Material fatigue

Both statements can be true at once. A warranty measures a visible dip on an empty bed; your body measures support under load — and support often fades before a visible dip appears. “Within tolerance” and “sagging” can describe the same mattress.

Good care helps at the margins and catches small problems early. What it can't do is reverse material fatigue in the center — that's a structural question, not a behavioral one. Rotation helps in narrower cases than most advice suggests.

Two things soften over time, and they're not the same. The comfort layer settles and loses surface firmness; the support core fatigues and loses its ability to hold your spine. The second matters more—and it shows up in the center first.

Durability comes down to whether the highest-load zone is built to last. The properties that matter are wire gauge, steel grade, foam density, and whether the center is reinforced. Coil count and price are poor predictors.

Most mattresses don't wear out evenly—one zone fails and takes the whole bed with it. The center third fatigues years before the rest, so the mattress feels worn out even though most of it isn't.

Body impressions form where you lie most. Some shallow settling is normal, but a deepening impression in the center is usually the support beneath failing—since body weight concentrates there and fatigues it first.

"Ten years" describes the warranty, not the structure. In practice, a uniformly built mattress begins losing center support at year three to four.

Sagging isn't material wear — it's structural failure. The center third carries most of body weight, and most mattresses are built uniformly edge to edge.