What gives you the impression that lying on a comfortable mattress is good for you? Comfortable and supportive aren't always the same thing, and neither one is particularly "good for you". Mattresses are really hard to get right. It depends on the shape of your spine, your weight, whether or not you have back problems (and which), and personal preference. There are trade-offs involved.

If you find evidence so important, I suggest you show me yours, as I find it to be strangely lacking. Ah, the irony.