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.