A research team in Freiburg has investigated how random sounds played during sleep affect the consolidation of new memories. The results show that non-targeted acoustic stimuli disrupt deep sleep. As a result, the slow brain waves typical of this sleep phase occur less frequently and spread more irregularly across the brain. It is precisely these changes that impair the consolidation of new memories.

For several years now, sleep research has been focusing intensively on the question of whether targeted auditory stimulation during sleep can improve the consolidation of new memories. A research team in Freiburg led by the neuropsychologists Prof. Dr Monika Schönauer and Dr Nora Roüast has discovered in a new study that auditory stimuli during sleep can also have undesirable consequences.

Randomly played sounds during sleep impair the consolidation of new memories because they disrupt deep sleep and thus alter the propagation of slow brain waves. The latter are considered a key component of memory formation because they significantly promote the exchange of information between different regions of the brain. “Our findings show that randomly played sounds can disrupt important processes during sleep. For memory formation, it is not only crucial that slow brain waves occur, but also how they propagate throughout the brain. It is precisely this propagation that is impaired by the sounds,” says Roüast.

Slow brain waves reach fewer regions of the brain

Twenty adults took part in the study. On two test days, they learnt both factual knowledge and a sequence of finger movements before taking a three-hour afternoon nap. Whilst they slept, the researchers recorded their brain activity and sleep stages using electroencephalography (EEG). On one test day, the participants heard randomly played sounds during their sleep in the form of a sequence of small clicks; on the other, it remained silent. After the nap, the researchers assessed how well the participants could recall the information they had learnt.

The randomly played sounds hardly shortened the total duration of sleep at all. Instead, they primarily altered the composition of sleep: the participants spent significantly less time in deep sleep and more time in lighter sleep stages. Furthermore, slow brain waves occurred less frequently and reached fewer brain regions. It was this altered spread in particular that was decisive for the significantly poorer memory performance. “Precisely because intensive research is currently being carried out into improving memory processes or using them therapeutically with the aid of sleep-based stimulation, our findings show that we must carefully consider potential side effects. Even the sounds themselves that have no melody or verbal content can influence and disrupt sleep physiology and the complex processes underlying memory formation,” emphasises Schönauer.

For more information:

Original publication: Roüast, Nora M., Kumral, Deniz, Gais, Steffen, Schönauer, Monika. (2026). Random auditory stimulation during sleep disturbs traveling slow waves and declarative memory. iScience, DOI: 10.1016/j.isci.2026.116601.
To the original publication: https://www.sciencedirect.com/science/article/pii/S2589004226019760?__cf_chl_f_tk=Yy7lHz0avlSRiVTnpkPNpDNGvKqhyeOrQ7s15SXHiI4-1783087729-1.0.1.1-.zah8LbiHk9HOpUX.0pgxVPjYBdYGHopARNICi9CeaI

To the Chair of Neuropsychology: https://uni-freiburg.de/neuropsychology/