Your limbs do not fall asleep due to lack of circulation, as is commonly believed. Usually it is due to the nerves being compressed and unable to send return signals to your brain. This is what can make you feel numb, and why your limb feels tingly or painful when you try to "wake it up" after it has "fallen asleep."
As a general rule your leg goes to sleep due to poor circulation caused by a lack of movement. There are other medical reasons of course, but in a normal healthy person that is reason.