Sleep does not isolate the brain from the world — rather, it affects how our brain balances the processing of different types of sensory information. A Swiss study shows that as we drift off to REM sleep, signals from the outside world gradually decrease while those coming from our own body are preserved, or even enhanced.
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.
