Sagging mattress: How to spot the hollow?

You wake up in the morning with a dull ache in your lower back, even though you've slept eight hours. When you turn over at night, you have to noticeably work your way out of a hollow. And when you pull back the sheet, you can clearly see two shallow indentations in the sleeping surface against the light. These are the classic signs that a mattress is sagging. The problem: The process is so slow over the years that it's hardly noticeable – the body gets used to the bad position before the mind realizes it. That's why it's worth consciously measuring once a year. In this guide, you'll learn how to objectively assess a body impression, where the line is between a normal indentation and a real material defect, what role the warranty plays, and how to significantly delay the formation of indentations in your next mattress.

The short answer: A mattress is considered sagged if the sleeping surface permanently retains an indentation when unloaded. This can be measured with the string method: A tightly stretched string across the mattress shows the distance to the deepest point. Up to about 1.5 cm, a slight indentation is normal and functionally unproblematic. From about 2 cm, the support effect noticeably decreases, and from 3 cm, the mattress is usually no longer usable and often a warranty case. Important: Only measure after the mattress has been unloaded for at least four hours and the cover has been aired.

How a body impression develops

Every mattress is subjected to the entire body weight for six to nine hours every night – that's over 2500 hours a year, distributed over about 40 to 60 position changes per night. The greatest load is not evenly distributed but concentrated on the shoulder and pelvic areas, where 60 to 70 percent of most people's body weight rests. Foam cells are compressed and relieved millions of times there. Over time, the cell walls lose some of their resilience – experts call this permanent deformation. Innerspring mattresses' steels fatigue, and latex's webs lose tension. Two factors significantly accelerate this: moisture and a defective base. Moisture from nightly sweating softens foam structures if it cannot dry out. And a slatted frame with broken or dislodged spring slats transfers the load pointwise instead of evenly, leading to premature fatigue at these specific spots. An important point for context: A slight, visible body impression after a year is not a defect but the normal settling behavior of the material.

The string method: how to measure correctly

Subjective impressions are particularly misleading on this topic. Therefore, use a measurement method that manufacturers and experts also use.

Step by step

Remove all coverings – sheets, protectors, toppers – and leave the mattress unloaded for at least four hours, preferably a whole day. Then stretch a strong string tightly across the mattress, exactly where the indentation appears deepest, usually at hip or shoulder height. The string must rest on both side edges and must not sag; this is easiest with two people. Hold a ruler vertically at the deepest point and read the distance between the bottom edge of the string and the mattress surface. Repeat the measurement at two to three points and note the largest value.

Photograph the measurement with the ruler visible and the date – this is the best basis for a complaint.

Other signs that your mattress is sagging

In addition to the measured indentation, there are everyday signs that often appear earlier. You wake up with lower back or neck pain that subsides after 20 to 30 minutes of movement – a typical pattern for a lying-related cause. You sleep significantly better in a hotel or at a friend's house. You involuntarily roll back into the same hollow at night. As a couple, you roll together towards the middle of the bed because a ridge has formed between the two sleeping zones – the so-called "visitor's crack" from the inside. The mattress creaks or you can feel springs. And a particularly reliable sign: You change position more often at night than before because no area remains comfortable for long. A simple self-test: Lie on your back and slide a flat hand under your lumbar spine. If it can move freely back and forth, there is a hollow space – the mattress no longer supports the lumbar lordosis. If your hand doesn't fit at all, you're sinking in too deeply. Further connections between the sleeping surface and discomfort can be found under Back pain in the morning.

Body impression or defect? The difference

This distinction determines whether a complaint has a chance of success. A body impression is a shallow, wide indentation that visibly recedes after several hours without load. It is part of the normal settling behavior of every mattress and is most pronounced in foam materials during the first few months. A permanent indentation, on the other hand, remains measurably unchanged even after a whole day without load. It often has sharp edges or a palpable ridge and is accompanied by a noticeable difference in firmness between the center of the indentation and the edge area. A third case is often confused: the cover effect. Some quilted covers appear wavy after prolonged use, even though the core is still intact. In this case, remove the cover and measure directly on the core. And finally: If only one half of a double mattress has an indentation and the people are of different weights, this is normal and not a material defect – it is an argument for two separate mattresses, as Mattress for couples explains.

Warranty and claims: what applies

In Switzerland, a statutory warranty of two years applies to sales contracts, covering original defects. Indentation after years of normal use generally does not fall under this – for that, the voluntary manufacturer's warranty is responsible. Many providers offer a 5 to 10-year warranty and define a concrete limit: often 2 cm, sometimes 2.5 or 3 cm of permanent indentation depth, measured in the unloaded state. Read the conditions carefully: The warranty is almost always tied to proper use, i.e., a suitable, intact slatted frame, adherence to the load limit, and that the mattress has not been permanently bent or stored in a damp condition. For a claim, you need: proof of purchase, photos of the measurement with a ruler, a description of the foundation, and ideally a photo of the slatted frame. Report the case in writing and set a deadline. The article Lifespan of a mattress explains what warranty promises mean in detail.

Common mistakes that accelerate indentation formation

Never rotate the mattress. If you always use the same side and the same orientation, you stress exactly the same zones – rotating it every three months distributes the load noticeably better. Ignoring the slatted frame. Broken spring slats are the most common unrecognized cause. Check each slat individually once a year for cracks and firm seating in the caps. Making the bed immediately in the morning. Moisture then remains in the core and softens the structure. Fold back the duvet and air for 20 to 30 minutes. Sitting on the edge of the bed. The edge area is the least resilient; daily sitting to get dressed permanently compresses it. Buying too small or too soft. A mattress that constantly operates above its load limit will fatigue faster. Covering an indentation with a thick topper. This temporarily mitigates the feeling but does not correct the lack of support underneath – the topper simply sinks in. How often a replacement is realistically due can be found under When to change a mattress: how often?.

Frequently asked questions about a sagged mattress

At what indentation depth is a mattress considered sagged?

Up to about 1.5 centimeters, it's considered normal settling behavior. Between 2 and 3 centimeters, the support effect is significantly reduced, and from 3 centimeters of permanent depth, a mattress is considered sagged and should be replaced. Many manufacturer warranties set the limit at 2 to 3 centimeters.

How do I measure the indentation in my mattress?

Remove all coverings and leave the mattress unloaded for at least four hours. Then stretch a string tightly across the sleeping surface so that it rests on both side edges, and use a ruler to measure the largest distance between the string and the mattress surface.

Can I save a sagged mattress with a topper?

Only to a limited extent. A topper improves the surface feel but cannot replace missing support – it will simply sink into the hollow. For an indentation up to about 1.5 centimeters, a topper can be a useful temporary solution, but for deeper hollows, replacement is the only reliable solution.

Why is my mattress sagged after only three years?

The most common causes are a defective or unsuitable slatted frame, permanent moisture due to lack of ventilation, exceeded load limits, or foam with low density. First, check the foundation – broken spring slats are the most frequently overlooked reason.

Does a sagged mattress cause back pain?

It can contribute to it because the spine is no longer properly supported in the hollow, and muscles have to work against it at night. Typical symptoms include discomfort immediately after waking up, which improves after 20 to 30 minutes of movement. If pain persists longer or radiates, have it checked by a doctor.

How can I best prevent indentation formation?

Rotate and flip your mattress every three months, check your slatted frame annually for broken slats, air your bed open for 20 to 30 minutes in the morning, and avoid daily sitting on the edge of the bed. These four habits significantly extend the usable lifespan.

This article is for general information only and does not replace medical advice.