It's often turned on at night in what's referred to as a reverse daylight photoperiod. It helps balance pH. At night just about every living thing in the display tank takes in oxygen and produces co2
in water forms carbonic acid and any acid in water lowers the pH of the water. By running the refugium lights opposite the display tank you let the plants and algae in the sump take the co2
from the display tank and convert it back into oxygen. This makes the pH more stable and keeps oxygen high when it would otherwise drop. Keeping the lights in the sump off during the day helps control heat and is good for the copepods. They're nocturnal.