During sleep, the brain does not "switch off" from the outside world but shifts its attention from external sounds to internal bodily signals. This is the conclusion reached by researchers in Switzerland, as reported by infohub.kz.
Scientists from the universities of Lausanne and Geneva studied how the brain processes information during sleep. In the experiment, they observed 25 volunteers over two nights, recording brain activity using electroencephalography (EEG). They compared the brain's response to external sounds and internal bodily signals, particularly heartbeats.
As participants transitioned from wakefulness to rapid eye movement (REM) sleep, the brain's response to sounds gradually weakened, while sensitivity to heart signals increased. This suggests that during sleep, the brain does not stop perceiving information but changes its priorities: instead of monitoring the outside world, it begins to pay more attention to internal processes.
Of particular interest to the scientists was the REM phase, during which most dreaming occurs. Although brain activity during this period resembles wakefulness, people respond significantly worse to external stimuli. The study showed that this sensitivity decreases gradually: first during wakefulness, then in the so-called tonic phase of REM sleep, and most strongly in the phasic phase, when rapid eye movements occur.
Based on the findings, the researchers developed a metric called the audio-cardiac index. It reflects what the brain is paying more attention to at any given moment—external sounds or internal bodily signals. According to the authors, in the future this index could help doctors assess the level of consciousness in patients who cannot communicate, such as those in a coma.
However, the study was small, involving only 25 participants. Larger studies will be needed to confirm the results and evaluate their practical applications.