sleep in whatever position you are most comfortable. Lying on your left side is the best position, BUT not if it causes you pain! I have bad hips and esp. now with all the relaxin in there making my joints looser, I ALWAYS have pain in them, esp. after sleeping on them. so I constantly am turning from side to side to even it up. and I do even sleep on my back sometimes, because it is what feels comfortable. They advice against lying on your back because the weight of your uterus can compress the major vein that takes blood back from your legs to your heart. this can cause your blood pressure to drop and in extreme cases to feel faint and pass out. if you do feel funny lying on your back, dont do it, or just prop yourself up on an extra pillow, or just stick a small pillow/cusion under one of your hips so your not 100% flat.

hope you have a good night sleep =)