I was watching a show, and i totally forget what plant was being observed but it was on the science channel. This specific plant needs to have dark periods, or else it pretty much dies. I dont know if that correlates to other plants, but im pretty sure most plants have dark periods built into the biological clock. I know for a fact that root systems grow the most during the dark. Its probably an overall better idea to have a dark period which mimics nature, opposed to giving them light all the time, i mean where on earth does that happen other then above the arctic circle. But in any case throughout the millions of years plants have been around they have created systems that work at night. Different plants to do different things at night, plants that utilize C4 metabolism dont even open there stomata during the day because its so hot where there located, so they open at night and store C02 for the day. These are just examples of plants that utilize the "night" and im sure almost 99% of plants do something during the off ours.