a US team (Seung-Schik Yoo, Matthew Walker and colleagues) kept volunteers awake for one night, then had them learn new information the next day while their brains were scanned. In their case, the hippocampus was clearly less active during learning, and they remembered less afterwards than people who had slept normally.

In other words, studying all night before an exam is a bad deal. You gain a few hours, but you lose part of your ability to remember, and the night that would have consolidated what you learned never happens.

The nightly clean-up

Sleep has another job, discovered more recently. In 2013, a team at the University of Rochester (Lulu Xie, Maiken Nedergaard and colleagues) observed that, in mice, the space between brain cells grows by about 60% during sleep. The fluid that bathes the brain then flows more easily and clears away certain waste products faster, including the protein beta-amyloid.

This work was done in animals, and its link with human brain health still needs to be clarified. But it gives a simple picture: at night, the brain does not just tidy up its memories, it also takes out the trash.

What about naps?

A nap can help. In 2003, Sara Mednick and colleagues at Harvard showed that a 60- to 90-minute nap, long enough to include deep sleep and REM sleep, improved learning of a visual task as much as a full night's sleep. More recently, a review of 11 studies (Frédéric Dutheil and colleagues, 2021, 381 participants) concluded that a daytime nap improves mental performance, especially alertness, with the effect mostly seen in the two hours that follow.

In practice, people often recommend either a short nap of 10 to 20 minutes, to feel refreshed without waking up groggy, or a long nap of about 90 minutes, which covers a full cycle. Avoid late-afternoon naps if you struggle to fall asleep at night.

Sleep to remember more: 5 simple tips

• Aim for at least 7 hours a night. That is the recommendation of the American sleep medicine and sleep research societies for adults (2015).
• Keep regular hours, even at weekends: the brain likes routine.
• Learn in the evening, review in the morning. That was the winning strategy in the Lyon study: one night between two sessions.
• Do not sacrifice the night before an exam. A rested brain learns better and remembers better.
• If you have been sleeping badly for weeks, talk to your doctor. Sleep problems can be treated.

Curious about your own memory? Our two memory games measure your number memory and your visual memory in a few minutes. Try them after a good night, then after a short one: the difference is often telling.

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The MyMindTests team writes about the brain from published studies. Every figure is sourced and checked before publication.