I have took several 3-4 hour naps in my smart. You have two comfortable options. One you can fold the passenger seat down flat and then from the drivers seat use the folded down seat to lay your head on. Use a pillow and within 10-12 in you would be out cold sleeping away in confort. (Note you must be tired 1st)
Second option is to fold down the passenger seat and then lay you legs in the hatch trunk space area and lay your upper body top of the right passenger seat. (You will be in a "L" shape) I have not officialy tried this second option but I believe it will work.
*Since the car is in high demand I recommend you have your windows tinted before attempting this or you will awake with a potential crowd watching you sleep.