How come sometimes when you're asleep, one of your legs "jumps"?!

How come sometimes when you're asleep, one of your legs "jumps"?

I'm not sure how to explain this, or if anyone will understand what I'm talking about...but when I sleep sitting in a chair or in my bed, even though my eyes are closed and I'm not just dozing off--like really asleep, my legs "jump" and then it wakes me. Why does that happen? Is it kind of like, when you dream that you are falling, your heart seems like it's in the sky and you get knots in your stomach which makes you wake up?

2 weeks ago
well, er, not "jumps"...more like moves a bit, i think--not quite sure how to explain it.

the stage of sleep is called the hypnogogic stage. this is where people sleep walk etc. the body is not quite into a deep sleep and thus can move about. it sucks when it wakes u, just try to ignore it. funny story-=the other day i was having a dream, and tried to slap someone,when i awoke to a noise and my husband said i just slapped him! lol try and explain that one-i had a good laugh but i also felt bad;)michele