Cold basement/fan setup

I have a cfl closet grow. This is my third grow and I am pleased with my previous harvests. This is my first time growing through winter and I am concerned about the temperatures in my basement. It stays around 65 during the day and cools down to 58 or so at night. The temps in my closet hover at about 75. How should I be using my fan to circulate air? Normally in the summer I used it as an intake at the bottom because the temperature was the same inside and out of the closet. Will the cold air hurt the plants? Should I shut the fan off at night? Any advice would be appreciated.

The cooler temps will be fine. I do my grow in basement and my room runs between 63F to 70F and i have great results. I have my exhaust fan set on a timer so kicks on and off and at night when lights are off i would get a digital temp gage that tells you your high and low and just start with running the exhaust for 15min every hour and next day see what your temps got down to and adjust the timer from there thats how i got my timer figured out. Hope this helps good luck on the grow

Also at night when lights are off i have a heater that runs alittle bit to help with keeping it warm so i can still exhaust when lights are off it just takes some messing with but don't be worried if you get down in the 60's they will be fine

Many thanks! What kind of heater do you use?