On the side
Sleeping on the side is way better than being curled-up, and it also extends an added bonus should you be lying down on the left in particular – it aids digestion thanks to the position in which the oesophagus passes into the stomach. Placing a pillow in between the legs can help the spine stay properly aligned.
On the stomach
Although this may be the only sleeping position some people are accustomed to, it is actually detrimental for sleep and health. It puts pressure on the whole body while straining the muscles, neck and jaw, aside from twisting the pelvis and lower back. It also restricts proper breathing and leaves you with numbness or tingling in the morning.