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Sleep as a continuation of the day: pre-sleep memories shape the content of dreams

Alexey Kryvenko, Medical Reviewer, Editor
Last reviewed: 09.08.2025

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06 August 2025, 05:49

Scientists from Wageningen University and the University of Groningen, led by Deniz Cumral, published a study in iScience demonstrating that targeted playback of sounds associated with recently learned material not only activates the corresponding neural ensembles during sleep, but also actually “embeds” elements of these associations into the content of dreams.

Experimental design and methods

Participants and training. The study involved 28 healthy volunteers (14 men, 14 women, 18–30 years old). During the day, each was shown 60 sound–image pairs (animals, objects, scenes), where each sound (1-s, frequency 500–1000 Hz) was rigidly associated with one image.
Targeted memory reactivation (TMR). Polysomnography (PEEG, EMG, EOG) was recorded during night sleep. Only in the NREM-2 and NREM-3 phases were half of the sounds (30 pieces) played through speakers (5–10 s interval between sounds, 45 dB level), the remaining half of the associations were not touched (control).
Dream recording. In the morning after waking up, participants filled out a standardized questionnaire: they described the plot of the dream and noted which animals or objects they dreamed about. Each mention of a specific image was considered an “incorporation” of TMR content.
Memory assessment: Immediately after sleep, subjects took a sound-image pairing test: they were played a sound and asked to name the corresponding image.

Neural reactivation and memory efficiency

EEG analysis: During TMR sounds, an increase in the power of slow waves (0.5–4 Hz) and sleep spindles (12–15 Hz) in the central areas (CPz, Cz) was observed by 25% above the baseline level (p < 0.005).
Increased connectivity: Coercion between the hippocampus and occipital cortex, as measured by phase locking, increased by 18% in response to TMR (p < 0.01).
Improvement of test performance. Participants correctly reproduced 82% of associations whose sounds were played in a dream, versus 68% for those not played (Δ14%, p = 0.002).

Incorporating content into dreams

Animals and objects whose sounds were heard in the NREM phase were 45% more likely to appear in dream descriptions (mean number of mentions 1.8 vs. 1.2 objects per participant, p < 0.001).
An analysis of the plot scenes revealed that 60% of the inclusions were metaphorical in nature: for example, the sound of a rooster led to a dreamlike “awakening” of a character in a dream.
Correlation with memory. The more associations encountered in the dream, the better the participant remembered the pairs (r = 0.52, p = 0.005), indicating a direct link between the “dream replay” and memory consolidation.

Mechanisms: from replay to dreams

Slow Wave Replay. Slow delta waves provide conditions for replaying daily experience, transferring information from short-term memory (hippocampus) to long-term memory (neocortex).
Formation of images. Integration of a real replay with REM sleep, where fragments of real associations are redistributed, gives birth to dream plots.

Authors' statements

"We have shown that dreams are not just a chaotic background, but a reflection of the actual processing of recently learned material. With TMR, both the content of dreams and the effectiveness of memorization can be controlled," comments Deniz Kumral.
"These findings open up prospects for the treatment of post-traumatic stress disorder through the "replay" of positive memories in dreams and the correction of nightmares," adds co-author Dr. Yves Extrasen.

Prospects and Applications

Improving learning. Integrating TMR protocols into educational technologies to accelerate the acquisition of complex material.
Neurorehabilitation. Support for memory recovery in patients with traumatic brain injury and dementia.
Dream Psychotherapy. Managing the Content of Nighttime Dreams to Treat Phobias and PTSD Through “Replacing” Traumatic Images.
Cosmetic neurology. Correction of pathological dreams (nightmares) in military personnel, resuscitation doctors and shift workers.

This study establishes a direct link between targeted memory stimulation during slow-wave sleep, replay of neural patterns, and dream content, opening new horizons in understanding sleep function and developing methods for controlling memory and dreams.