This is an old thread, but it comes up high on google, so in case anyone else reads it, you might find this link of interest:

Instinctive sleeping and resting postures: an anthropological and zoological approach to treatment of low back and joint pain
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1119282/
(BMJ. 2000 December 23; 321(7276): 1616–1618.)

Michael