Dreams incorporate recently encoded waking experiences, and the sleeping brain spontaneously reinstates memory-related neural activity, but the relationship between these two phenomena is not understood. This problem asks (i) in what form the sleeping brain recapitulates and interprets recent waking experience in dream content — literal, fragmentary re-instantiation versus transformation and recombination into novel contexts — and how that form relates quantitatively to neural memory-reinstatement signatures in the same sleep periods; (ii) whether dreaming itself causally contributes to memory processing or is an epiphenomenal byproduct of reactivation; and (iii) how memory reinstatement mechanistically gives rise, if at all, to generated conscious experience during sleep, including its relation to sensory gating at rapid eye movements. The question stands in full generality, unrestricted by sleep stage, task, or modality, as posed explicitly by Kumral, Palmieri, Gais, and Schönauer (iScience, 2025) and, on the gating side, by Koroma, Lacaux, Andrillon, Legendre, Léger, and Kouider (Current Biology, 2020). These questions remain unresolved in the literature to date.

Background

Sleep alternates between rapid eye movement (REM) sleep — a state with waking-like, desynchronized brain activity, vivid dreaming, and periodic bursts of eye movements — and non-REM (NREM) sleep, dominated by slow oscillations (slow, large-amplitude synchronized cortical waves) and sleep spindles (short bursts of rhythmic brain activity around 12-15 Hz). A dominant account holds that newly formed memories are progressively consolidated during sleep by spontaneous reactivation (also called replay): neural firing patterns or activation patterns laid down during waking learning reappear in compressed or reorganized form in the sleeping hippocampus and cortex, first observed in rodents by Wilson and McNaughton (Science, 1994) and, for REM sleep specifically, in the temporally structured hippocampal replay of Louie and Wilson (Neuron, 2001). In humans, memory reactivation can be triggered experimentally by re-presenting a cue (a sound or odor) that was paired with the learned material during wake — a technique called targeted memory reactivation (TMR) — and it can be detected spontaneously with multivariate pattern analysis (MVPA), which uses classifiers or similarity analyses to ask whether scalp EEG or fMRI activity during sleep carries information about which of several pre-sleep learning materials was encoded (Schönauer et al., Nature Communications, 2017; Abdellahi et al., eLife, 2023, for REM TMR). Dream content has been studied with the serial awakening paradigm, in which sleepers are woken repeatedly at defined sleep stages and asked to report any experience that was present just before awakening (Siclari et al., Nature Neuroscience, 2017, used it with high-density EEG to localize dream generation to a posterior cortical 'hot zone'). Subjective dream reports have long been known to include fragments of the previous day — the 'day-residue' — and, with a delay of about a week, the 'dream-lag' effect (Nielsen and Stenstrom, Nature, 2005; Stickgold et al., Science, 2001). Behavioral work links such incorporation to memory: dreaming of a learning task predicts better overnight retention of that task (Wamsley et al., Current Biology, 2010; replicated in Wamsley and Stickgold, Journal of Sleep Research, 2019), and a meta-analysis of 16 studies found a moderate overall association between task-related dreaming and memory improvement that was significant for dreams collected from NREM but not REM awakenings (Hudachek and Wamsley, Sleep, 2023). On the neural side, decoding of dream categories from sleep neuroimaging is possible (Horikawa et al., Science, 2013), and sensory stimuli can be deliberately incorporated into dreams (Salvesen et al., Sleep Medicine Reviews, 2024, systematic review). The problem sits at the junction of these two literatures, which remain largely parallel: the memory-consolidation literature measures reactivation without access to subjective experience, while dream research has reports without a mechanistic account of their memory sources. Kumral et al. (iScience, 2025) bridged them in a proof-of-concept study: participants listened to one of several audiobooks before sleep, and blind raters could identify which audiobook a participant had heard from anonymized dream reports alone, while MVPA showed that EEG activity in the beta frequency range (roughly 13–30 Hz) during REM sleep carried information about the audiobook; participants who dreamed of the audiobook showed stronger neural reinstatement, yet the amount of reinstatement did not correlate with memory retention. Complementarily, Koroma et al. (Current Biology, 2020) showed that informative speech in a multi-talker environment is amplified during REM sleep without eye movements but selectively suppressed at the timing of rapid eye movements — periods that predict dream reporting — raising, but not testing, the idea that sensory gating protects ongoing dream content. The closing paragraph of Kumral et al. states the open questions this task poses: 'in which form are life events recapitulated and interpreted by the sleeping brain? Does dreaming itself have a functional role in memory processing?'

Problem Statement

Determine how recent waking experience reappears in the dreaming brain. Concretely, in what form does the sleeping brain recapitulate and interpret memories of the preceding day in dream content — as literal, verbatim re-instantiation of encoded episodes and fragments, or as transformation, abstraction, and recombination into novel contexts (the verbatim-versus-transformed classification to rest on an explicit operational criterion, such as a stated coding scheme or similarity/derivation metric) — and how does this form relate, quantitatively and mechanistically, to the neural memory-reinstatement signals (such as REM beta-band decoding of pre-sleep material) measurable in the same sleep periods? Building on that, establish, on the available evidence, whether dreaming itself performs a causal function in memory processing or is instead an epiphenomenal byproduct of memory reactivation during sleep, and what experiment would decide between these alternatives; and characterize the mechanistic relationship between memory reinstatement and the generation of conscious experience during sleep — including whether internally generated dream content is protected by the eye-movement-locked sensory gating observed during REM sleep. The problem is posed in this generality by its sources and is not restricted to a sleep stage, sensory modality, or experimental task; a complete answer must cover (a) the form of dream incorporation of waking memories, (b) its relation to measurable neural reinstatement, (c) the functional-versus-byproduct status of dreaming, and (d) the mechanistic link between neural reinstatement and the generation of conscious dream experience, including the eye-movement-locked sensory gating question — the four parts that together constitute the one overarching objective: a mechanistic account of how memory processing in the sleeping brain becomes, or fails to become, conscious dream experience.

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