At night is when the micro-fuana thrive and the corals are utilizing the water column for food.i personally have mine setup thru the apex to turn off at 8pm when my lights go out and turn back on when my lights turn on.
I actually turn mine on at a reverse light cycle.
1) I don't hear it during the day
2) It keeps the 2 degrees out of my tank at it's peak with the lights on.
3) I don't have to fumble and shut it down for feeding during the daytime.
4) Oxygenates the water probably when it's at the lowest point of the day (PH etc.)