On my side, usually the right side but I shift back and forth between right and left until I fall asleep. Usually I wake up lying on the right side though... it's just comfier for some reason. Don't sleep on my stomach at all; only time I've slept on my back is when I was recovery from an appendectomy a few years ago - sometimes it was too painful to lie on either side for long periods of time because of the stitches/surgical scars. Otherwise I usually find lying on my back too uncomfortable :B