I have done the "underfloor storage" mod on my JKU that has the side benefit of making a flat platform in the back for sleeping. I ordered an "SUV air-mattress" from Walmart on line that has two compartments for inflating. The larger compartment is just right for the back of the Jeep and is very comfy. Our local stores didn't have such a thing in stock.

 

I have a pic of spanishgoose around here somewhere, showing the proper sleeping technique for the JKU...