We’ve known for some time now that during sleep, the brain undergoes a memory spring cleaning of sorts, during which thoughts collected during that day are either converted into long-term memories or discarded.

It makes sense that this brain cleanse would occur at night when the brain is idle, considering there’s no way that even the enormous human brain could possibly have enough room for everything that we take in on a given day.

Until recently, researchers didn’t understand the mechanism for how the brain chooses what to keep. But, in a study recently published in the journal Science, researchers have demonstrated for the first time that during the day the brain goes through a series of steps in an effort to tag certain memories to be stored that night. 

How the Brain Stores Memories

Researchers found that when the brain is idle during the day, simultaneous waves of neurons come together in what’s called sharp wave ripples that literally shout to the rest of the brain that this memory is important and needs to be tagged for storage later that evening.

While sharp wave ripples happen less often during the day, their role is crucial because at night a series of 2,000-4,000 sharp wave ripples occur, causing the brain to condense its collection of memories.

“Sharp wave ripples are a pattern that occurs in the hippocampus, and in the wake state, this is the pattern that selects what will be put in for permanent storage and what will be triaged,” says lead study author György Buzsáki, the Biggs Professor of Neuroscience at the New York University School of Medicine.

These sharp wave ripples make up the “most synchronous pattern in the mammalian brain,” and they occur when the brain is offline, either idling during the day or at night.

“It takes what’s good about sharp wave ripples in the brain to the next level,” says Jim Gnadt, co-lead of the Integrative and Quantitative Neuroscience Team for the NIH BRAIN Initiative, who was not involved in the study.

Read More: What Happens in Your Brain When You Make Memories?

How Rest Impacts Memory Storage

It seems that the brain cuts up the experiences that happen druing our day and combines them with other experiences that also occurred. Parts are removed, and thoughts are condensed.

“Many parts of our waking experience are cut out and bound together with other experiences using this pattern in the hippocampus,” says Buzsáki.

As part of the study, researchers allowed rodents to run through a maze many times and looked at their brains while they were doing it. They found that when the mice were at rest drinking sugar water, that was when the sharp wave ripples occurred. The longer the rodents stayed by the water cooler, the more sharp waves that occurred in the brain. After the animal went to sleep and there were multiple sharp waves over night, the maze run that was replayed the next day was the one that occurred just before their water cooler idle rest. 

Read More: Understanding Why Certain Memories Flood Back (And Others Don’t)

The Brain’s Two Algorithms

It turns out that the brain has two modes or algorithms: an acquisition mode and a storage mode. It’s not that the brain is at rest when we sleep, it’s that it’s storing what was tagged during the day. “Sharp wave ripples happen when we’re not attentive, but they are as important as the active state,” says Buzsáki.

That’s part of the reason that breaks are so important to the brain functioning at a higher level. Going for a run or taking a coffee break is the best way to remember or learn something complicated. You have to be at rest in creative work in order to have the best results.

Gnadt says that this research takes the whole idea of how memory works to the next level. “It gives us a better idea of why during sleep the brain grabs onto certain memories and not others,” he says. Our brains are constantly doing the work of collecting and storing and this is another tool in understanding the mechanism for how it all comes together.

Read More: Why Do Smells Trigger Such Powerful Memories?

Article Sources

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Sara Novak is a science journalist based in South Carolina. In addition to writing for Discover, her work appears in Scientific American, Popular Science, New Scientist, Sierra Magazine, Astronomy Magazine, and many more. She graduated with a bachelor’s degree in Journalism from the Grady School of Journalism at the University of Georgia. She’s also a candidate for a master’s degree in science writing from Johns Hopkins University, (expected graduation 2023).