If you strictly sleep on your side, a soft mattress will cradle your curves and give under your hips and shoulders to promote a neutral spinal alignment. However, when you're sleeping on your back or stomach, a soft mattress allows your spine to sag into the bed. Beds that offer more support have proven to be more beneficial for people with back pain. So, if you want a bed that offers plushness and support, go for a medium firmness level.