Fix your mattress · Updated September 2026
Mattress Sagging vs Body Impressions: What's the Difference?
Every mattress develops shallow body impressions where you sleep — foams compress where load is repeated, and that is normal wear, not a defect. A sag is different: a deeper dip that does not recover, that you can feel while lying flat, and that is no longer supporting you. The distinction matters because it decides whether you rotate and carry on, or start a warranty claim.
What a normal body impression is
A shallow, gradual settling where your body spends the night — most visible under the hips. Impressions appear in foam, hybrid and pillow-top mattresses alike, and they are the single most common reason people believe a mattress has failed when it has not.
The practical test is feel, not appearance. If the mattress still supports you evenly and you are not rolling into a hollow, an impression is cosmetic.
What a genuine sag is
A sag is a deeper depression that does not recover when you get up, that you can feel as a slope or hammock while lying flat, and that pulls you towards it during the night. Sags change how the mattress supports your spine, which is what separates them from cosmetic impressions.
Tell-tale signs while sleeping: rolling towards the middle or towards your usual spot, morning stiffness that was not there when the mattress was new, and a visible dip along the surface when the bed is stripped.
How to measure it the way warranty departments do
Strip the bed completely — no topper, no protector, no sheets. Lay a straight edge (a broom handle works) across the mattress so it spans the dip, and measure from the straight edge down to the mattress surface at the deepest point.
Measure with nobody on the bed. Warranties define sag as a depression measured unloaded, and most set a threshold depth in their terms. Check your own warranty for the exact figure rather than relying on a general rule.
- Strip the bed fully before measuring.
- Span the dip with a rigid straight edge.
- Measure to the deepest point, unloaded.
- Photograph the measurement with the straight edge in place for a warranty claim.
Why the base matters to the verdict
A mattress on sagging slats or a worn box spring can show a dip that is entirely the base. Before claiming a sag, put the mattress on the floor and measure again. If the dip disappears, the base was the problem — and most warranties require a compliant foundation anyway.
What to do next
Normal impression: rotate the mattress 180 degrees, carry on, and rotate regularly from now on. Genuine sag under warranty depth: contact the manufacturer with your photos and measurement. Sag below the warranty threshold but uncomfortable: a topper can mask a shallow dip temporarily, but it is a bridge to replacement, not a repair.