We turned ours off when the temperature went down at night, and turned it on when we got up, before the air temperature started to increase.
As SRS says, they worked in low-humidity areas, and they should be mounted to suck in outside air, as artbrooks notes.

They helped in the more arid parts of Texas.

They would not be any good in Bobert country, or "Swamp People" country (anyone here watch that program, besides me?).