During REM sleep, the human brain does not completely disconnect from the outside world. Instead, it appears to reorganize its sensory priorities: as its ability to respond to external sounds gradually declines, its sensitivity to signals coming from within the body — particularly the heartbeat — becomes stronger.
That is the finding of a study published in Current Biology by researchers from the University of Lausanne (Unil/Chuv) and the University of Geneva (Unige), led by Marzia De Lucia and Sophie Schwartz.
The researchers used electroencephalography (EEG) to monitor the brain activity of 25 volunteers as they transitioned from wakefulness into REM sleep, including its two stages, tonic and phasic REM. The results showed that as the brain became increasingly disconnected from sounds in the surrounding environment, its neural response to heartbeats became progressively stronger.
The phenomenon does not appear to reflect a general suppression of sensory processing. “It is not a global suppression of stimuli,” says Marzia De Lucia, the study’s coordinator. “Rather, it is a brain that turns its listening inward.”
Building on this finding, the researchers developed a new audio-cardiac index designed to measure which type of signal — external or internal — the brain is prioritizing at a given moment.
The new measure could provide fresh insight into how the brain processes information during sleep and across different states of consciousness. Its potential applications may also extend beyond sleep research. According to the researchers, the audio-cardiac index could eventually serve as a marker for identifying and distinguishing complex disorders of consciousness, including coma and minimally conscious states.
The findings suggest that even when the brain appears increasingly disconnected from the outside world, it may not stop listening. During REM sleep, it may simply shift part of its attention inward.
