All ventilation/circulaltion fans should be on 24/7, unless you are running co2. When temps drop, relative humidity rises. If you have a decent sized grow (ie running a couple thousands watts HID w/ two dozen plants or so) then temps drop enough to form dew. You don't want dew to form so you got to keep that air circulating.

Even a small grow benefits from constant fresh air and a stem-strengthening breeze.