My daughter (5yrs old) does this on occassion...mostly when she's overly tired. I believe they're night terrors. She'll just act like she's very unsettled and uncomfortable. You know, whimpering, tossing/turning, kicking, etc. I try to leave her alone b/c that's what I've been told to do based on advice from friends, books, doctors, etc. I've been told it's not good to wake someone having a night terror...however...if it's bad enough, I'll actually pick her up and carry her around for a minute or two. That seems to help b/c it distracts her and gets her out of it.
Her terrors have gotten much better with age. They've never actually been really really bad, but, there have been times where I had to interfere. Around age 4yrs, they were at their peak with terrors around 4 nights a week. Now that she's older, they've gotten better unless, like I stated earlier, she's overly tired.