thats a highly debated topic, I do not use a dark cuz i have limited room in my cfl grow, they stay nice and short, the growth that happens at night is alot of stretching the stems/branches, which is fine if you got an hps but cfls light penetration and cfls are bad at being effective, after like 3 inches from the light to your plant you get a drastic decrease in light that makes it to the plant.