Pretty sure when the lights are on. Pretty sure they soak up the water when the light is on, and grow when the lights are off.
Best time is before lights on. This can't be done by most growers so most water right at lights on. In nature it is least likely to rain in the afternoon, so watering right before lights out is fine but before lights on is best. If you water your backyard garden or lawn on a hot day the evaporating water acts like steam which hurts plants, which is why sprinklers and irrigation systems are set to run at night. The same goes for mj.
