As the human body evolved, it become more upright. That's great for seeing predators more easily on the savannah or keeping the hands free to make fire, but it also means that our backs can come under strain. Unfortunately, many people are spending 8 hours a day with the object that makes that back pain even worse: an old mattress.
People often put off the idea of buying a new mattress, but it should really be something you take care of once you notice any signs of wear, especially if you tend to suffer from back pain.
The sagging of the aged mattress
Mattresses can have trouble staying firm as they enter their later years. With night after night of use, a mattress will eventually start to sag towards the middle as the springs or inner pockets become too compressed. This process will take place gradually, but you might eventually find that you're sleeping with your torso at a lower level than your head and your feet.
It might sound nice to sink into a mattress like that, but lying on a sagging mattress will mean that your spine is not being kept in a straight line; instead, it will be curved, somewhat as if you were lying in a hammock. This is not good for your back, and it can cause pain to develop over time.
The uneven distribution of weight
You might be the type of sleeper who moves around a lot at night, or you might sometimes have someone else sharing the bed with you. In such cases, it's common for the mattress to sag down unevenly across certain sections. This will mean that your body isn't enjoying proper support. A mattress should ideally distribute your weight evenly across its surface to keep your spine straight instead of bent as you slumber. If this doesn't happen, more weight can be shifted to one part of the back, which is not how your body was designed.
The errant spring
Additionally, older sprung mattresses can eventually develop springs that are pushed out of alignment. Those springs will dig into your back as you sleep, creating unnatural pressure points that can disrupt the proper alignment of the spine and cause pain along your back's larger muscles.
The muscular issues
Speaking of muscles, keep in mind that a healthy night's sleep is vital for your muscles to repair themselves after each day. You might be aware of how important the back muscles are if you're an Olympic bodybuilder, but even everyday people use their back muscles all the time, and they need a chance to recover. If your back muscles become weak or strained, back pain can be the result. If you're using an older mattress that doesn't give you the restful nights of sleep you need, your muscles aren't going to recover after each day.