Autonomic storms during REM sleep can lead to physiological activation including erections in men and vaginal lubrication in women.

“During REM sleep, what we later discovered is that you go through these bizarre what we call autonomic storms, which sounds dramatic, but it actually is when you measure them. That you'll go through periods where your heart rate, decelerates and drops and your blood pressure goes down and then utterly randomly, your heart rate accelerates dramatically, and what we call the fight or flight branch of the autonomic nervous system or the sympathetic nervous system, badly named 'cause it's anything but sympathetic, it's very aggravating, that all of a sudden fires up and then it shuts down again. And it's not in any regular way. And it's when you get those autonomic storms, you get very activated from a physiologic perspective that you can have these erections and you have vaginal discharge et cetera.”