Hey there mellonc,
If you’re seeing the same issue with multiple mattresses, it makes sense that the problem isn’t the mattress. It is in all likelihood the foundation or support system underneath. You’ve already gone through three reasonable mattresses, Sealy Posturepedic, Avocado Green, and Sleep On Latex, and even swapped out the foundation once, and the issues still persists.
Athough, when you put the mattress on the floor, it felt perfectly flat, which really rules out the mattress as the cause. Even though your level shows the foundation is flat and your bed rail heights are nearly identical, the fact that the problem sticks around no matter how you set things up suggests that there may be subtle flexing or sagging happening under load that just isn’t obvious when the bed is empty.
This kind of thing is not uncommon, especially with slatted foundations or frames that don’t provide completely even support across the entire surface. They can look fine to the eye and even register as level with a tool, but once you lie down, they might flex just enough to throw things off. And unless you’re on the heavier side where body impressions could potentially be a factor, the issue is almost certainly with the base, not the mattress. I have had folks use their phone to video under the bed while they get on and stay on it for a while to determine if there is subtle or significant flexing going on with the slats.
The hump in the middle you’re noticing could also be part of the problem. That often happens when the center of the bed doesn’t get used as much, like if you and a partner sleep on opposite sides, so it stays firmer while the sides soften slightly over time. Some foundations also have stronger center support beams, which can exaggerate this effect and create that raised feeling in the middle.
At this point, I’d suggest trying a more rigid, non-flexing foundation like the Terra Foundation from mattressfoundations.com. It’s designed to provide even support across the whole mattress surface, which is especially important for latex and hybrid beds.
If you’re not ready to buy a new base just yet, you could try laying a sheet of plywood over your current foundation to see if that flattens things out. Since your mattress lies flat on the floor but not on your bed, that’s usually the clearest sign the support system is the root of the problem. The plywood is not quite as simple as buying a section of plywood that throwing it on though. You should have the lumber supplier (homedepot, lowes, whoever) round and sand the edges and then you should wrap it in a flat sheet or something. You dont want to damage the mattress in any way, otherwise you may void your warranty.
The foundation of a mattress is the unsung hero of your bedding set-up. Way too often we read stories just like yours and someone buys a 3K mattress and $149 foundation on Amazon. I am not suggesting that is what happened here, but it is a common issue we see. The main point being, the foundation does not match the needs of the mattress and the occupants sleeping on it.
It really does matter, particularly for those who have other sleeping issues.
Hope this helps,
Maverick