The amount of REM sleep you get may influence which details of your memories stick with you, new brain research suggests.
Previous research has shown that sleep helps strengthen memories, but the question of how sleep shapes the content of memories has been more difficult to pin down. Now, a study published Oct. 1 in the journal Communications Biology suggests that time spent in different stages of sleep can influence this aspect of memory storage.
The sleep cycle is divided into four stages. one rapid eye movement (REM) stage and three NREM stages, including “deep sleep” characterized by slow brain waves. To test how these sleep stages affect our memory, researchers asked 32 healthy young people to learn 96 word-picture pairs (such as action words associated with images of animals and plants) and recorded their brain activity with electroencephalography (EEG), which monitors brain waves flowing across the surface of the brain.
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The volunteers were then monitored with EEG while they slept overnight and took a memory test the next morning. The researchers used a technique called representational similarity analysis to compare brain patterns before and after. These data allowed the scientists to focus on both detailed memories associated with specific images, such as pictures of beagles, and broader categorical memories that encompass images of all animals, for example.
“By using EEG, we were able to track how memory-related brain activity changed from before to after sleep,” first study author Jing Liu, a research assistant professor at Hong Kong Polytechnic University, told Live Science via email.
The research team discovered a pattern. Brain waves associated with individual images weakened after sleep, while broader category signals remained stable.
This change was stronger when REM sleep occupied more of an individual’s total sleep time than deep sleep. This pattern, Liu explained, may help the brain connect new memories with what it already knows, while slow-wave sleep may help keep those memories in their original, more detailed form.
“Even if people remembered the same thing after they woke up, the brain patterns behind that memory had changed,” she added. This suggests that sleep not only strengthens memories, but that REM and slow-wave sleep may contribute in different ways to reorganize how memories are represented in the brain.
Taken together, these results provide further evidence that memory consolidation, the brain process that stabilizes and reconstructs new memories, involves both storage and transformation. Rather than storing memories of experiences as they happened, the brain may subtly reconstruct memories during sleep, balancing accuracy and generalization. The researchers noted that this difference may help explain how knowledge networks in the brain evolve over time.
However, this pattern does not necessarily mean that deep sleep and REM sleep work in opposition to each other. Rather, the two phases support different aspects of memory, Dr. George Dragoi, a professor of psychiatry and neuroscience at Yale University who was not involved in the study, told Live Science via email.
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“Our results demonstrate the complementary roles of REM and slow-wave sleep in different types of memory, including general knowledge and memory for facts and specific experiences,” he said.
He added that good sleep quality is widely associated with healthy cognitive function, so maintaining a regular sleep schedule may help support these processes. “Longer REM periods may promote the kinds of memory changes highlighted in this study,” he suggested.
However, Liu cautioned that the results showed an association rather than a cause-and-effect relationship.
”[EEG] “We cannot pinpoint the exact brain regions that are causing these changes,” she said, adding that a combination of EEG and recordings taken directly from electrodes placed inside the skull could reveal the circuitry behind the effects. They also noted that future research could, for example, reactivate certain memories during sleep by playing sounds or cues related to previous learning, or interrupt certain sleep stages to see if this changes how flexible people are at applying what they have learned.
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