I can't imagine not ever being able to sleep again. I know how it feels staying up all night and having to do something the next day and I certainly do not like it at all. Perhaps instead of the characters being each other's torturers, it is the feeling of drowsiness associated with not getting sleep that is the true evil. (Do Garcin, Estelle, and Inez even feel anything in Hell... Are they just souls like we read in Inferno?) Sleep provides a time period for the brain and body to rest and it also allows a means for momentary escape from society around us. The characters aren't even able to close their eyelids, so imagine going all of eternity without taking a break ever. The fact that they are trapped in a plain room without anything to do for 24 hours a day, 7 days a week adds to the hellish nature of their situation. They really and truly are in Hell.
Also, this is a picture of the set. It's very boring like I visualized it in my head. I can only imagine the endless possibilities of doing absolutely nothing in this room.