My grow box was in an unheated outbuilding where temps were often well below freezing during the early parts of the grow so I wanted to run the HPS light 24 hours as a heat source. A side benefit was the giant size of the plants...looks like the total dry yield for 4 autos in a 4x4 ft room and 400w HPS will be over a pound! Check out my Dinafem White Widow thread to see one of the four plants (other 3 were Barney's Critical Rapido). I think next time I'l run a short dark cycle even if it means getting a small heater.