When your foot falls asleep it's usually because of a loss of blood flow to the foot which can happen when sitting down and not moving often enough or even when you're sleeping you can pinch the blood vessel or compress the blood vessel or nerve enough to cut off blood supply to the foot.
That can then lead to your foot falling asleep and causing a tingling feeling or numbness feeling in your feet.
If your foot does get numb though and you experience pain along with the food numbness then you might have diabetes.
Being diabetic can cause the diabetics persons feet go numb as well and loose feeling often in their foot so I would get checked out just to be sure.
If the foot falls asleep too often and it just suddenly started happening then I would get tested for diabetes and at least tell a doctor about it to make sure nothing is wrong.