Foot falling asleep. I think you are looking at this backwards. The foot falling asleep is a symptom of something else- whether a vitamin deficiency, a pressure on a nerve, improper sleep position- the foot falling asleep is not the cause of a permanent problem but the cause of the foot numbness should be checked out and hopefully resolved before it becomes permanent.
It depends. The permenant side effect is the possibility it falls asleep permenantly so you should have it checked out. A foot should not fall asleep that frequently. Either you are doing something to cause it...I.E sitting in a way you are causing a compression neuropathy or you have a nerve that is being compressed thru exertion or an anatomical compression.