Research Reveals Sleeping Brain's Sensitivity to Urgent and Harsh Sounds

During sleep, the brain faces a complex balancing act: it must disconnect from external sensory inputs to facilitate restorative processes, yet remain alert enough to awaken if necessary. Recent research from the University of Geneva (UNIGE) and the Institut Pasteur sheds light on how the brain processes specific sounds during sleep, particularly what are known as "rough" sounds—such as screams or alarms—that are characterized by rapid variations in sound intensity.

Scientists discovered that these rough sounds are systematically processed by the sleeping brain, prompting particular brain wave responses. This finding was published in the journal Scientific Reports and enhances our understanding of perceptual disorders like hyperacusis — hypersensitivity or intolerance to certain sounds — as well as the impact of nighttime noise pollution on brain health.

Rough sounds exhibit a distinct acoustic property where sound intensity modulates rapidly—between 40 and 100 times per second. Unlike speech, which has syllables typically occurring at 4 to 8 Hz, rough sounds affect the auditory system at much higher frequencies, causing shrill and often unpleasant sensations. These sounds, which include alarms, screams, and infant cries, are effective because they automatically draw our attention and signal potential danger, primarily activating the amygdala, a brain region involved in emotional responses and attention.

The study involved 17 volunteers sleeping in specialized rooms while their brain activity was monitored via EEG. Researchers played various sounds, including human cries and false cries, at low volumes, with modifications in pitch and roughness to observe brain reactions without waking the participants. They found that rough sounds, regardless of pitch, activated the brain’s alert systems and were associated with an increase in sleep spindles—short bursts of brain activity triggered by sensory stimuli during sleep.

Roughness in sounds is rare in everyday environments but is typically associated with urgent messages in high-stakes communication. Overexposure to these sounds can evoke intense emotional reactions, sometimes irrational or aggressive. Understanding how these sounds influence the brain is crucial not only for psychiatric and neurological conditions such as hyperacusis, tinnitus, epilepsy, and Alzheimer’s disease but also for assessing the health impacts of urban noise pollution.

The findings highlight the importance of understanding auditory processing during sleep and suggest that environments with frequent loud or harsh sounds may affect mental and physical health adversely. As urban living exposes us to increased noise levels, studying the brain’s response to these sounds helps inform strategies to mitigate their effects on mental well-being and sleep quality.

Source: https://medicalxpress.com/news/2025-06-brain-harsh-urgent.html