The ideal time to water is about an hour after lights come on, but overall, water early in the day / lights on cycle.Its best because the plant gets the chance to uptake this water during the day due to plant transpiration. At night, the plants are not drinking like they do in the day, so by watering at night, your roots sit in a more saturated soil and for longer. This is one of the main reasons you'd want to water at lights on.