According to a physiotherapist I saw over the summer, it's best to sleep on your side, second best, your back. Stomach bad.
He told me the best is to sleep on your side with your hips and knees flexed, so you form a sort of step shape.
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The key to not messing yourself up is to keep your spine straight while you sleep. That means that if you lie on your side you should make sure your head doesn't tilt more toward your down shoulder than your up shoulder, and don't pull it forward like you are curling up in a ball (fetal position). It's hard to get used to keeping your posture straight (even in bed), if you don't do it already it feels pretty weird but you get used to it.
It will improve your sleep drastically though.