Does the wind stop when the sun goes down? Nope, it can even get worse. With that said I would say yes leaving the fan on would be good plants as I understand it do a lot of working at night and circulating the air replaces the surrounding O2 rich air with Co2 heavy air and that is good!

As far as keeping the room worm. I would suggest some in-flow and out flow ducting with a couple of bends and that should keep light out while allowing fresh air movement. You don't want light leaks but you do need fresh air with Co2. Or a heater(fire hazard) or a heat pad.

I am very new to this hobby so take my info for what you want.

But I always try to remember that I am not just mixing a can of paint. A plant is a seasonal life and what your doing in that box, closet, basement, shed or warehouse is recreating the growing season the calendar may read January but as far as the (hopefully) girls know its July. So care for them as Mother Nature does for us in those summer months.
Good luck :hippy: