Brain Redirects Attention Inward During REM Sleep

Sleep does not isolate the brain from the outside world—instead, it changes how the brain balances processing of different types of sensory information. A study from researchers at the universities of Lausanne and Geneva shows that as people drift into REM sleep, neural responses to external sounds gradually weaken while responses to internal bodily signals, such as heartbeats, are preserved or even strengthened. 

The study, led by Marzia De Lucia in collaboration with Sophie Schwartz was published in Current Biology.

The brain constantly sorts, prioritizes and combines signals from the environment and the body, a process that is harder to study as consciousness shifts, such as from wakefulness to sleep. "REM sleep provides an ideal context for addressing this question," said 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 raises the arousal threshold, making it harder for environmental sensory information to reach the brain, even as it continues processing other signals.

REM sleep alternates between tonic REM and phasic REM, the latter marked by rapid eye movements, brief muscle twitches, and more variable heart and respiratory rhythms. Sensitivity to external stimuli decreases gradually from wakefulness to tonic REM, reaching its lowest point during phasic REM. "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," explained Andria Pelentritou, the study's other co-first author. 

The researchers recorded neural activity in 25 volunteers over two nights of sleep using electroencephalography, comparing electrical potentials generated by auditory stimuli to those triggered by heartbeats across wakefulness and the different sleep stages. They found that as the neural response to environmental sounds weakens, the processing of cardiac signals strengthens. "It's not a global suppression of stimuli. Rather, the brain turns its listening inward," De Lucia said.

The team then calculated an audio-cardio index, indicating the degree to which the brain is tuned toward 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 behaviorally," De Lucia said. The index could eventually help distinguish between difficult-to-assess states of consciousness, such as coma or minimally conscious states.