Somnambulism = sleepwalking. Somnas = sleep. Ambulism = walking.
Generally you get nightmares during REM sleep, and you're paralysed for safety. I look at them as the conscious mind sorting itself out.
However, in deep sleep, you are not paralysed. Sometimes in this period people have sleepwalking, sleeptalking, and night terrors. These are rooted firmly in the subconscious mind.
Night terrors differ from nighmares in that:
- There is no paralysis.
- They stem directly from your subconscious.
- People experiencing night terrors can be seen, in bad cases, moving about with looks of sheer terror on their faces, perhaps screaming and such.
- People in them might sweat, have very high pulse rate, and basically be very panicked.
I've read that it's dangerous to wake people from night terrors, but I'm not sure why.