New research explores how some individuals remain undisturbed by external noises during sleep. The study suggests the brain may selectively focus on internal bodily signals, especially during REM sleep. Conducted by researchers at the University of Lausanne and the University of Geneva, the investigation highlights the brain’s focus shift from external to internal attention while sleeping.
Examining Brain Activity
The researchers examined the brain’s response to sounds and internal signals across different sleep stages. They recorded brain activity in 25 healthy adults. Using electroencephalography (EEG), they measured responses to sounds and heartbeats while participants were both awake and asleep. The study revealed that as participants entered REM sleep, their brains responded less to sounds but remained highly responsive to heartbeats.
REM Sleep Focus Shift
The research highlighted a specific phenomenon during phasic REM sleep, marked by rapid eye movements and vivid dreaming. During this period, the brain’s response to external sounds significantly diminished, while attention to the heartbeat remained strong. This showcases a processed balance shift from ‘exteroception’ (external awareness) to ‘interoception’ (internal signal awareness). Consequently, people who sleep through external noises are not ignoring them but prioritizing internal information.
Audio-Cardio Index Development
The team introduced an “audio-cardio index,” comparing responses to sounds and heartbeats across different consciousness states. This index illustrates a gradual transition from external sound processing in wakefulness to internal heartbeat processing in deeper REM sleep, aiding in distinguishing consciousness states.
Consciousness and Sleep Implications
This research does not imply a complete lack of sensitivity to surroundings during sleep. Auditory processing still occurs, although it is selective and reduced. The findings hold broader implications. Understanding this balance might help evaluate consciousness in cases where individuals cannot respond, such as under anesthesia or with consciousness disorders.
The study is detailed by Cataldi, J., Pelentritou, A., Schwartz, S., and De Lucia, M. in Current Biology.