Asian believe ghost/spirit/etc they pins you down when U can't move like in lucid dream.
Azazyel is right that it is hard to control in deep sleep. And I have first hand experience in it. There were a couple of times that I accually hear what is going on around me but I know I was sleep. It got a little freaky when I tried to wake up but couldn't. I tried to scream and stuff but nothing. This happen when I fall asleep on the living room leather sofa. First I wasn't sure, but the second time the same happen so no more sleeping on sofa for me.
Another dream/nightmare I had when I was a kid. It happen the same thing over and over often enough that I remember it even when I am dreaming it. I was scared at the chinese pottery that has human figure and this white face with two pink dimple on it's cheek. I know it's not suppose to be scarry, but I was scared of it. Especially when they moves. But since they have no legs, they can't climb up stairs. So I would hide in the artic. Anyway, when I starts to remember my dream.. I know I was dreaming. So I went down stair, scared but I did it because I know I was dreaming and I kicked the sh*t out of those pots with white smiley face. It's funny to see them hopping as they try to run from me. But in the dream, I know that they has killed everyone else.
Another dream I had is a really dream come true. I always love to fly even thought I afraid of height. But this time, I know I was dreaming.. so I willed myself to fly. Wow, it was amazing. I flew over houses. I saw my feet flew over the tree's most highest tip. Once I even flew over a canion with the birds. I felt the wind in my hair. But then it become harder and harder to fly because I was falling deeper into sleep.
One of those flying dream, I certainly realize that I was in the water. But I didn't need any oxigent or anything. Like I was walking at the bottom of the clearest ocean. I jumped off a cliff so high that I got tired of falling so I swam. Then I saw the water thirl like an under water tonato and I swam into it.. wow the spin was awsome. And in this dream I also swam long an under water canion.
The awsome thing about these dreams are they felt very real and I can not get hurt. U might think I had the discovery channel turn on or something, but I was asleep in my room with everything off. No sound no nothing but silence.