Yup, I've experienced this plenty of times. The worst part is that you can't consciously recognize the anxiety or fearful thoughts at the beginning ... you are at the behest of your unconscious mind. I never wake up until it has already spiraled out of control and there's no rational thought left even when I'm awake at that point.

Ehh, they come and go for me. :/ I mean, I have had them for months at a time, stopped sleeping and was miserable ... then they go away for awhile. Wish I had better advice. I'm just, giving my experience I guess.