Sleep rewires the brain to strengthen memories

Why is a good night’s sleep so crucial for a sharp memory? A team of neuroscientists has taken a major step toward answering this question. 

By monitoring brain activity in rats for up to 20 hours after a learning task, they discovered that sleep reorganizes neural patterns in a way that helps memories take hold and prepares the brain for new information. 

The study, led by researchers from the Institute of Science and Technology Austria (ISTA), offers new insights into how the brain engraves long-term memories during times of rest.

Monitoring brain cell activity during sleep

Sleep’s role in helping animals remember things – like where food is hidden – has long been known. But scientists still don’t fully understand how this happens at the neuronal level. 

To investigate, the ISTA team focused on spatial memory, by training rats to locate food rewards in a maze while recording brain activity using a wireless system. 

The setup allowed them to monitor hippocampal neurons, which play a key role in spatial navigation and learning, for up to 20 continuous hours of sleep following the task.

“We showed that the neuronal assemblies in the early stages of sleep reflect recently learned spatial memories. However, as sleep progresses, neuronal activity patterns gradually transform into those seen later, when the rats awaken and remember the locations of their food rewards,” explained Professor Jozsef Csicsvari, the senior author of the study.

The hippocampus as a map maker

Earlier research has shown that the hippocampus creates internal “maps” of an animal’s environment by firing specific neurons at specific locations. These mental maps are updated during learning and become especially focused around important activities, like where food is found. 

The hippocampus continues to play a vital role during sleep by replaying memory traces, a process called reactivation.

Previous work by the Csicsvari group showed that the more frequently the hippocampus reactivated a reward location during sleep, the better the animal remembered it later. When this replay was blocked, the rats struggled to recall the location – suggesting that reactivation is key to cementing memory.

Memory patterns shift during sleep

Until now, such studies were limited to short sleep sessions lasting just a few hours. In the new study, the team extended the recordings to cover a full overnight sleep. 

The extended monitoring revealed a previously unseen phenomenon: memory-related neural patterns shifted gradually during sleep.

“Our findings were unexpected,” said co-first author Lars Bollmann. “We showed that the activity patterns of neurons linked to the reward locations reorganized during the long sleep.”

Initially, when the rats entered sleep, the same neurons that were active during learning remained active, forming what the researchers called a “stable subgroup.” 

But as sleep progressed, some of these neurons stopped firing and new neurons took their place. By the end of the sleep period, the firing pattern had evolved to resemble the one seen when the rats successfully remembered the reward locations the next day.

This representational shift occurred specifically during non-REM sleep, the researchers found, while REM sleep seemed to prevent or counteract this reorganization.

Making room for new memories

What’s the point of this neural reshuffling? The researchers suggest it could be a way for the brain to optimize its memory storage.

“It is possible that memory representations must be formed quickly during learning but that such representations are not optimal for long-term storage,” Csicsvari explained. 

“Therefore, a process may take place in sleep that optimizes these representations to reduce brain resources to store a specific memory.”

In other words, by trimming down the number of neurons needed to represent a particular memory, the brain may free up resources for encoding new experiences. 

Supporting this idea, the researchers found that fewer neurons were linked to a given reward location after sleep than before.

“Any new memories must find a way to be integrated into existing knowledge,” Csicsvari adds. “Frequent repetitions of the new memories as well as partial change in the neuronal code may thus help optimize their integration into existing memory representations.”

Understanding sleep and memory

This study marks a significant advance in understanding how sleep strengthens memory. By showing that the brain reorganizes memory representations over the course of sleep – and by linking these shifts to improved recollection – the research sheds light on one of the most essential functions of sleep.

While the findings come from animal models, they offer promising avenues for future research into human memory and the consequences of sleep disorders. 

As scientists continue to explore how memories evolve overnight, they may eventually uncover strategies for enhancing learning, supporting brain health, and even treating memory-related conditions.

The study is published in the journal Neuron.

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