Is it better to have a softer or firmer mattress?

Firmness is best matched to sleep position, not personal preference alone. Side sleepers generally need a softer mattress (around a 3–5 out of 10 on the firmness scale) to cushion hips and shoulders, while back and stomach sleepers typically perform better on a firmer surface (6–8 out of 10) that keeps the spine neutral.

A softer mattress lets the shoulders and hips sink in enough to keep the spine aligned for side sleepers, but that same give causes the lower back to bow on stomach sleepers, which strains the lumbar. Body weight also shifts the math: heavier sleepers (230 lbs and up) need a firmer mattress at any position to avoid bottoming out, while lighter sleepers (130 lbs and under) often find a medium-firm mattress feels harder than the label suggests.

Side sleepers: recommended mattress firmness is 3–5 out of 10 to relieve shoulder and hip pressure points.
Back and stomach sleepers: recommended mattress firmness is 6–8 out of 10 to maintain lumbar support.
Sleepers over 230 lbs typically need one to two firmness levels above the standard recommendation for their position.
Combination sleepers who shift positions most often match best with a medium firmness (5–6 out of 10).
Mattress firmness and mattress support are distinct: a soft comfort layer can sit atop a firm support core, addressing both pressure relief and spinal alignment.