See how tiny Isabel was?
The following is completely based on unscientific results. :-)
In the above picture you can see a crib and a wedge. This wedge is made entirely out of foam and was put together almost 6 years ago by the OT/ PT staff in the Pediatric unit of Swedish Hospital. Our first child was diagnosed with acid reflux (amongst other ailments) and after we were told this "they" said we would be getting a small wedge. In my mind and my husband's we were both imagining this small thing that maybe lifted Isabel's head 5 inches off her mattress. When they brought this thing in we started laughing. For Isabel was super tiny and this damn thing was HUGE! However it proved to be a great thing. Isabel was completely snug in her wedge and we could safely put a blanket on her to keep her warm in our very cold upstairs, plus it helped her reflux. Without a second thought we put Sam in it when we brought him home from the hospital. Sam was also a child that always slept in his car seat even when we weren't in the car. And now Lucia is sleeping in it. For some reason I believe that small children do better sleeping slightly upright. I think that their little bodies are trying to figure out how to function and it is more comfortable to not lay entirely flat. I have heard about other parents who's children also will only sleep in their respective car seats. My advice to fellow parents is if the kid wants to sleep in their car seat, then let them. I don't think it will hurt them in the long run. Sam sleeps on a twin mattress without any troubles these days.