Summary: A new study demonstrates that as the human brain transitions into deep REM sleep, it dynamically reprioritizes sensory processing away from external environmental sounds and toward internal cardiac signals.
The research revealed that as responsiveness to external sounds weakens during REM sleep transitions, the brain’s processing of internal cardiac signals systematically intensifies. To quantify this neural pivot, the team developed an audio-cardio index, offering a novel physiological marker for assessing altered states of consciousness.
Key Facts
- Internal Sensory Reprioritization: As the human brain shifts into REM sleep, it does not globally shut down sensory processing; instead, it selectively downregulates external environmental inputs while upregulating internal visceral signals.
- Gradual Disconnection Across REM Phases: Environmental sensory disconnection reaches its peak during phasic REM sleep (marked by rapid eye movements and muscle twitches), whereas tonic REM represents an intermediate state between wakefulness and complete sensory gating.
- Reciprocal Audio-Cardio Shift: High-density EEG recordings confirm an inverse biological relationship: as electrical responses to external auditory tones attenuate, heartbeat-evoked neural responses become significantly stronger.
- Diagnostic Audio-Cardio Index: The authors developed a quantitative ratio comparing external-to-internal sensory processing, providing an objective biomarker capable of evaluating depth of consciousness in non-responsive clinical populations, such as comatose or minimally conscious patients.
Source: University of Lausanne
Are you someone who would keep sleeping even if a bomb went off right next to you? Scientists from the universities of Lausanne and Geneva offer a partial explanation for this phenomenon.
In a study published in Current Biology on August 3, 2026, the team led by Marzia De Lucia from the Department of Clinical Neurosciences at CHUV and senior lecturer at the Faculty of Biology and Medicine of the University of Lausanne, in collaboration with Sophie Schwartz, professor at the University of Geneva, shows how the brain progressively reorganizes the way it processes sensory information as it shifts from wakefulness to REM sleep.
Disconnecting Without Unplugging
Constantly bombarded with stimuli from both the environment and our body, the brain is a true master at integrating signals. What remains a mystery for neuroscientists is the precise mechanism that allows it to sort, prioritize, and combine all this information – and even more mysterious is how this process changes as we move from one consciousness level to another, for example from wakefulness to sleep.
“REM sleep provides an ideal context for addressing this question”, explains Jacinthe Cataldi, one of the study’s co-first authors.
“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.” This disconnection comes with a higher arousal threshold: sensory information from the environment has a harder time reaching the brain, which nevertheless continues to process other types of signals.
A Gradual Transition
REM sleep is not, however, a uniform state. It constantly alternates between two phases: tonic REM and phasic REM. It is during this second phase that rapid eye movements appear, which give this sleep stage its name. Phasic REM is also characterized by brief muscle twitches and more variable heart and respiratory rhythms.
The two phases also differ in their degree of sensory disconnection: sensitivity to external stimuli progressively decreases from wakefulness to tonic REM and all the way to phasic REM, where it reaches its lowest level.
“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”, explains Andria Pelentritou, the study’s other co-first author.
The neuroscientists recorded neural activity in 25 volunteers over two nights of sleep using an electroencephalogram (EEG). They compared the electrical potentials generated by auditory stimuli to those triggered by heartbeats across wake and the different sleep phases, leading to a discovery: as the neural response to sounds from the environment weakens, the processing of cardiac signals strengthens. “It’s not a global suppression of stimuli. Rather, the brain turns its listening inward”, summarizes Marzia De Lucia.
Audio-Cardio Index
The scientists then calculated an index indicating which type of signal the brain preferentially processes — in other words, this index can indicate to which degree the brain is tuned towards environmental sounds relative to heartbeats.
“This audio-cardio index could serve as a marker for altered states of consciousness, particularly in situations where the person cannot respond behaviourally”, says Marzia De Lucia. This index could eventually help better distinguish states of consciousness that are difficult to assess, such as coma or minimally conscious states.
Key Questions Answered:
A: Rather than turning off all neural processing, the brain selectively alters its sensory gating mechanisms. During REM sleep, especially phasic REM, it suppresses incoming external auditory signals while simultaneously enhancing its processing of internal body signals, such as heartbeats, shifting its perceptual focus inward.
A: REM sleep alternates between two sub-states: tonic REM and phasic REM. Phasic REM is characterized by rapid eye movements, muscle twitches, irregular cardio-respiratory rhythms, and maximum sensory disconnection from external sounds. Tonic REM is a quieter phase with slightly higher environmental sensitivity.
A: Because the audio-cardio index measures how the brain balances external sensory inputs against internal cardiac signals without requiring behavioral responses, clinicians could use it to evaluate covert consciousness in non-responsive patients, such as those in a coma or a minimally conscious state.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this sleep and auditory neuroscience research news
Author: Géraldine Falbriard
Source: University of Lausanne
Contact: Géraldine Falbriard – University of Lausanne
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Sensory processing reallocation from auditory to cardiac signals in REM sleep” by Jacinthe Cataldi, Andria Pelentritou, Sophie Schwartz, 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 information from the external environment (exteroception) and the internal bodily milieu (interoception). However, how the balance between these two processing streams shifts across vigilance states with differing levels of environmental responsiveness remains poorly understood.
Here, we examined neural responses to external auditory and internal cardiac signals across wakefulness and rapid eye movement (REM) sleep microstates—tonic and phasic REM—which are characterized by progressively reduced responsiveness to external stimulation. High-density electroencephalography (EEG) was recorded in healthy participants (n = 25).
Auditory evoked potentials (AEPs) and heartbeat evoked potentials (HEPs) served as indices of exteroception and interoception, respectively, and were compared across vigilance states. AEPs progressively decreased from wakefulness to tonic REM and were most attenuated during phasic REM. In contrast, HEPs were preserved across REM microstates and were enhanced relative to wakefulness, indicating sustained—and even amplified—processing of cardiac signals during REM sleep.
To quantify the relative weighting of external and internal signals, we introduce an audio-cardio index, defined as the ratio of auditory to cardiac neural responses. This index decreased systematically across vigilance states, revealing a graded shift from auditory-oriented processing during wakefulness to cardiac-oriented processing during phasic REM, with tonic REM occupying an intermediate position.
Together, these findings demonstrate that while responsiveness to auditory stimuli diminishes during phasic REM, the brain continues to prioritize physiologically relevant cardiac signals. The exteroceptive-interoceptive balance may thus provide a novel, mechanistically grounded marker of altered consciousness, which is particularly informative in contexts where behavioral responsiveness cannot be assessed.
