I run oscillating circ fans 24/7 in each corner of my 7' x 8' x 9' flowering space. The exhaust blower is controlled by a thermostat.
During lights-off, there's no heat from lighting to trigger the exhaust blower thermostat, which can cause a buildup of humidity during the dark period hours.
To counter that effect, I run a dehumidifier during lights-off. The dehumidifier itself makes a little heat and raises the room temp a little, often enough to trigger the exhaust blower a few times per dark period, but it doesn't matter if it doesn't run.
Since the plants are not actively transpiring CO2 in the dark, it's not as important to bring in fresh CO2 during lights-off as it is to keep temperature controlled and humidity in check. Seems like keeping humidity to about 50% controls powdery mildew, which thrives in cool, humid conditions with limited air circulation.
Without a dehumidifier, it would be wise to force an otherwise thermostatically controlled exhaust fan to run a few times per dark period to keep humidity below about 50%, particularly when ambient air around the grow op is cool i.e. winter. However, circ fans should run all the time. Air movement helps prevent powdery mildew and other evils.
