Theta phase-locked memory reactivation during REM sleep reduces memories’ emotional tone

REM sleep appears to be involved in emotional memory processing, yet the nature and neural underpinnings of this involvement remain largely unclear. Recent findings suggest REM-related theta activity may play a central role. To test this, we developed an automated protocol to target non-arousing acoustic memory cues to specific phases of REM sleep theta oscillations. During sleep, participants who had undergone a fear conditioning procedure combined with a memory recollection and emotional evaluation task, were re-exposed to conditioned stimuli, timed to either the upswing or downswing of theta waves. Both up- and down-targeted memory cues significantly enhanced the theta dynamic. Moreover, theta phase-locked memory reactivation significantly reduced the emotional tone of conditioned memories compared to sham reactivation (no acoustic memory cue), without affecting recognition performance or dream qualities. This constitutes the first evidence that REM sleep theta oscillations are causally involved in emotional memory processing, specifically in attenuating memories’ emotional charge. These findings have important implications for understanding emotional memory consolidation and may lead to therapeutic applications in disorders featuring maladaptive emotional memories.