Brain Focus Shifts During REM Sleep

If you’ve ever wondered how someone can sleep through loud noises, new research suggests the answer may lie in the brain’s focus during sleep. During rapid eye movement (REM) sleep, the brain seems to turn its attention away from external sounds and towards internal signals, particularly the heartbeat.

The Study

Scientists at the University of Lausanne and the University of Geneva led a study to explore how the brain processes external and internal signals during wakefulness and different REM sleep phases. They recorded the brain activity of 25 healthy adults using electroencephalography (EEG). Participants listened to simple tones while awake and asleep, allowing researchers to compare responses to sounds and heartbeats.

Key Findings

The research revealed a clear pattern. As participants transitioned from being awake to REM sleep, the brain’s response to external sounds weakened. This effect was strongest during ‘phasic’ REM sleep, a period marked by bursts of rapid eye movements and vivid dreams. Conversely, the brain’s response to heartbeats remained strong, even more so than during wakefulness. The brain reallocates its focus from external to internal signals, shifting the balance between ‘exteroception’ (external processing) and ‘interoception’ (internal awareness). This suggests deeply sleeping individuals might not ignore external sounds, but instead prioritize internal body signals.

Implications

The researchers developed an ‘audio-cardio index,’ comparing brain responses to sounds and heartbeats. This index showed a shift from sound-focused processing while awake to heartbeat-focused processing in deeper REM sleep. Though sleepers remain somewhat receptive to external sounds, the findings indicate the brain selectively reduces focus on the outside world while monitoring vital internal functions. These insights could extend beyond sleep, potentially aiding in assessing consciousness in individuals unable to respond behaviorally, such as those under anesthesia or with consciousness disorders.

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