Plant growth doesn't halt when the lights are turned off, so I don't think there's much of a call for 24hr light. In photosynthesis, the plant combines water and carbon dioxide with the energy of light, forming glucose. Meanwhile, the plant performs cellular respiration within their mitochondria, just like animals, in which oxygen is combined with glucose to form adenosine triphosphate (ATP). The plant then uses this ATP for all sorts of metabolic functions. These are the anabolic functions which build and transport the structures, proteins, etc that grow the plant. Normal respiration takes place 24 hrs a day. There should be enough photosynthesis already occurring with your grow lights in 12 hrs for the plant to respire all day. If the lightwasnt intense enough, longer periods would help, I imagine. But the idea of indoor growing is that the lights are optimal brightness already.