These two words get used interchangeably in reviews and complaints, but manufacturers treat them as completely different events, and so should you. A body impression is the top comfort layer moulding to the shape you sleep in. It appears in the first weeks, it is shallow, and crucially it recovers: strip the bed, leave it a few hours, and the surface comes back up. Sagging is the support core underneath losing its ability to hold weight. It deepens over months, it does not recover when you get out of bed, and it changes your spinal position rather than just the surface feel.
THE TEST THAT SEPARATES THEM. Strip the mattress bare and leave it uncovered for a few hours with nothing on it. Then lay a straight edge — a broom handle, a length of timber, a taut string pulled tight between two hands — across the width of the mattress over the deepest part of the hollow. Measure straight down from the straight edge to the mattress surface. This is exactly how brands measure it, and every one of them measures with no weight on the bed. If the hollow has largely disappeared after a few hours uncovered, you are looking at a body impression. If the gap is still there and measurable, it is sag.
HOW DEEP IS TOO DEEP. There is no universal number, because each brand sets its own. Tempur-Pedic covers a visible indentation greater than 0.75". Purple and Casper set the line at 1". Nectar, DreamCloud, Helix, Brooklyn Bedding and Avocado all require more than 1.5" before they call it a defect. Those figures come from each brand's own warranty document. A 1" hollow is a valid claim on one bed and explicitly excluded on another, which is why the measurement matters more than how bad it feels.
WHY THE FEELING MISLEADS YOU. A shallow impression in a soft comfort layer can feel dramatic — you sink into a channel and have to climb out of it to roll over — while a genuine 1.5" sag in a firm mattress can feel like nothing more than a slightly awkward morning back. Feel tells you whether you are comfortable. Only the straight edge tells you whether the mattress has failed.
THE THIRD POSSIBILITY EVERYONE MISSES. If the hollow follows a line rather than a body shape — a long trough across the middle, or a step you can feel with your hand — the mattress may be fine and the base may be failing. A broken slat, a missing centre support leg or a sagging box spring produces a dip that looks identical from on top. Always look underneath before you conclude the mattress is finished.
WHAT TO DO WITH THE ANSWER. A body impression within the normal range is not repairable and does not need repairing; it is the mattress doing its job. A measurable sag inside the warranty threshold is a claim, and the claim is worth more than a topper. A measurable sag below the threshold on a mattress several years old is usually the beginning of the end, and the decision becomes whether to buy time with a topper or replace.