For me, it's usually my adhd brain dredging up bad memories. I'm ashamed to admit my go to 'treatment' used to be taking a couple shots of whiskey before bed. That (I think?) caused vivid nightmares which made me avoid going to sleep even more.

Eventually, I realized this wasn't healthy and I switched to doing strenuous exercise. Hard to be wound up when you're exhausted! It's not a cure all. Sometimes I wind up doing so many sets of pushups a night that I feel it the next day, but it's definitely better than nothing.