Heart Rate Variability During REM Sleep is Associated with Reduced Negative Memory Bias.

Emotional memories change over time, but the mechanisms supporting this change are not well understood. Memory consolidation during sleep has been shown to selectively prioritize negative experiences while forgetting neutral memories. Whereas studies examining the role of vagal heart rate variability (HRV) during waking in memory consolidation have shown that vagal HRV is associated with enhanced memory of positive experiences at the expense of negative ones. However, no studies have explored how HRV during sleep contributes to emotional memory processing. Accordingly, we aimed to investigate the neural and vagal contributions during sleep to the processing of neutral and negative memories. To do so, we examined the impact of pharmacological vagal suppression, using zolpidem, on overnight emotional memory consolidation in a double-blind, placebo-controlled, within-subject, cross-over design. Thirty-two participants encoded neutral and negative pictures in the morning, then were tested on picture recognition before and after a night of sleep. Zolpidem or a placebo drug were administered in the evening before overnight sleep, monitored with electroencephalography and electrocardiography. Results showed that higher vagal HRV in Non-Rapid Eye Movement Sleep slow wave sleep (NREM SWS) and Rapid Eye Movement Sleep (REM) was associated with greater overnight improvement for neutral pictures in the placebo condition. Additionally, higher vagal HRV during REM was associated with an emotional memory tradeoff (i.e., greater memory for neutral at the expense of negative images), indicating a potential role for REM vagal HRV in forming a positive memory bias overnight. As previously reported, zolpidem reduced vagal HRV during SWS and increased NREM sigma power, and this vagal suppression eliminated the positive memory bias. Lastly, we used a stepwise linear mixed effects regression framework to investigate how NREM sigma power and vagal HRV during REM independently explained the variance in the emotional memory tradeoff effect and found that including vagal HRV significantly improved the model's fit. Overall, these results suggest that neural and vagal signals synergistically interact in the processing of emotional memories, with REM vagal HRV playing a specific role in contributing to the positive memory bias.