Thanks for the question, James. A sagging mattress can be a real pain. By that I mean that you can wake up in actual pain because middle support is incredibly important for your back and spine.
Sleep experts nearly unanimously agree that you should have a solid foundation and firm surface to place your bed on. So the first thing I’d check is the box spring. You are using one, right? If not, simple fix. Just order a new box spring with your mattress and you should be good to go.
If you currently have a box spring though, or are using a platform bed with built in slats, I’d take a look at the slats to make sure they’re still intact and supportive. If they are, you can rule out the foundation as being a problem.
Next, take a look at the mattress itself. Some mattresses, particularly innerspring mattresses can suffer from durability issues several years into their life. What happens it that the top padding layer gets pushed down into the coils and stuck there, leaving a permanent indentation and sunken in feeling towards the middle of the mattress. Depending on how long you’ve had your bed, this is a pretty common thing and likely could be your issue.