girlfriend, I totally hear you on the sleep deprivation and the feeling helpless.
We've had them since DD was about 18months (I know that's no help in the time frame thing but hopefully that someone else out there understands too). My DD is an extremely heavy sleeper, like once she's asleep you CANNOT wake her until she's ready. She kicks and screams and hits, but has no idea. Ours are from about 10.30 -2am usually....actually is is often the exact time I get into bed and shut my eyes lol.
I second what Sarah said about waking them just before (I learned a lot of coping from her and knowing what Z went through). When D was smaller I used to be able to wake her and it helped immensely.
Now sadly the only time she's really able to be woken is once she starts the terrors. It does lessen the drama over the night though if I am able to wake her before. Changes in her life seem to set them off. We have moved twice this year and she started preschool for the first time....this particularly bad few weeks...I really have no idea this time.

woops I meant it's better if I wake her before...not after the terror starts even though that's the only time she's easily woken....thanks Sally I read it back after reading your post.