Well yeah, it’s just during the night in general. Hell I’m not even awake in the mornings, lol.

This puts physical pressure on the sacral nerve, causing the erection. In turn, the erection closes the sphincter to the bladder more tightly because getting urine in the vagina during sex would change the pH and possibly kill sperm you have deposited/will deposit, which makes not tightening that sphincter an evolutionarily disadvantageous trait. This does make it something of a self-reinforcing cycle, though.

Exactly what I was asking for, thank you.