[user104658]

Originally Posted by Will.

But it's not, my leg goes numb! There is no pin and needles!

Yeah but when the feeling comes back you get pins and needles... Surely?

But yeah anyway it's the weight combined with sitting in a bad position, pitting pressure on a nerve. Not dangerous in itself but, like I said, unrelated to the numbness, if you don't sit properly you'll end up with hip / knee problems in later life .