Yes, you will be sleeping on the ground (in a tent). To
make your night as restful as possible, be sure to pack these three
essentials: a sleeping bag, a pillow, and a pad to put under your sleeping bag.
There are several different types of pads out there, but your best bet
is one of the self-inflating types (e.g., ThermaRest) or one made of
dense foam; these are lightweight and pack up nicely. You will probably
want to avoid the blow-up air mattresses you would use in the swimming
pool or at the beach; they tend to get holes in them and after walking
all day, you aren't going to feel like blowing it up. Also, you are
limited to 35 pounds of luggage, so the large inflatable mattresses
that come with an air pump are probably going to be too heavy and too
large.