50-70% of all dreams include residue from the previous day, especially in the early stages of sleep, while later stages refer to more distant memories

Memory reactivations during sleep: a neural basis of dream experiences? Claudia Picard-Deland et al. Trends in Cognitive Sciences, March 22 2023. https://doi.org/10.1016/j.tics.2023.02.006

Abstract: Newly encoded memory traces are spontaneously reactivated during sleep. Since their discovery in the 1990s, these memory reactivations have been discussed as a potential neural basis for dream experiences. New results from animal and human research, as well as from the rapidly growing field of sleep and dream engineering, provide essential insights into this question, and reveal both strong parallels and disparities between the two phenomena. We suggest that, although memory reactivations may contribute to subjective experiences across different states of consciousness, they are not likely to be the primary neural basis of dreaming. We identify important limitations in current research paradigms and suggest novel strategies to address this question empirically.