I haven't read all of the replies but so many taken Clonidine for nightmares. It's a BP meds which is why it works. Lowering your BP helps you stay asleep. I have it in my internal pain pump for neuopathy and right away saw a HUGE difference in nightmares. I was WAY less active (I was hurting myself in my sleep, waking up running, things like that and that fully stopped) and though I still had nightmares, the severity seemed to lessen, the amount lessened, and the times waking up from nightmares really lessened.
For about 3 weeks or so my stress level has been through the roof and stayed there and so though the same amount of clonidine was being delivered to my body, nightmares became present again. I started waking up several times a night, every night. Became active again. So it isn't full proof but it is the one med I see raved a lot on here for nightmares, being called "the nightmare med", and people swear by it.
I did see Seriquel mentioned. If you go on it specifically for nightmares/insominia/etc night time stuff, go for the immediate release. I take Seriquel XR (extended release) for daytime anxiety and emotional deregulation and it does jack for nightmares in my case. Not saying it can't. Just saying if that is the reason you are wanting it, got for immediate.release and give.your body time to adjust.
What is good about Clonidine is I didn't have to miss work or anything. Nothing for my body to get used to. I was very worried about my BP as it normally hangs low (110/60 I think but don't remember exactly). We checked my BP 3 times a day for a week and the top number never went lower then 90. On the low end but ok. I have never felt dizzy or any other side effects. Personally I want as little side effects as possible.