One of the more common complaints that people have about their mattresses is their tendency to sink in the middle. This problem is often referred to as sagging. A sinking or sagging mattress can be quite uncomfortable for a person trying to sleep, as she feels trapped in a valley in the centre of the mattress. It is even more pronounced as a problem for couples who tend to roll together toward the centre of the bed. Why you can't repair a mattress that sinks in the middle, you may be able to prevent or delay the problem.
Rotation or flip
If you do not rotate or flip your mattress regularly, it will naturally wear out when you sleep on the same spot over a long period of time. Although people tend to toss and turn a bit in bed, they are generally creatures of habit when it comes to sleep positions. A person who sleeps on his right side will generally end up sleeping on his right side for the majority of the night. Over time, the mattress will begin to form a body impression where the person usually sleeps. Rotating or flipping the mattress every so often will help delay this inevitable effect and prevent the sagging associated with it.
Improper centre support
If the bed frame does not adequately support the centre of your mattress, it may sink in the middle. This is especially true for larger beds such as double- or king-size mattresses. A centre support is generally built into the metal frame that attaches to either side of the rails. Platform beds do not typically have this problem.
Most mattresses sit atop a box spring support. Despite the name, box springs are not usually springs at all, but are stiff metal supports and wood that form a solid foundation for the mattress to lie on. If these supports are broken or damaged in the centre, the middle of the mattress may sink.
Over time, mattresses simply wear out. Depending on the quality of product you purchased and how much use it gets, a mattress can begin to show serious signs of wear in anywhere from one to 10 years. The materials used to pad the tops of mattresses can compress over time and form sinkholes in the mattress. This is especially noticeable in pillow top mattresses. Also, the springs that provide the support inside the mattress will eventually weaken and not be able to spring back to their original positions after a time. Either problem can cause a sunken area to appear in the centre of the mattress.