2008 study. Do a Google search for:  Kim Bergholdt, Rasmus N Fabricius, Tom Bendix
then click on the link from www.medscape.com/

2003 study. Do a Google search for: mattresses Francisco Kovacs
then click on the link from www.medscape.com/

1981 study. Do a Google search for:  S R Garfin, S A Pye
then click on the link from www.medscape.com/

I hope even more extensive research is done in the future and becomes available to the general public. This is just what I could find tonight.

Matt

These will only give you one page of each study. You can access the other pages of the 2008 and 1981 studies by registering with medscape.com. It's free.

Matt

Mattress related lower back pain is a relatively simple to fix.  90% of the time it is from side sleepers sleeping in between positions instead of being on their side with their hip and shoudler pointing straight up and down (at 12 o'clock).  If the mattress is too firm it creates pressure in the shoulders that makes the person want to twist slightly and that creates the LBP.  The other 10% of the time it is a stomach sleeper on too soft of a mattress.

Batman wrote:
Good point. Position is important. I noticed when my feet get cold I'll cross my legs. This causes pain in my lower back. I've been trying to be conscious of this when I lie down. Summer is here so now this is easy to fix.

Matt

Try elevating your feet.  Elevating your feet above your heart spaces out the vertebrae in your back and relieves lower back pain.  Most people buy adjustable beds for this reason. People with chronic back pain find themselves sleeping in their recliners if they have not figured out how to elevate their legs in bed. The do make foam wedges for your legs now.

I don't think back pain is all that simple.  I do sleep on my left side primarily, but when my back hurts it is after sleeping on my back, if the mattress isn't supporting my body evenly.  I suspect that my low back muscles tighten up trying to keep my butt from sinking too low.  I know it happens when a bed's foam starts to give too much under my hips.  I know that if I sleep on a more supportive bed the pain goes away very quickly(found this out from staying out of town...bed being offset coil, spring box spring, minimal foam, five year old hotel bed...years of pain went away in two nights!).  I will read the studies and see what I learn, tho.  Thanks for sharing!
Kait

http://www.rebuildyourback.com/backpain/1963964.php 
This sums up the studies pretty well.  Clear as mud!  Generally, new beats old, medium firm beats extra firm, and almost everything is better than a really hard futon.
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Kait