Taking my mattress as an example. My mattress is made of foam, and rightly so because after a long and tiring day, it provides me with comfort and support. The foam is compressible, which helps it to take the shape of my back and cushion the curves and bends of my body. Now imagine if it was made of solid wood. Provided I sleep on it every night for a considerable amount of time, it would eventually start to shape according to my body, but it would very uncomfortable, it wouldn't provide me much support or comfort, if at all. The only thing common between my foam mattress and the wood mattress would be that after a long and tiring day, I would still most probably go to sleep on either of them, because they're much better to sleep on than the cold hard ground.