A team of scientists from the universities of Lausanne and Geneva found out why some people can sleep through almost anything, even a loud noise right next to their bed. According to the study published in Current Biology, it has to do with how our brain shifts its attention during REM sleep.
Their study was led by Marzia De Lucia from the Department of Clinical Neurosciences at CHUV, together with Sophie Schwartz from the University of Geneva. The findings show that as we fall into REM sleep, our brain doesn’t just “switch off” from the world, it changes what it pays attention to.
Disconnected, But Not Shut Down
Our brains are constantly flooded with information, both from the world around us and from inside our own bodies. Normally, the brain is very good at sorting through all of this and figuring out what matters. But scientists still don’t fully understand how the brain does this, especially as we move between different states of consciousness, like falling asleep.
REM sleep turns out to be a great state to study this. As co-first author Jacinthe Cataldi explains, “REM sleep provides an ideal context for addressing this question.” That’s because although the brain is quite active during REM sleep, similar in some ways to being awake, it also becomes strongly cut off from the outside world. This means sounds and other outside stimuli struggle to get through, even though the brain keeps working on other kinds of signals.
A Gradual Shift, Not a Sudden Switch
REM sleep isn’t one single state. It flips back and forth between two types: tonic REM and phasic REM. Phasic REM is when the classic rapid eye movements happen, along with small muscle twitches and shifting heart and breathing rhythms.
These two REM phases also differ in how disconnected we are from our surroundings. As we move from being awake, into tonic REM, and then into phasic REM, our sensitivity to outside stimuli keeps dropping, hitting its lowest point during phasic REM. Co-first author Andria Pelentritou explained that the researchers used this known pattern to compare how the brain responds to outside sounds versus signals from inside the body, specifically, heartbeats.
Listening Inward
To explore this, the team recorded brain activity in 25 volunteers over two nights of sleep using EEG (a way of measuring electrical activity in the brain). They compared how the brain reacted to sounds from outside with how it reacted to the person’s own heartbeat, across different sleep stages.
The result was striking: as the brain’s response to outside sounds got weaker, its response to heartbeats got stronger. As De Lucia puts it, “It’s not a global suppression of stimuli. Rather, the brain turns its listening inward.” In other words, the brain isn’t simply ignoring everything, it’s redirecting its attention toward the body.
A New Way to Measure Consciousness?
Based on this, the researchers created what they call an “audio-cardio index.” This measures how strongly the brain is tuned toward outside sounds compared to internal signals like heartbeats.
The team believes this index could eventually be useful well beyond sleep research. It might help doctors better assess consciousness in patients who can’t respond or communicate, such as those in a coma or a minimally conscious state, situations where it’s currently very hard to know how aware someone actually is.
De Lucia, M., Cataldi, J., Pelentritou, A., et al. (2026). Title of study as published in Current Biology [details from press release]. University of Lausanne (CHUV) and University of Geneva. Current Biology. https://doi.org/10.1016/j.cub.2026.07.024