Scientists have explained what happens in the brain of people who do not wake up to an alarm, according to Newsweek. During a certain phase of sleep, the brain responds far less to outside sounds and instead focuses more on signals coming from within the body, particularly the heartbeat.
The research was carried out by scientists from the University of Lausanne and the University of Geneva. They studied how the brain processes external sounds and internal bodily signals during wakefulness and different phases of rapid eye movement (REM) sleep, the phase most associated with vivid dreams.
The study involved 25 healthy adults. Using electroencephalography, researchers recorded their brain activity while participants listened to simple sound signals during wakefulness and sleep, then compared the results with the brain's response to its own heartbeat.
Brain shifts priorities during sleep
The researchers found that as people moved from wakefulness into REM sleep, the brain's response to sounds gradually weakened, becoming weakest during the REM phase marked by rapid eye movements and the most vivid dreams. At the same time, the response to the heartbeat not only persisted but grew stronger than during wakefulness.
The scientists said this suggests the brain does not simply shut down but redistributes attention, monitoring the external environment less and controlling the body's internal state more.
Why some people are hard to wake
The study's authors explained the process as a shift in balance between exteroception, the perception of signals from the environment such as sounds, and interoception, the processing of internal bodily signals such as the heartbeat. That is why people who sleep deeply even through loud noise may not be ignoring the sounds but unconsciously prioritizing signals from their own body.
The researchers also developed what they called an audio-cardiac index, comparing the brain's response to sounds and to the heartbeat. It showed that as people sink deeper into REM sleep, the brain increasingly orients toward internal signals, which the scientists said could help identify different states of consciousness more precisely.
The researchers stressed that a person is not completely deaf to the surrounding world during REM sleep. The brain continues to process sounds, but does so far less actively, prioritizing control over processes important for survival. The scientists said the findings could also help assess levels of consciousness in patients under anesthesia or with severe disorders of consciousness.
