It depends.

If I'm on my "cycle" or I have a stomach ache, I sleep on my stomach.

If my back hurts, I'm on my back.

Any other time I'm on my left side. If I sleep on my right side, I'm facing the wall and it feels weird. Plus my cat will come to me and chew on my hair if I sleep on my right side.

And when I'm feeling a little off, I'll sleep in my closet.