I don't know if they are nightmare or not, but our Sadie will do the same thing. Awake, she is our quitest dog, barely barks. But she falls alseep and strats growling andbarking. it is sorta funny, but after a minute or two, if she has not stopped, we do pet her to calm her down. I think dogs dream like we do, and it that it is normal. If touching her or gtnley shaking her, doesn't not stop the twitching, that is what could signal a problem.