I chose the 8 inch twin as this size is difficult to find in a comfortable mattress. I needed this for a trundle that rolls out on the floor with slats and no box spring . I was surprised that this had springs in it, because it came tightly shrink wrapped. I didn’t need to wait anytime for it to reach the full 8 inches. It seemed to “spring” right into shape after I opened the package. It is certainly considered a medium firmness, and is very comfortable. Exposed to the air, the top of the mattress does feel cooler than my other mattresses. The fit for a trundle is perfect, and it disappears as it should when tucked away. Most slimmer twin mattresses wouldn’t be too comfortable for adult weight, as it traditionally doesn’t have enough support. My husband is tall and about 200 pounds and said he was very comfortable laying on it. Now we have the extra sleeping arrangements in a tight space for unexpected guest. This was the perfect solution.