A Swiss study reveals that during REM sleep, the brain reduces processing of external auditory stimuli while enhancing internal signals like heartbeats. Researchers from the University of Lausanne and Geneva used EEG to track 25 volunteers, discovering a gradual shift from wakefulness to phasic REM sleep. As environmental sounds weaken, cardiac signals strengthen, suggesting the brain 'listens inward.' This reorganization occurs through two REM phases—tonic and phasic—with phasic REM showing the lowest external sensitivity. The team developed an 'audio-cardio index' to measure this shift, which could help assess altered consciousness states like coma. The study, published in Current Biology, highlights how neural processing reallocates resources during sleep.

The findings emphasize REM sleep's role in prioritizing self-generated information. Lead author Marzia De Lucia notes this isn't a global suppression but a selective focus. The gradual transition between sleep phases allows researchers to compare neural responses to external and internal stimuli. Phasic REM, marked by rapid eye movements and muscle twitches, shows the most pronounced disconnection from external inputs. This mechanism might explain why dreams often incorporate bodily sensations.

The audio-cardio index could serve as a biomarker for consciousness levels. De Lucia suggests it may aid in diagnosing patients unable to respond behaviorally. The research bridges sleep science and consciousness studies, offering insights into how the brain filters information across states. Further research could explore applications in neurology or sleep disorder treatments.

The study involved collaboration between neuroscientists at CHUV and the University of Geneva. Sophie Schwartz and Andria Pelentritou contributed to experimental design and data analysis. Results were published in 2026, with the DOI 10.1016/j.cub.2026.07.024.