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. 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 authors note that some level of auditory processing remains during REM sleep.