REM sleep may reshape what we remember

The amount of REM sleep you get may influence which details of your memories remain in storage, a new brain study suggests.

Previous research had found that sleep helps fortify our memories, but the question of how it shapes the contents of these memories has been harder to pin down. Now, a study published Oct. 1 in the journal Communications Biology hints that the time spent in different stages of sleep may influence this aspect of memory storage.

The sleep cycle is split into four stages: one stage of rapid eye movement (REM) and three non-REM stages, including "deep sleep," marked by slow brain waves. To test how these sleep stages impact our memories, the researchers asked 32 healthy young adults to learn 96 word-picture pairs — such as an action word linked to an image of an animal or plant — while their brain activity was recorded with an electroencephalogram (EEG), which monitors brain waves that wash over the surface of the brain.

The volunteers were then monitored with EEG as they slept overnight and had their recall tested the next morning. The researchers compared the before-and-after brain patterns using a technique called representational similarity analysis. These data enabled the scientists to focus both on detailed memories tied to specific images — like a photo of a beagle — and on broader, categorical memories, covering all the animal images, for instance.

"By using EEG, we could track how brain activity linked to memories changed from before to after sleep," first study author Jing Liu, a research assistant professor at The Hong Kong Polytechnic University, told Live Science in an email.

The team uncovered a pattern: Brainwaves linked to the individual images weakened after sleep, while the broader category signals remained stable.

The shift was stronger when REM made up more of an individual's total sleep time, compared to deep sleep. Liu explained that this pattern suggests REM sleep may help the brain link new memories with what it already knows, while slow-wave sleep helps keep those memories in their original, more-detailed form.

"Even when people remembered the same things after waking, the brain patterns behind those memories had shifted," she added. This suggests sleep not only strengthens memories but may reorganize how they're represented in the brain, with REM and slow-wave sleep contributing in different ways.

Together, these results add to evidence that memory consolidation — the brain's process of stabilizing and reorganizing new memories — involves both preservation and transformation. Rather than storing memories of experiences exactly as they happened, the brain may be subtly restructuring them during sleep, balancing accuracy with generalization. The distinction, the researchers noted, could help explain how knowledge networks in the brain evolve over time.

However, the pattern doesn't necessarily mean that deep sleep and REM sleep work in opposition to one another. Rather, the two phases support different facets of remembering, Dr. George Dragoi, professor of psychiatry and neuroscience at Yale University who was not involved in the study, told Live Science in an email.

"The results here point to a complementary role of REM and slow-wave sleep in different types of memory," he said, such as general knowledge and facts versus memories of specific experiences.

He added that keeping regular sleep schedules may help support these processes, since good sleep quality is broadly linked to healthy cognitive function. "Longer REM periods may promote the kind of memory transformation this study highlights," he suggested.

Liu, however, cautioned that the results show associations, not causation.

"[EEG] prevents us from precisely identifying the brain regions driving these changes," she said, adding that combining EEG with recordings taken directly from electrodes placed inside the skull could clarify the circuitry behind the effect. She also pointed to future studies that might try to reactivate specific memories during sleep — for instance, by replaying sounds or cues linked to earlier learning — or interrupt particular sleep stages to see whether that changes how flexibly people can use what they've learned.

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