Summary: A recent study from the University of Freiburg shows that randomly timed sounds played during sleep can actively impair memory consolidation. Using real-time electroencephalography (EEG), researchers found that brief ambient clicks disturb deep slow-wave sleep, blocking the physical propagation of slow brain waves across the cortex and breaking the information transfer necessary for forming long-term memories.
Key Facts
- The illusion of undisturbed sleep: On the day participants were exposed to auditory stimulation, the random clicks did not noticeably wake them or significantly reduce overall sleep duration. That means a person may assume a soundscape is harmless simply because it does not cause awakenings, while important neural processes are nonetheless being disrupted.
- Altered sleep architecture: Although total sleep time remained similar, the clicks changed the internal balance of sleep stages. Participants exposed to random sounds spent significantly less time in restorative slow-wave (deep) sleep and more time in lighter, less restorative stages.
- Blocked slow-wave propagation: Slow brain waves are a hallmark of deep sleep and serve as a carrier for coordinated communication across distant cortical regions. The study found that acoustic stimulation reduced the occurrence of slow waves and prevented them from traveling across wide brain areas.
- Impaired memory performance: After their naps, participants who had experienced sound-click stimulation recalled fewer facts and performed worse on motor sequence tasks compared with their performance after quiet naps.
- Propagation matters: As Dr. Nora Roüast explains, it is not enough that slow waves appear; they must propagate smoothly and broadly across the cortex to support memory consolidation. Ambient clicks fragmented this propagation and undermined consolidation.
- Implications for consumer devices: With the rise of sleep apps, white-noise machines, and wearable sleep-tech, Professor Monika Schönauer warns that even simple, non-melodic sounds can change sleep physiology and produce cognitive side effects. Careful calibration and real-time monitoring are essential when designing therapeutic auditory stimulation.
Source: University of Freiburg
Researchers have long investigated whether targeted auditory stimulation during sleep can boost memory consolidation. A team led by neuropsychologists Prof. Dr. Monika Schönauer and Dr. Nora Roüast extended this work by examining potential negative effects of non-targeted, randomly timed sounds.
Their study shows that randomly presented clicks during sleep can harm the process that stabilizes newly acquired memories. These clicks disrupted deep slow-wave sleep and altered the spatial reach of slow brain waves—oscillations considered crucial for transferring information from temporary stores to long-term cortical memory networks.
“Our results demonstrate that randomly played sounds can interrupt important sleep processes,” Roüast says. “For memory consolidation, what matters is not only that slow waves occur, but also how they travel across the brain. It is precisely that traveling that the sounds impair.”
Slow waves reach fewer brain regions
Twenty adults participated. On two separate test days they learned factual information and a finger-movement sequence, then took a three-hour afternoon nap while their brain activity and sleep stages were monitored with EEG. On one day, randomly timed click sounds were played during sleep; on the other day the nap was silent. After each nap, memory for both the facts and the motor sequence was evaluated.
The random clicks did not meaningfully shorten overall sleep duration but did change sleep composition: participants exposed to sound spent less time in slow-wave sleep and more time in lighter stages. Slow-wave events became less frequent and, critically, they spread to fewer brain regions. This reduced spatial propagation was closely linked to the poorer memory outcomes that followed.
“Because there is considerable interest in using sleep-based auditory stimulation to enhance memory or as a therapeutic tool, our findings show we must carefully weigh potential side effects,” Schönauer emphasizes. “Even non-musical, non-verbal sounds can alter sleep physiology and interfere with the complex processes underlying memory formation.”
Key Questions Answered:
A: Yes. The Freiburg study showed that randomly timed clicks caused little change in total sleep time and rarely produced awakenings, so sleepers were unaware of the disturbance. However, the brain responded to the noise by shifting away from deep slow-wave sleep toward lighter stages. This hidden disruption was enough to impair memory consolidation.
A: Slow brain waves are large, synchronized oscillations that dominate deep sleep. They coordinate communication between brain regions, enabling the transfer of newly encoded information from temporary storage (such as the hippocampus) into lasting cortical networks. If ambient noise prevents these waves from spreading, the coordinated transfer is disrupted and learning may be lost.
A: Not necessarily. Some research protocols use phase-locked auditory stimulation—sounds precisely timed to the brain’s slow-wave peaks—to strengthen slow waves. The risk arises when devices play uncalibrated, random sounds throughout the night. Without precise timing and monitoring, even simple sounds can alter sleep physiology and potentially impair memory.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full.
- Additional context was added by editorial staff.
About this auditory neuroscience, sleep, and memory research news
Author: Beate Suppinger
Source: University of Freiburg
Contact: Beate Suppinger – University of Freiburg
Image: The image is credited to Neuroscience News
Original Research: Open access. “Random auditory stimulation during sleep disturbs traveling slow waves and declarative memory” by Roüast, Nora M., Kumral, Deniz, Gais, Steffen, Schönauer, Monika. iScience
DOI: 10.1016/j.isci.2026.116601
Abstract
Random auditory stimulation during sleep disturbs traveling slow waves and declarative memory
Sleep is essential for memory consolidation, and auditory stimulation has been explored as a way to enhance that process. Yet the broader effects of such stimulation on sleep physiology and memory are not fully established.
In this study, random or sham auditory stimulation was applied during an afternoon nap to assess effects on neural activity and memory consolidation. Stimulation produced a selective reduction in slow-wave sleep, a lower slow-wave count, and impaired declarative recall that correlated with shallower sleep depth.
In addition, stimulation altered the traveling dynamics of slow waves: trajectories were shorter, reached fewer regions, and showed reduced spatial spread—particularly in frontal regions. Disruptions in these traveling-wave features predicted memory deficits.
These results highlight the importance of dynamic slow-wave properties for declarative memory consolidation and point to methodological limits for using auditory cues during sleep, since uncalibrated sounds may disrupt complex neural processing patterns.