If you've ever wondered how someone can snooze through a blaring alarm, a thunderstorm or a noisy neighbor, new research suggests the answer may lie in what the brain chooses to pay attention to during sleep.

Scientists have found that during rapid eye movement (REM) sleep, the stage most closely linked to vivid dreaming, the brain appears to shift its focus away from the outside world and toward signals coming from within the body, particularly the heartbeat.

The study, led by researchers at the University of Lausanne and the University of Geneva, examined how the brain processes external sounds and internal bodily signals across wakefulness and different phases of REM sleep.

To investigate, the team recorded the brain activity of 25 healthy adults using electroencephalography (EEG), a technique that measures electrical activity in the brain.

Participants listened to simple tones while awake and during sleep, allowing researchers to compare responses to sounds with responses triggered by their own heartbeats.

The findings revealed a clear pattern. As participants moved from wakefulness into REM sleep, the brain's response to sounds became progressively weaker.

The effect was strongest during "phasic" REM sleep, a period associated with bursts of rapid eye movements and some of the most vivid dreams.

By contrast, the brain's response to heartbeats remained strong throughout REM sleep and was even enhanced compared with wakefulness.

Rather than simply shutting down during sleep, the brain appeared to be reallocating its attention from the external environment to the body's internal signals.

The researchers describe this as a shift in the balance between "exteroception" and "interoception."

Put simply, exteroception is the processing of information from outside the body, such as sounds, while interoception refers to awareness of internal signals including the heartbeat.

Their results suggest that people who remain deeply asleep despite external noise may not be ignoring sounds so much as prioritizing information from within their own bodies.

During phasic REM sleep in particular, the brain seems less concerned with monitoring the outside environment and more focused on internal physiological processes.

The team also developed what they called an "audio-cardio index," a measure comparing the brain's responses to sounds and heartbeats.

The index showed a gradual shift from sound-focused processing when awake to heartbeat-focused processing in deeper REM sleep, helping distinguish different states of consciousness.

The research does not mean sleepers become completely deaf to their surroundings. The authors note that some level of auditory processing remains during REM sleep.

Instead, the findings suggest the sleeping brain selectively reduces attention to external inputs while continuing to monitor bodily functions that could be important for survival.

Beyond explaining why some people seem impossible to wake, the findings could have wider implications.

The researchers believe this balance between external and internal processing may eventually help scientists assess consciousness in situations where a person cannot respond behaviorally, such as under anesthesia or in disorders of consciousness.

Newsweek reached out to the research team lead for comment via email.

Reference

Cataldi, J., Pelentritou, A., Schwartz, S. and De Lucia, M. (2026). Sensory processing reallocation from auditory to cardiac signals in REM sleep. Current Biology, [online] 0(0). doi:10.1016/j.cub.2026.07.024.

Contact Newsweek editors on this story: Kara Dolman and Emma Lee-Sang