A dip on your side of the bed is the single most common reason people start shopping. It is also the symptom most often misdiagnosed, because the mattress gets blamed for something happening underneath it. Work through these four causes in order — they run cheapest to most expensive to fix.
CAUSE 1 — THE BASE UNDER THE BED. Check this first, every time. Get underneath with a torch and look for a cracked or bowed slat, a slat that has slipped out of its bracket, a missing centre support leg, or a box spring that has gone soft in the middle. On a queen or larger, a centre support running to the floor is not optional; without one the middle of the frame flexes and the mattress follows it. Published slat spacing limits are tighter than most frames deliver: Purple requires no more than 3.5" between slats, Tempur-Pedic requires cross slats at least 3" wide and no more than 4" apart, and Helix voids coverage above 5.5". A fix here can cost the price of one piece of timber.
CAUSE 2 — A COMFORT-LAYER IMPRESSION. If the base is sound, strip the mattress bare and leave it a few hours. An impression that largely recovers is the comfort layer conforming to you rather than the mattress failing. Rotating head to foot on schedule spreads the load and is the only genuinely effective preventive step. Note that most modern foam and hybrid mattresses are one-sided and must never be flipped.
CAUSE 3 — SUPPORT CORE FAILURE. If the base is sound and the hollow is still measurable after hours uncovered, the support core is giving way. Measure it with a straight edge and no weight on the mattress. This is the failure mode warranties are written about, and it is not repairable at home.
CAUSE 4 — WHERE YOU SIT, NOT WHERE YOU SLEEP. A dip along one edge, near where you sit to put shoes on, is edge-support breakdown rather than sleep-surface failure. It is caused by concentrated load on a small area, and no amount of rotating brings it back. It rarely affects sleep, but it makes getting in and out feel unstable.
IF BOTH SIDES DIP AND THE MIDDLE STAYS HIGH. Two hollows with a ridge between them is a classic two-sleeper wear pattern on a mattress that has never been rotated, and it is uncomfortable out of proportion to its depth because you both roll away from a hump. Rotating from new prevents it; once the ridge is established it does not reverse.
THE ORDER TO SPEND MONEY IN. Fix the base first — it is cheap, and every warranty excludes sag caused by an unsupportive foundation, so a failed base can also disqualify a genuine claim. Then measure. Then check the warranty threshold. Only after all three does a topper or a replacement make sense.