There it goes again!

I take my sit and a couple minutes into the lecture, I notice something. Something that I do regularly and chances are you do it too. I start looking for students doing it. I see one, then another, then another and right there another one! Thats when I made a note to self to get to the bottom of this. "I gotta google this!" I said.
"Why do people jiggle or shake their feet/leg?" As of now I have yet to get any scientific reason(s) but lots of people on the net have put forward their own personal reason(s) why they "think" they "shake their leg!"

Please help me settle this on going ignorance about why we shake our legs by posting your answer in the comment section below. The more scientific your answer the better.