Summary: A new study shows that as the human brain moves into deep REM sleep, it shifts sensory priority away from external sounds and toward internal cardiac signals. Researchers quantify this shift with a novel “audio-cardio index” that may serve as a physiological indicator of altered consciousness.
Key Facts
- Selective sensory reprioritization: During the transition into REM sleep, the brain does not shut down sensory processing entirely. Instead, it reduces responsiveness to external environmental inputs while increasing processing of internal bodily signals.
- Gradual change across REM microstates: REM alternates between tonic and phasic phases. Sensitivity to external stimuli declines progressively from wakefulness to tonic REM and is lowest during phasic REM, which features rapid eye movements and muscle twitches.
- Reciprocal auditory–cardiac relationship: High-density EEG recordings reveal an inverse pattern: auditory-evoked potentials decrease as heartbeat-evoked potentials strengthen across REM sleep.
- Diagnostic audio-cardio index: The study introduces a quantitative ratio comparing auditory and cardiac neural responses. This index could become an objective biomarker to assess depth of consciousness in patients who cannot behaveally respond, such as those in coma or minimally conscious states.
Source: University of Lausanne
Would you keep sleeping even if a very loud noise occurred nearby? Scientists from the universities of Lausanne and Geneva provide a partial explanation: as REM sleep deepens, the brain intentionally redirects attention inward.
Published in Current Biology on August 3, 2026, the study was led by Marzia De Lucia at the University of Lausanne in collaboration with Sophie Schwartz at the University of Geneva. The team investigated how the brain reorganizes sensory processing when shifting from wakefulness into REM sleep, focusing on the balance between exteroception (external sensory inputs) and interoception (internal bodily signals).
Disconnecting without fully switching off
The brain constantly integrates a flood of signals from the outside world and from the body itself. Neuroscientists still seek a clear explanation for how the brain sorts and prioritizes this information, and how that prioritization changes across different states of consciousness. REM sleep offers an ideal window into these mechanisms because its electrical activity resembles wakefulness while behavioral responsiveness to the environment is markedly reduced.
“REM sleep lets us study how sensory gating can selectively reduce external input while maintaining or even increasing processing of internal signals,” explains Jacinthe Cataldi, co-first author of the study. The researchers used this context to compare the brain’s responses to external sounds with its responses to heartbeats as participants moved through wakefulness and REM microstates.
REM microstates: tonic versus phasic
REM sleep contains two alternating microstates. Tonic REM is a quieter phase with fewer rapid eye movements and relatively higher sensitivity to environmental cues. Phasic REM features bursts of rapid eye movements, brief muscle twitches, and more variable cardio-respiratory rhythms, and it shows the strongest sensory disconnection from the external world.
To track these changes, the team recorded high-density EEG in 25 healthy volunteers over two nights. They measured auditory-evoked potentials (AEPs) in response to external tones as an index of exteroceptive processing, and heartbeat-evoked potentials (HEPs) as an index of interoceptive processing. The comparison across vigilance states produced a clear pattern: AEPs diminished from wakefulness through tonic REM and were most attenuated during phasic REM, while HEPs were preserved and even enhanced during REM sleep relative to wakefulness.
“It’s not that the brain goes offline,” says Marzia De Lucia. “Rather, it changes which signals it prioritizes — turning its sensory ‘listening’ inward as REM progresses.”
Introducing the audio-cardio index
To quantify this shift, the authors created the audio-cardio index: a ratio that compares neural responses to auditory stimuli with neural responses to cardiac signals. This index steadily decreases from wakefulness to tonic REM and declines further during phasic REM, reflecting the graded reallocation of processing toward interoception.
Because the audio-cardio index is derived from neural responses rather than behavior, it has potential clinical utility. It could help clinicians detect residual or covert consciousness in patients who cannot communicate or move, such as those in comatose or minimally conscious states, by revealing how the brain balances external and internal sensory streams.
Frequently Asked Questions
A: Instead of a blanket shutdown, the brain selectively adjusts sensory gating. During REM—especially phasic REM—it suppresses neural responses to external auditory inputs while enhancing processing of internal bodily signals like the heartbeat, effectively shifting perceptual focus inward.
A: Tonic REM is a relatively stable, quieter phase with modest environmental sensitivity. Phasic REM contains rapid eye movements, muscle twitches, irregular cardio-respiratory rhythms, and the greatest reduction in responsiveness to external sounds.
A: The audio-cardio index offers an objective measure of how the brain weights external versus internal inputs without requiring behavioral responses. Clinicians could use it to assess covert perception or residual consciousness in non-responsive patients, aiding diagnosis and prognosis.
Editorial Notes
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full and additional context was provided by staff.
About this research
Author: Géraldine Falbriard
Source: University of Lausanne
Contact: Géraldine Falbriard – University of Lausanne
Image: Image credit: Neuroscience News
Original Research (open access): “Sensory processing reallocation from auditory to cardiac signals in REM sleep” by Jacinthe Cataldi, Andria Pelentritou, Sophie Schwartz, and Marzia De Lucia. Current Biology. DOI: 10.1016/j.cub.2026.07.024
Abstract
Sensory processing reallocation from auditory to cardiac signals in REM sleep
The brain continuously integrates external (exteroceptive) and internal (interoceptive) information. How the balance between these two streams shifts across vigilance states with different levels of environmental responsiveness remains incompletely understood. Using high-density EEG in 25 healthy participants, this study compared auditory-evoked potentials (AEPs) and heartbeat-evoked potentials (HEPs) across wakefulness and REM microstates (tonic and phasic REM).
Findings show that AEP amplitudes progressively decline from wakefulness to tonic REM and are most suppressed during phasic REM, while HEPs are preserved and enhanced during REM relative to wakefulness. The audio-cardio index, defined as the ratio of auditory to cardiac neural responses, decreased across vigilance states, indicating a graded shift from auditory-focused processing during wakefulness to cardiac-focused processing during phasic REM.
These results reveal that, although responsiveness to external stimuli diminishes during phasic REM, the brain continues and even amplifies processing of physiologically relevant cardiac signals. The changing balance between exteroception and interoception may provide a mechanistic marker of altered consciousness useful in clinical contexts where behavioral assessment is limited.