I had one of the best sleeps of my life on a bed of soft sand, in the curve of a dry creek that I really should not have slept in due to flash flood risk. Since then I've wondered if ancient people might have made mattresses of sand, but they were so simple and evanescent that they left no trace.
"The Egyptians built using stone but wrote day to day on papyrus.
The Sumerians built with mud and wrote on clay tablets.
This is why we have lots of Egyptians buildings but not much of their writing and the opposite with the Sumerians."