I'll let one of the SWCG folks chime in with a definitive answer, but if it were my pool I would run it during the day--I would want the chlorine production happening when it's needed most, which is when the sun is out burning up what chlorine is in the water. If you run it at night, you will have to make sure that there's enough residual chlorine in the pool ALL DAY so that you don't fall below your minimum level by night time. Makes more sense to me to run it during the day...but that's just me thinkin'

Janet