I was finding it too difficult to crawl in the back, and there was no good way to open the rear hatch from the inside.
i have never slept in a car. i have considered the possibilty if the home a/c went out in really hot humid weather. the prius would be perfect, and i'd just use a pillow and blanket in the front seat.
my dad used to sleep in his continental at rest stops on the way to florida. 40 winks and back on the road. refused to spend a hundred on a hotel.
On most Prius if you remove the headrests and slide the front seats all the way forward you can recline them flush with the back seat. That way you have plenty of room to stretch out (especially if you remove center console) and you still have room for storage behind back seats.