Weakness when going to sleep?

EVery once and a while, when I am trying to go to sleep, it feels like my body becomes weak and I lose control of my arms and legs. It feels like I am on the brink of going to sleep, but the loss of total control doesn't let me. I'll lay there like an hour or so and kinda be asleep, but I just cant get fully asleep, till I get up and move around a little.MY arms and legs dont become numb, but it feels like I am not in complete control. When I get up and walk around a while, it gets better and eventually I can get back in bed and go to sleep. Can someone give me an idea of what this is and the causes?