A mattress that's too firm is generally the lesser problem — an overly firm surface causes pressure discomfort, but an overly soft one lets your hips sink below your waist, which is the mechanical source of most lower back pain from a bad mattress.
A mattress that's too soft loses its ability to keep your spine in a neutral position, and that misalignment accumulates over hours of sleep. A mattress that's too firm creates localized pressure at the hips and shoulders but still holds the spine roughly level. For back and combination sleepers especially, a measured firmness of around 6.5–7/10 is where support outweighs pressure — softer than that, and hip sinkage tends to become the dominant complaint.
- Too-soft mattresses allow hip sinkage past the waist, disrupting spinal alignment across the full sleep surface.
- A medium-firm mattress measured at 6.8/10 firmness keeps hips level for back sleepers without excessive shoulder pressure.
- Center compression under a 44-pound load: 1.55 inches on a medium-firm hybrid, versus deeper sinkage on soft-rated models.
- Side sleepers are the exception — too-firm surfaces cause shoulder pressure, making a 6.0–6.5/10 range more appropriate for that position.