Why has my mattress developed a body-shaped dip?

The comfort foam has taken a permanent set — its cell structure has collapsed under repeated load and will not recover. Low-density foam does this within a couple of years. Rotating slows it down; nothing reverses it.

Why it happens

Polyurethane foam is an open-cell structure of thin plastic struts. Under repeated compression those struts buckle and eventually stay buckled. The foam still looks intact but no longer springs back to its original height, and the dip follows the shape of whoever sleeps there.

Density predicts this better than any other specification, and it is the number most often left off the listing. Firmness describes how a mattress feels on day one; density describes whether it will still feel that way in year five.

A missing transition layer makes it worse. That medium-density layer between the soft top and the hard core stops you sinking through onto the support layer. Cheap mattresses omit it, which is why they feel supportive briefly and then bottom out.

What to do about it

1. 1Rotate head-to-foot every few months. If the mattress is designed to be flipped, flip it — many modern ones are not.
2. 2A firm topper masks the symptom and can buy a year, but it sits on top of collapsed foam rather than restoring it.
3. 3When replacing, ask for foam density in kg/m³ specifically. If a retailer cannot supply it, that is itself informative.