More than enough to roll over, but overall not a lot. I'm only 5'9" and my wife is 5'6", so we'll be laying across the van. The headliner is significantly lower at the edges because of the rear air. I can measure it though to give you a better idea. What I purchased today was a mattress topper. It's about 4 inches thick, so when you lay on it, it compresses quite a bit. Very comfy though.
http://www.campingworld.com/shopping/product/perfect-harmony-mattress-toppers/9275
http://www.campingworld.com/shopping/product/perfect-harmony-mattress-toppers/9275