In a non hybrid car I would not leave the engine on all night while I sleep. I have already left the ac on while I take a 30 minute knapp and I have not noticed any problems. My prius is big enough that I can sleep in the back. I require a cpap machine becuase I have sleep apnea. The machine requires less that 80 watts. I have 12V DC to 120V AC adapter and have sleep in the passsanger seat using my cpap machine while my girlfriend drives. I need to sleep in the back with the car on. I will be using my cpap machine and will have the Air Conditioner at about 75. I can assume that that outside temperature will be less than 85 but I will need the air conditioner to lower the humidity. I also hope to sleep with the heat on when the weather is cold. QUESTION Will leaving the on while I sleep void my warranty? Will having the car on while I sleep cause me to have major repairs. If keeping the car on is a reasonable option is there temperature differentials I should avoid. For example it might be ok to use the heat when the overnight low is 45 but a bad idea when the low is 10 below zero. Lets suppose that the lifespan of my prius is 200,000 miles. If sleeping with the car for 6 hours will lower that by 100 miles then the cost of sleeping in the car is between $10 and $15, far cheaper than a motel. But is 100 miles a reasonable estimate?