Man, all men know erections just happen with you sleep. It's incredibly inconvenient when the demons need me to answer the door in the morning and it's just raging down there.
It erects so you can keep on erecting, otherwise it stops working properly and eventually shrinks down; sleep is basically maintenance mode for a lot of things, erections are a subroutine of that.
Tbh there's rarely a dream I'm not self-aware in anymore so I try and lucid dream the demon into something else. It doesn't work so this comic is on point.