New brain research suggests that the length of REM sleep may influence which details of your memories are retained.
Previous studies have shown that sleep helps strengthen our memoriesbut how it shapes the content of those memories is more difficult to determine. Now the study, published Oct. 1 in the journal Communication biology hints that the time spent in different stages sleep may affect this aspect of memory storage.
The sleep cycle is divided into four stages: one rapid eye movement (REM) stage and three non-rapid eye movement (NREM) stages, including “deep sleep,” characterized by slow brain waves. To test how these stages of sleep affect our memories, the researchers asked 32 healthy young adults to learn 96 pairs of words and pictures (for example, an action word associated with a picture of an animal or plant) while their brain activity was recorded using an electroencephalogram (EEG), which tracks brain waves washing the surface of the brain.
The volunteers were then monitored with an EEG while they slept through the night, and their memory was tested the next morning. The researchers compared before and after brain patterns using a technique called representational similarity analysis. These data allowed the scientists to focus both on detailed memories associated with specific images (such as photographs of a beagle) and on broader, categorical memories encompassing, for example, all images of animals.
“Using EEG, we could track how brain activity associated with memories changed before and after sleep,” said the study's first author. Jing Liuassistant research professor at Hong Kong Polytechnic University, told Live Science via email.
The team found a pattern: brain waves associated with individual images weakened after sleep, while signals from the broader category remained stable.
The shift was greater when REM sleep accounted for a larger portion of a person's total sleep time compared to deep sleep. Liu explained that this pattern suggests that REM sleep may help the brain associate new memories with what it already knows, while NREM sleep helps preserve those memories in their original, more detailed form.
“Even when people remembered the same thing after waking up, the brain structure underlying those memories changed,” she added. This suggests that sleep not only strengthens memories, but can also reorganize how they are represented in the brain, with REM and slow-wave sleep contributing differently.
Taken together, these results provide evidence that memory consolidation—the brain's process of stabilizing and reorganizing new memories—involves both storage and transformation. Instead of storing memories of events as they happened, the brain can subtly restructure them during sleep, balancing accuracy and generalization. The researchers noted that this difference may help explain how knowledge networks in the brain develop over time.
However, this pattern does not necessarily mean that deep sleep and REM sleep are at odds with each other. Rather, the two phases support different aspects of memory. Dr. George Dragoiprofessor of psychiatry and neuroscience at Yale University, who was not involved in the study, told Live Science via email.
“The results point to the complementary roles of REM sleep 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 maintaining a regular sleep schedule can help support these processes, as good sleep quality has a lot to do with healthy cognitive function. “Large periods of REM sleep may contribute to the kind of memory transformation that this study highlights,” he suggested.
Liu, however, cautioned that the results show associations, not causation.
“[EEG] prevents us from accurately identifying the brain regions causing these changes,” she said, adding that combining EEG with recordings taken directly from electrodes placed inside the skull could clarify the circuitry underlying the effect. She also pointed to future studies that might try to reactivate certain memories during sleep—for example, by playing sounds or cues associated with previous learning—or interrupt certain stages of sleep to see if that changes how flexibly people can use what they've learned.
