If you sleep on your side, you might need a softer mattress that allows your shoulders and hips to sink into the mattress which helps keep your spine aligned. Mattress choice also depends on your weight. A lighter-weight side sleeper might need a more-soft mattress, while a heavier side sleeper would likely do better with a less-soft mattress.
Best mattresses for side sleepers: Latex mattresses, memory foam mattresses.
If you sleep on your front, you might need a firmer mattress. Firmer mattresses prevent your hips from sinking in too deep, which can create pressure on your lower back. This is generally true regardless of the weight of the sleeper.
Best mattresses for front sleepers: Pocket sprung mattresses.
If you sleep on your back, you might need a medium mattress. Medium mattresses push into your lower back, eliminating the gaps between your back and the mattress which can cause discomfort. Mattress choice also depends on the weight of the sleeper. A heavier back sleeper might find a medium-firm mattress more comfortable, while a lighter-weight back sleeper might prefer a medium-soft mattress.
Best mattresses for back sleepers: Combination pocket spring and memory foam mattresses, pillow top mattresses.