Leg wants to move, yet i'm tired?

Anyone have any idea what this is? Anybody get it too? Just curious? Most of my friends don't have a clue what im on about.. Sometimes its when i'm just sat down or I'm in bed. My leg will just get this feeling that it independantly just needs/wants to move however my entire body, seemingly apart from my leg is tired.It wants to run or move around but im just too tired to move it. How can i be exhausted however my leg is full of energy and could run 10 miles all the sudden. You can't really just ignore it either. Like you can't really ignore an itch. It like a niggle and a not painfull ache just a kind of ache?? Hard to describe really? ...