I got hit with this a lot while going to OSU. Attending university and trying to keep up as an honors student while dealing with PTSD, bipolar disorder, and a few other tidbits does strange things to the mind. When summer came along, I dealt with it by going on a camping trip where I let my body decide my schedule: if I felt sleepy, I slept; if not, I didn't, and I paid no attention to the clock, so I could be found canoeing or swimming at 3 a.m.... right after a 2 a.m. breakfast or lunch. During terms, sleep deprivation was usually effective (and a great way to get class projects done!). But most potent for an attack was when I learned to step in and take control of my dreams.
Only a few times was I truly terrified, once I knew what was going on. One was when I couldn't breathe -- I was aware enough of my body to know I actually wasn't breathing, not merely dreaming that I couldn't breathe. It was terrifying enough I was fighting to keep from peeing my bed -- but then it occurred to my "dreamwalker" mind that if I did, that might make me jump up, which might get me breathing. Short version: it worked. Another was when I was laying there frozen and getting cold, and sensed that around me the sleeping porch was getting cold, and the wood was starting to shrivel and crack. I have a vague memory of ordering a demon to get lost in the name of Jesus, then yelling -- and I really did yell; it woke up just about the whole house (I had thirty guys or so grumbling at me for days). The worst was on a camping trip, when I woke up (or dreamed I'd woken up) and could see out my tent window that the hills that had been there were gone... then I felt myself sliding, and understood that I and tent were on a vertical surface, and I was slipping, and would fall... my choice of dreamwalker image was very fortunate: I'd determined that in any dream, if I could get naked, I'd have power over the dream; since I was sleeping naked, I already had power. I commanded the cliff to be a hill again. I yelled as in Tae Kwon Do when delivering a punch as I gave that command, and felt the surface whirl and slam back to horizontal, which scared the crap out of me so I kept rolling, in dream and real now, out of my sleeping bag and out of my tent to stand "sky clad" in the moonlight. I learned in the morning -- about an hour and a half later -- that I really had yelled, too; other campers described it as a defiant and commanding yell they heard. With the hills the way they were, it echoed, too.
Then there was the one where I was encased in carbonite, a la Han Solo in Star Wars. That didn't stay terrifying very long, because it was just too funny. I think that was the only time I ever woke up giggling.