The sleeping brain remembers: Memory consolidation during sleep

Dateline Portland, Oregon—Amelia J. Wilcox, Ph.D.: We all know sleep is important, and here’s another reason why: When you sleep, your brain takes fragile new memory traces from the day and consolidates them into memory.

Why we sleep:
Some say we evolved to sleep in the dark of night so that we wouldn’t accidentally injure ourselves or become a night predator’s meal. While that may be part of the story, our brains have learned to take advantage of the quiet time. Housekeeping takes place—glial cells clearing the neurotransmitter detritus of the day. We dream—a fascinating topic for another blog post. And we work to consolidate memory from the day.

Sleep and memory consolidation—the secret weapon of studying:

Once our understanding of memory formation was mostly theoretical. We couldn’t see it happen, but we knew that it did, and so we explained it in ways we could understand. Our human memory was described as something of a filing system. One place for the recollection of a birthday party, another for the names of US presidents, and so on.

But the arrival of increasingly sophisticated brain imaging techniques has helped us make exciting discoveries about memory consolidation, the brain and sleep. Our theoretical understanding has evolved as a result. 

Now when I teach my students about the consolidation of new memories, I can tell them with certainty that the hippoCampus is where new memories go to camp for the day. (Corny, I know.) These memory traces are fragile. There is no certainty they will be consolidated. These fragile memory traces strengthen and reach their final destinations in the brain while we are sleeping. 

Here’s how it works: Say you work hard to learn new information during the day and then spent the night in an fMRI, a sophisticated imaging machine that charts brain activity in real time. This is what would be seen. Your hippocampus would fire, a distant region of your brain would fire in reply, hippocampus fires again, then that region again, or perhaps another one or two or more would fire. This neural conversation continues throughout the night, allowing new memories become strong and move to their home bases in your brain. And bases is not a typo—aspects of each memory live in disparate places in the brain—it is not the tidy filing system we once conceptualized it to be.

How to optimize memory consolidation:
There are many ways to optimize memory consolidation, and most of those include conscious and effortful engagement with the material you are working to learn. Dig in to the information. Don't try to learn it all at once. Study it every day. Ask yourself questions, grapple with the answers. Write, ponder, teach someone else what you know. And at the end of each day, sleep. Because we now know new and fragile memory traces stored in the hippocampus strengthen and reach their final destinations in the brain while we are sleeping. 

Yet another reason to get a good night’s sleep:
What is the take home message here? This is another reason sleep matters. If you are getting less sleep than you need, you are shortchanging yourself in the memory department. Research demonstrates this fact.

We know students pulling all-nighters to cram for tests often do less well than their peers who study over time. They almost invariably have poorer memory of the material once the test is over—a real problem when you are working to deepen your knowledge in an academic discipline, and have foundational material that is necessary to know well in order to understand what comes next. We now understand the consolidation process that comes with sleep is an essential part of the memory formation process.

Could there be more efficient study time? I often encourage my students in the days before an exam to consider spending the last bit of time before sleep every night reviewing notes (on paper--not on a screen—read my blog post about blue light and sleep disruption to understand why). Their brains will work to solidify those notes as they sleep—a twofer!

This is another reason that “catching up” on sleep over the weekend appears not to be a real thing—we don’t make up sleep debt. And the opportunity to consolidate new material appears to be an opportunity lost if that night’s sleep is too short or disrupted.

Sleep perchance to dream? Sleep certainly to remember!