Slow-wave sleep and REM sleep differentially contribute to memory representational transformation.

Sleep consolidates memories. Yet how sleep preserves precise memories while transforming them into abstract and categorical knowledge remains unclear. Using electroencephalography and representational similarity analysis, we examined memory representational transformation across overnight sleep. We focused on item-level representations, which reflect specific details of individual memories, and category-level representations, which capture shared conceptual features across items from the same category. Our results showed that after sleep, item-level representations were reduced, while category-level representations were preserved. Notably, a higher ratio of rapid eye movement (REM) to slow-wave sleep (SWS) predicted greater item-level reduction and category-level enhancement. Additionally, theta (4-7 Hz) and beta (15-25 Hz) power during REM sleep were positively associated with these memory representational transformations, whereas slow oscillation-related (1-1.25 Hz) power during SWS showed the opposite pattern. Our findings suggest the differential roles of SWS and REM in balancing memory preservation and transformation.