No sleeping position is inherently bad for your body, if you have a pillow and mattress that can support the sleeping position. Back sleepers want a medium firm mattress that supports their bodyweight without sinking, and a pillow that is soft enough that the head and neck are cradled by the pillow. Side sleepers want a softer mattress, so pressure is taken off their shoulder, and a thicker pillow that keeps your neck in a good alignment with your back. Stomach sleepers want a firm mattress, and a low but supportive pillow so your neck isn’t bent upward while you are sleeping.
-- Dr Christopher Ernst, D.C.