Exhaust should run 24/7 (if not running co2) for the following reason:

Cool air holds less moisture than warm air so when the lights go off the moisture will condense and cause dew, especially right after watering. Dew covering your walls and windows and equipment is generally not good but is especially bad for big flowering buds for obvious reasons.

Running your exhaust at night constantly vents the humid air out of the room.
Perp Reviewed by Perp on . Fans On or Off during Lights Out?? I have not been running my fans during "lights out" and I am growing in a closet where the only airflow is provided by fans. The only air that comes in is through the duct work during that time. Is this ok? I'm not like suffocating my plants for 6 hours or anything right?? I'm trying to figure out what happened to my seedlings and just trying cover all the bases. Rating: 5