A new Swiss study reveals how the brain reorganizes sensory processing as it shifts into REM sleep, turning its listening inward to preserve cardiac signals. Key takeaways:Researchers found that during REM sleep, the brain’s response to external sounds decreases while its processing of internal cardiac signals strengthens. A study published in Current Biology shows that as individuals drift into REM sleep, signals from the outside world gradually decrease while those coming from the body are preserved or even enhanced. “REM sleep provides an ideal context for addressing this question,” says Jacinthe Cataldi, co-first author of the study, in a release. REM sleep alternates between two phases: tonic REM and phasic REM, which is characterized by rapid eye movements, brief muscle twitches, and more variable heart and respiratory rhythms.