I assumed SERTA had a good product, I needed a very firm mattress due to some back weakness. I got a rather expensive extra firm twin SERTA Perfect Sleeper mattress and it was fine for the first few months. I only weigh 135 pounds and soon began to sense the mattress was turning into a soup bowl. It kept getting worse. The only way I could get the firmness I needed was to try to sleep right on the mattress edge where it was still quite firm. And then I was about falling off the bed. I flipped it over to see if using it upside down would help and it did for about a month, but that too became a soup bowl, like what you would expect in a cheap motel. I went back to where I bought it and their lame excuse was that the firmness does break down some with time. Really! Just a half year and as light as I am? They said they would stand behind it if it sagged 1 1/2” below the level line from edge to edge. What a joke that is. The sagging happens when skinny me lies on it. It does not show much sag with no weight on it. I’m totally disgusted with this mattress. If you need a firm mattress, DO NOT BUY SERTA !