butterhead lettuce

Does anyone know if butter head lettuce has a photo period requirement for growing indoors? I am growing some and it is very healthy, but it is also open and not really binding into a head. This my first try at it and it looks great I just can't find any info for growing it indoors in rock wool. It is under two 65 watt compact floros at 3500 kelvin and 24hrs. I am going to add a timer to save some money and thinking it may do better under 18 hours. Any info would be greatly appreciated.I am feeding grow big, carboload, and bioroots. Next feeding I am adding calmag and micro from general hydro.