Fix your mattress · Updated September 2026
Why Is My New Mattress Hurting My Back?
A new mattress causing discomfort is common enough that every trial period exists because of it. There are three usual causes, and they have different fixes: the mattress is still breaking in, the firmness is wrong for how you sleep, or something under the mattress is undermining it. Working out which one you have is the whole job.
Cause one: break-in and body adjustment
New foams are stiffer than used foams, and your body is also accustomed to the shape of your old mattress — including its sags. Sleeping on a flat, supportive surface after years on a worn one can feel strange or even sore for the first couple of weeks while your posture adjusts.
Give it three to four weeks of consistent sleep before judging. Discomfort that is gradually fading is adjustment; discomfort that is stable or worsening is a mismatch.
Cause two: the firmness is wrong for your position
Side sleepers need enough give for the shoulder and hip to sink; a mattress that is too firm pushes back on those points and twists the spine. Back and stomach sleepers need the hips held up; a mattress that is too soft lets the hips drop and arches the lower back. Either mismatch can read as back discomfort.
A useful morning test: soreness concentrated at the shoulder or hip points towards too firm; a dull lower-back ache that eases once you are up and moving points towards too soft.
- Side sleeper, sore shoulder/hip → likely too firm.
- Back/stomach sleeper, lower-back ache → likely too soft.
- Combination sleeper → err towards the middle of the firmness scale.
Cause three: the foundation changed
A new mattress on an old base is a common setup mistake. Worn slats, a tired box spring or wide slat gaps change the feel of the mattress above them, usually making it feel softer and less supportive than intended.
Check the maker's foundation requirements — many warranties are conditional on them — and test the mattress on the floor for a few nights if you suspect the base.
What to do about each
Break-in: wait, and rotate the mattress once to even out early compression. Wrong firmness: if the mattress is too firm, a topper is a genuine fix; if it is too soft, a firm topper or a sturdier base helps. Foundation: fix or replace the base first — it is the cheapest variable.
Use the trial period like a deadline, not a safety net. If you are at week five and still uncomfortable, start the exchange or return process rather than hoping week six is different.
When it is not the mattress
A mattress can be too soft, too firm or worn out, and fixing that is worth doing. But pain that persists for weeks, pain that wakes you at night, or pain with numbness or tingling is not a furniture problem — that is worth raising with a clinician.