REM sleep involves cycles of cardiovascular activation and silence, leading to increased heart rate variability.

“But REM sleep also seems to have this strange function where you go through these cycles where you get a massive activation of the cardiovascular system, and then it falls silent, and then a massive escalation again. And you get this really remarkable increase in heart rate variability. And heart rate variability has been used as a metric of health outcome, that if you have high current rate variability, it's very good. Predicts lots of health outcomes. That's what you get during REM sleep.”