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In a Nutshell
- Random trains of clicks played during naps cut deep sleep short and roughly halved the memory-supporting slow brain waves, even though total sleep time barely changed.
- Only visual-spatial memory took a measurable hit; word memory and finger-tapping skill were left intact.
- The clicks also stunted how far slow waves traveled across the brain, hitting the frontal regions hardest, and those weaker waves tracked with poorer recall.
Falling asleep to a podcast, a white-noise app, or a partner’s late-night TV might feel harmless. A new study from the University of Freiburg in Germany suggests otherwise. Random sounds during sleep can weaken brain activity most closely tied to memory storage, and it does so without ever waking a person up.
Researchers played random trains of clicks through earphones while young men napped. Nobody was jolted awake, and total sleep time barely moved. But underneath that calm, the slow brain waves that roll through deep sleep and help lock in memories were being cut short. By the time the nappers woke, part of what they had learned from a visual task was simply gone, according to research published in the journal iScience.
That result lands awkwardly for a popular line of research built on the opposite hope, that carefully timed sounds during sleep can strengthen memory. Freiburg’s findings add a cautionary note: even well-meant noise piped into a sleeping brain can exact a real cost.
How the Sounds-During-Sleep Study Worked
Twenty healthy young men, ages 18 to 31, each visited a sleep lab twice. Before every nap, they studied material meant to test two kinds of memory. One was a declarative task, covering visual-spatial recall, such as tracing a maze and remembering word pairs. The other was a finger-tapping sequence that measures how well someone learns a physical skill.
Each man then got a three-hour nap. On one visit, random trains of clicks played through earphones the whole time, starting as he drifted off. On the other visit, silence. A cap of scalp electrodes tracked the brain’s electrical activity throughout, and memory tests came right after waking. Because every participant underwent both conditions, spaced at least 6 days apart, each person served as his own control. That setup makes the results harder to chalk up to random luck.
Deep Sleep Took the Hit, Not Sleep Time
By the usual measures, the men slept fine even with sound playing. They fell asleep just as fast, slept nearly as long, and scored the same on a reaction-time test after waking. A look at the shape of their sleep told a different story. Time spent in deep sleep dropped sharply when sounds were present, replaced by lighter sleep. Their brains stayed asleep, but shifted into a shallower, weaker gear.
Slow waves, the large rhythmic pulses that sweep across the brain during deep sleep, showed up only about half as often during noisy naps. Those waves are thought to move memories from short-term into long-term storage, so fewer of them means fewer chances to file information away. A separate memory-related rhythm called sleep spindles held steady through the noise, which helps explain why one type of memory suffered while others walked away untouched.
The Memories That Slipped Away
After waking, the damage showed up in one place: visual-spatial recall, things like remembering a route through a maze or recognizing abstract shapes. That score fell meaningfully after a noisy nap. Word memory and the finger-tapping skill both held up fine.
Researchers suggest one explanation. Visual-spatial memory relies heavily on deep sleep, slow-wave sleep, and the hippocampus, a brain region central to the formation of new memories. Physical-skill learning rides more on lighter sleep stages, which the noise left mostly intact. And the link was direct: the less deep sleep a man got, the worse his visual recall.
Digging further, the team tracked how slow waves actually moved. Slow waves do not just flash on and off in one spot; they ripple from region to region, a journey believed to help the brain coordinate information while it sleeps. During noisy naps, the waves that did occur were shorter, covered less ground, and reached fewer areas, especially the frontal regions tied to higher-level thinking.
Those stunted waves were associated with poorer memory, even after accounting for the loss of deep sleep. The statistics suggested that the memory payoff of deep sleep runs in part through the quality of these traveling waves. Cut the waves short, and part of that payoff goes with them.
What Sounds During Sleep Mean for Rest
Study authors are careful not to condemn every sound-based sleep tool. Other work shows that sounds precisely synced to the rhythm of slow waves can strengthen memory. Timing and content seem to be the dividing line: random noise is one thing, a signal tied to specific learned material and locked to a brain rhythm is another.
Still, the caution stands. As the authors put it, sound reaching the ear during sleep can produce “functional gains or losses, with the ultimate memory benefit of sleep potentially being the sum of both influences.” The unsettling part is how ordinary the sleep looked on paper, same duration, same wake time, same alertness, while visual memory quietly eroded. Sleep duration alone is a poor gauge of sleep quality.
For anyone who drifts off to ambient noise, and for scientists building sleep-enhancement gadgets, one point is hard to miss. What unfolds inside a sleeping brain is more fragile than simply staying asleep would suggest.
Disclaimer: This article summarizes a single peer-reviewed study for general readers and is not medical advice. The research involved 20 healthy young men napping in a lab, so the findings may not apply to everyone, including women, older adults, or people with sleep disorders. The link between slow-wave travel and memory is correlational, not proven cause and effect. Anyone with concerns about sleep or memory should consult a qualified healthcare professional
Paper Notes
Limitations
Several limits are worth keeping in mind. Only male participants took part, a deliberate choice by the researchers because evidence at the time of data collection suggested that the female hormonal cycle might affect sleep-related memory. They caution that sex-based differences in the results cannot be ruled out, and that a broader sample would be needed to confirm the findings more widely. The sample itself was small, at twenty people. Because the sounds began before participants fell asleep rather than only after, everyone was aware of them, which may have prompted efforts to tune them out that lingered into sleep in ways hard to measure. The volume was estimated using biological norms rather than measured directly with a sound meter, leaving some uncertainty about the exact loudness. The authors add that the sound may have been a bit louder than is typically used in memory-boosting sleep studies, which could have exaggerated the disruption. Finally, the tie between how slow waves traveled and how well people remembered is correlational, so future work will need to test that link more directly.
Funding and Disclosures
According to the paper, the work was supported by BMBF funding and a German Research Foundation Emmy Noether grant to Monika Schönauer and Steffen Gais (SCHO1820/2–1, GA730/3-1), as well as a German Research Foundation Walter Benjamin grant to Nora M. Rouëast (RO6828/1-1, RO6828/1–2). The authors declare no competing interests.
Publication Details
Authors: Nora M. Rouëast, Deniz Kumral, Steffen Gais, and Monika Schönauer. The authors are affiliated with the Institute of Psychology, Neuropsychology, University of Freiburg; BrainLinks-BrainTools, University of Freiburg; the Bernstein Center Freiburg; and the Institute of Medical Psychology and Behavioral Neurobiology, University of Tübingen, all in Germany.
Paper Title: “Random auditory stimulation during sleep disturbs traveling slow waves and declarative memory”
Journal: iScience
DOI: 10.1016/j.isci.2026.116601
Received: December 5, 2025 | Revised: April 30, 2026 | Accepted: June 11, 2026
