First of all before looking for evidence that your mattress has run its course, you need to look at how long you have had it. A mattress is meant to last you 7-10 years, if it's over that then you definitely need to upgrade. With that said let's look into the following signs you need a new mattress:
Your mattress is sagging
This was one of the first signs that made us realise it was going to be time to get a new mattress pretty soon. Sagging can occur when the coils in a mattress start to weaken or the memory foam begins to lose its shape.
Your mattress is making noises
Squeaking sounds and screeching metal sounds might be a sign that your mattress should be replaced. This is more likely with mattresses that have coils, as it means they maybe starting to age and no longer giving the support needed.
You can't get comfortable
I found that I was tossing and turning more in the night and just in general couldn't get at all comfortable. This can mean your mattress has lost its comfort and support.
You wake up in pain
I started waking up with pain around my neck and shoulder area. It got to the point where the day before we got our new mattress I felt more comfortable sleeping on the sofa.
You sleep better on another mattress
If you are finding that you wake up uncomfortable and in pain and then go to a hotel and find sleeping on a different mattress doesn't cause this issues then your mattress may be the problem.
Your mattress smells
Not the nicest thing to think about but your mattress can start to smell. This can happen over time as mildew, mold, and fungi build up in the mattress –If you take a whiff of your mattress and smell some serious must, it's time to get rid of it!