Usually your feet and legs sometime fall asleep or go numb due to blood flow being cut off momentarily to the feet or legs and sometimes a pinched nerve in that area is a cause of feet and legs falling asleep.
To keep your feet and legs from falling asleep you should avoid sitting in a position for too long and get up and walk around as much as you can and stretch your legs and move your feet around.
Sometimes kidney disease, liver disease or chronic inflammation, or hormonal imbalances can cause you to lose feeling in your feet.
So if the feet falling asleep happens often a doctors visit would be a good idea to get checked out for any conditions that you could potentially have.