When Is a Mattress Finished: A Sag-Growth Model and a Measurable Replacement Rule Based on the Depth That Matters to the Spine

Mattress replacement advice is given in years — eight, ten, 'when it feels wrong' — but years are not what the spine experiences; sag is. This paper models unloaded centre sag as a saturating creep curve with stated parameters for foam and innerspring construction, and converts sag into an estimated spinal misalignment through the hip-to-shoulder differential. Foam reaches the stated 25 mm sag criterion at 6.6 years and the 2.5 degree misalignment criterion at 18.4 years; innerspring reaches the sag criterion at 16.4 years and does not reach the alignment criterion within the thirty-year horizon modelled, standing at 1.21 degrees at year ten. At ten years the two constructions carry 29.8 mm and 19.3 mm of sag respectively. Because the governing quantity is a depth any household can measure with a straight edge and a ruler, the paper replaces the calendar rule with a measurement rule, and shows that the two rules disagree in both directions — some mattresses are finished years before their calendar date, and some are sound years after it. This study is simulated, not measured.