Brain Focus During REM Sleep on Internal Signals

Have you ever wondered why some people can sleep through loud noises? New research suggests the answer is in how the brain focuses during sleep. Scientists have discovered that during rapid eye movement (REM) sleep, the brain tends to pay less attention to external noises, directing its focus towards internal signals, especially the heartbeat.

The study, conducted by researchers from the University of Lausanne and the University of Geneva, explored how the brain processes external sounds and internal bodily signals during wakefulness and different REM sleep phases.

Methodology

Researchers recorded the brain activity of 25 healthy adults using electroencephalography (EEG), which measures the brain’s electrical activity. Participants were exposed to simple tones both while awake and during sleep. This allowed a comparison between responses to sounds and responses triggered by heartbeats.

Findings

The results showed a noticeable pattern. As participants entered REM sleep, the brain’s response to external sounds weakened. This effect was strongest during “phasic” REM sleep, marked by bursts of rapid eye movements and vivid dreams. However, during REM sleep, the brain’s response to heartbeats remained strong and was even increased compared to wakefulness.

Shifting Attention

Rather than shutting down, the brain reallocates its attention from external to internal signals during sleep. This process is described as a shift between “exteroception” (processing of external information) and “interoception” (awareness of internal signals, like the heartbeat).

The research suggests that those who remain deeply asleep despite noise may not be ignoring sounds but prioritizing internal body information. Particularly during phasic REM sleep, the brain focuses less on external noises and more on physiological processes.

To quantify this shift, the team developed an “audio-cardio index.” This index compared brain responses to sounds and heartbeats, showing a gradual move from sound-focused processing when awake to heartbeat-focused processing during deeper REM sleep.

Although sleepers are not entirely deaf to their surroundings, the brain selectively reduces attention to external noises. Meanwhile, it continues monitoring vital bodily functions essential for survival.

Impact and Future Implications

The findings offer insight into why some people are difficult to wake. Moreover, the balance between external and internal processing may help assess consciousness in situations where behavioral responses are absent, such as under anesthesia or in consciousness disorders.