A study using electroencephalography found that the extent of memory retention after sleep is associated with time spent in slow wave sleep. The location where the brain shows neural activity associated with successful memory recall shifted from the parietal areas to the anterior temporal lobe after sleep. The paper was published in Neuropsychologia.
Slow wave sleep is the deepest stage of non-REM sleep, characterized by large, slow brain waves and a highly reduced level of consciousness. During this stage, the brain significantly alters its activity, allowing neurons to rest and recover from daytime demands. It plays a central role in consolidating memories, especially facts and skills learned throughout the day.
Slow wave sleep also supports physical restoration by promoting the release of growth hormone and facilitating tissue repair. The immune system becomes more active during this phase, helping the body fight infections and maintain long-term health. Because the brain is less responsive to external stimuli, slow wave sleep provides the most uninterrupted and restorative rest.
This stage helps clear metabolic waste from the brain, contributing to long-term cognitive health. People who get too little slow wave sleep experience reduced attention, poorer memory, and diminished emotional regulation the following day. Slow wave sleep also helps stabilize neural circuits, making future learning easier.
Study author Simon Faghel-Soubeyrand and his colleagues wanted to assess how sleep may support the reorganization of memory recall networks in the brain. They note that our memories are not static, but that they change in both strength and quality over time. Previous studies indicate that, as these changes happen, the location of brain activity during recall changes. The optimal time for this consolidation to occur is during sleep, when the brain is sheltered from external stimulation. They conducted a study to examine this using high-density electroencephalography (EEG).
Study participants were 24 university students. Their average age was roughly 23 years. Five of them were male.
The participants completed two separate sessions of word-image memory tasks. Word-image memory tasks require people to learn and later recall or recognize associations between written words and corresponding pictures. The two sessions were 7–8 days apart. One set of sessions was “object sessions,” where the images participants learned to associate with words were either a car or a guitar. The other set of sessions was “scene sessions,” where participants associated words with either a house or a corridor. Within a session, images were paired with 100 unique verbs.
Each of these sessions was followed by two recall sessions. One recall session happened before sleep, while the second happened after sleep. In general, the first recall session happened in the evening. The participants then slept through the night. The experimenter would wake them at 7:30 the next morning, and after a few minutes to fully awaken, they would again try to recall what they learned. During these activities, study authors took EEG recordings of the participants’ brain activity.
Results showed that the location in the brain where the strongest signs of recall activity occurred shifted from before to after sleep. While the primary activity during memory recall was in the parietal area of the brain before sleep, it moved towards the anterior temporal lobe after sleep. Individuals who spent more time in slow wave sleep tended to recall more of what they learned after sleep, and the shift in the location of brain activity during recall tended to be greater in these individuals.
Overall, results indicated that the profiles of brain activity during successful recall of memories changed overnight and that the extent of these changes was higher in individuals who spent more time in slow wave sleep.
“Together, these findings suggest a link between SWS [slow wave sleep] and the consolidation and functional reorganisation of episodic memory networks,” study authors concluded.
The study contributes to the scientific understanding of the way memory retention and recall function. However, the study was conducted on a small sample of students. Results on other demographic groups and older individuals might differ.
The paper, “Slow wave sleep is associated with a reorganisation of episodic memory networks,” was authored by Simon Faghel-Soubeyrand, Polina Perzich, and Bernhard P. Staresina.