Answers, built from engineering.

When you lie down, your weight isn't spread evenly. The torso and pelvis—the heaviest part of you—rest on the center third, putting roughly 60–70% of your body weight on that one zone, while the shoulders and legs press far less on the ends.

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.

A mattress feels uneven when one area has lost support while the rest hasn't—usually a softer, lower center against firmer edges. Sometimes the unevenness is the foundation underneath, not the mattress itself.

First-night support and long-term support are different things. A new mattress feels supportive before any material has fatigued; over time the center loses springback and the support fades, even though the showroom feel was real.

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.

Hybrids sag for the same reason other mattresses do: the center coils fatigue under concentrated load. The foam layout doesn't prevent it, and a high coil count won't save it—the center has to be built stronger than the rest.

Waking with lower back pain has several causes, but if it's worst on waking and eases as you move, the sleep surface is a prime suspect. A sagged center lets the hips sink and bends the spine out of neutral all night.

The middle feels softer because that's where you sleep—and where support has worked hardest. The center third fatigues years before the edges you rarely lie on, which still feel firm because nothing has worn them down.

Yes—a sagging mattress can cause or worsen back pain. When the center loses support, the hips sink below the spine and the back bends out of alignment all night. The tell is pain worst on waking that eases during the day.

"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.