When most of us sleep through a loud neighborhood or heavy traffic, we assume our brains are simply ignoring external noise. But neuroscience research reveals something more sophisticated is happening: during rapid eye movement (REM) sleep, the brain doesn't just shut down sensory processing—it selectively reallocates it from outside the body to inside.
A new study published in Current Biology 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. This discovery introduces a novel physiological marker for assessing altered states of consciousness with potential clinical applications.
The Study: How the Brain Turns Hearing Inward
Researchers from the universities of Lausanne and Geneva led by Marzia De Lucia and Sophie Schwartz wanted to understand how the brain balances information from the external environment (exteroception) versus internal bodily signals (interoception). They asked a simple but profound question: What happens to this balance as we transition from wakefulness into 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."
The team recorded neural activity in 25 healthy volunteers over two nights using high-density electroencephalography (EEG). They measured auditory evoked potentials (AEPs)—the brain's electrical responses to external sounds—and heartbeat evoked potentials (HEPs), which track how the brain processes internal cardiac signals.
Two Phases of REM, Two Different States
REM sleep is not a uniform state. It alternates between two distinct phases: tonic REM and phasic REM.
Tonic REM is a quieter phase with slightly higher environmental sensitivity. The brain begins disengaging from external stimuli but hasn't fully committed to internal processing yet.
Phasic REM, on the other hand, is marked by rapid eye movements, brief muscle twitches, irregular heart and respiratory rhythms, and maximum sensory disconnection from external sounds. This is where the brain's focus shifts most dramatically inward.
"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 Key Finding: An Inverse Relationship
The results were clear and systematic. As participants transitioned from wakefulness into tonic REM and then phasic REM:
- Auditory responses attenuated progressively, the brain became less responsive to external sounds
- Heartbeat-evoked potentials were preserved across all sleep stages and actually enhanced relative to wakefulness during REM
"It's not a global suppression of stimuli," summarizes Marzia De Lucia. "Rather, the brain turns its listening inward."
This reciprocal audio-cardio shift reveals that the brain isn't simply powering down sensory systems, it's actively choosing which signals deserve attention. External sounds are filtered out, while physiologically relevant cardiac information is amplified.
The Audio-Cardio Index: A New Biomarker for Consciousness
Building on these findings, the researchers developed an audio-cardio index, a quantitative ratio comparing external-to-internal sensory processing. This simple metric tracks how much the brain prioritizes environmental sounds versus heartbeats across vigilance states.
Why does this matter clinically? Because consciousness is often difficult to assess in patients who cannot respond behaviorally, such as those in comas or minimally conscious states. Traditional tests require some level of motor response, but the audio-cardio index works without requiring any behavioral output. It measures how the brain balances external sensory inputs against internal cardiac signals purely through neural activity patterns.
"This audio-cardio index could serve as a marker for altered states of consciousness, particularly in situations where the person cannot respond behaviorally," says De Lucia. "This could help distinguish states of consciousness that are difficult to assess."
What This Means for Neuroscience and Medicine
The findings have several implications:
For understanding sleep: The research shows that sensory gating during REM is selective rather than global, a nuanced process that preserves internally generated signals while filtering external noise. This helps explain why we can sleep through disturbances without waking, yet still maintain vital physiological awareness.
For clinical assessment: In patients with altered consciousness, whether from stroke, traumatic brain injury, or other conditions, the audio-cardio index could provide an objective measure of covert consciousness without requiring patient cooperation.
For future research: The exteroceptive-interoceptive balance may serve as a fundamental marker for understanding how the brain weighs different types of information across conscious and unconscious states.
Key Takeaways
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The brain doesn't shut down hearing during REM, it redirects it. External auditory processing diminishes while internal cardiac signal processing strengthens.
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Phasic REM represents maximum sensory disconnection from the outside world, where the audio-cardio index shows the most dramatic shift toward cardiac-oriented processing.
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This reprioritization is gradual and systematic as the brain transitions through wakefulness, tonic REM, to phasic REM, providing a window into how consciousness changes at the neural level.
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The audio-cardio index offers practical clinical utility for assessing consciousness in non-responsive patients where traditional behavioral assessments fail.
As we continue to explore the mysterious states of sleep and consciousness, this research illuminates one of the brain's most elegant adaptations: turning inward without unplugging from vital internal signals.
This article is based on original research published by the University of Lausanne in Current Biology (DOI: 10.1016/j.cub.2026.07.024). The study was conducted by a team including Jacinthe Cataldi, Andria Pelentritou, Sophie Schwartz, and Marzia De Lucia.
