how cold is too cold?

im in the 4th week of 12/12, i have to bring in cold air from outside in order to keep the tent grow temps below 80. i try to keep them around 69-75 when the lights are on. when they are off their chilling @ 73.
we are havin crazy hella cold weather, and with the window closed and a fan just pulling the cold air from that are @ 64. do i need to remove the fan and let em warm up? or is 65 acceptable.?

65 is great. My room is 66 or so night, 76 day. Some say the plant does best when there is about a 15º swing between night and day. As long as you aren't dropping below 50 you are fine.