When the lights go off or the sun goes down. This ensures all of the nutrient spray has been absorbed before the light is able to react with it. When you spray with the light on, or when your plants are still wet when the lights turn on, there is a negative reaction. The water also acts like a magnifying glass when the light shines through to the leaf. Liquid Light by Dutch Master requests that you spray the solution with the lights on. This product has been engineered for this. Also the product is very light to begin with (it’s hard to burn). But DO NOT use this product outside or inside when using 1000 watt lights. Only 600 watt lights and below for Liquid Light (it too has its limits in the light).