Sometimes it happens to me, and then the body part falls asleep. The best thing to do is to exercise. I know that you might hear that a thousand times, but walking around the house or at work is not what i mean. You should walk/run with incline, so like on hills... It keeps everything flowing, and builds up your butt.
Exersise. Get up, and just move around to get the blood flowing!
This sounds strange but i think you should first see if theres any string or bandage tightly around, but if its happening with your hands as well then. umm if its going purple go to the hospital immediately. but maybe just put up with it till morning and then go see a doctor.