University of Lausanne researchers recorded EEG in 25 volunteers and found that as sleep moves into REM, responses to external sounds weaken while processing of heartbeats grows stronger.

Sleep is often described as a disconnection from the world. The eyes close, the body goes still and most sounds fail to wake us. But the brain does not simply switch off. A new study suggests that during REM sleep, the stage most associated with vivid dreaming, it redirects its attention inward.

Researchers at the University of Lausanne, working with colleagues at the University of Geneva, report in Current Biology that as sleep progresses toward REM, the brain’s response to external sounds weakens while its processing of the body’s own heartbeat strengthens.

“It’s not a global suppression of stimuli. Rather, the brain turns its listening inward,” said Marzia De Lucia, a senior lecturer in the university’s Faculty of Biology and Medicine and a member of the Department of Clinical Neurosciences at Lausanne University Hospital, who led the team.

A natural experiment in consciousness

REM sleep, short for rapid eye movement sleep, offers a unique window into how the brain handles information. “REM sleep provides an ideal context for addressing this question,” said Jacinthe Cataldi, a co-first author of the study. “Even though its neural activity shares some similarities with that of the awake brain, REM is characterized by a profound disconnection from the outside world.”

REM itself comes in two flavors. In tonic REM, the brain is in REM sleep but the eyes are relatively still. Phasic REM is marked by bursts of rapid eye movements, brief muscle twitches and more variable heart rate and breathing.

During REM sleep, most of the body’s muscles are temporarily paralyzed, which prevents people from acting out their dreams, while the brain shows patterns of activity resembling wakefulness. Healthy adults cycle through several periods of REM each night, with the longest typically occurring toward morning. Disruptions to REM sleep have been linked to conditions including depression and certain neurodegenerative diseases.

The researchers recorded electroencephalography, or EEG, brain activity in 25 volunteers over two nights of sleep. Andria Pelentritou, the other co-first author, explained the approach: “We took advantage of this well-known gradual transition to compare the neural response to external auditory stimuli with the response to internal inputs — in this case, heartbeats.”

From outside to inside

The team compared the brain’s responses to sounds and to heartbeats during wakefulness and during both types of REM sleep. Sensitivity to external sounds dropped from wakefulness to tonic REM and was lowest during phasic REM. At the same time, the brain’s processing of heartbeat signals grew stronger.

The brain constantly receives signals from inside the body, a sense known as interoception, which includes awareness of the heartbeat, breathing and gut. Researchers can measure the brain’s response to each heartbeat in EEG recordings. Such heartbeat-evoked responses have been linked to self-awareness and emotional processing.

The authors interpret the shift as a reallocation of attention rather than a general shutdown of the senses. During REM, the brain appears to turn away from the outside world and toward the body.

An index for hard-to-assess states

The researchers developed what they call an audio-cardio index, which measures how much the brain favors sounds over heartbeats. Because it relies on brain responses rather than behavior, it might be useful for people who cannot communicate.

Current assessments of consciousness in patients with severe brain injuries rely largely on bedside examinations of whether patients respond to commands or track objects with their eyes. These exams can miss awareness in patients who cannot move. Brain-based measures, from EEG to functional MRI, have been developed to detect covert consciousness, but none is yet a perfect test.

De Lucia said the audio-cardio index could serve as a marker for altered states of consciousness, particularly in situations where the person cannot respond behaviorally. She added that it could eventually help distinguish states of consciousness that are difficult to assess, such as coma or minimally conscious states.

Distinguishing these conditions is a major challenge in neurology. Studies have found that a significant fraction of patients diagnosed as being in a vegetative state show signs of awareness when tested with brain imaging. Tools that can detect hidden consciousness could influence treatment decisions and family conversations about care.

Dreams and the body

The findings may also bear on why dreams feel the way they do. Dreams often involve strong emotions and bodily sensations, and some researchers have proposed that signals from the body help shape dream content. A brain that is more attuned to the heart during REM could provide a link between bodily states and the emotional texture of dreams, although the study did not examine dreams directly.

The study was relatively small, and the audio-cardio index will need to be tested in patients with disorders of consciousness before it can be used clinically. Still, it offers a new view of sleep: not as the brain going dark, but as the brain shifting its focus from the world outside to the rhythms within.

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Notes

1. “Sensory processing reallocation from auditory to cardiac signals in REM sleep.” Current Biology, 3 August 2026. DOI: 10.1016/j.cub.2026.07.024.
2. University of Lausanne. “During REM sleep, the human brain ‘listens’ to bodily signals.” EurekAlert!, 3 August 2026. https://www.eurekalert.org/news-releases/1138039.

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