How cold is too cold?

So i have air from outside coming into my grow room and its starting to get cold outside. Ive just switched to flowering and with the lights off for 10 hours its gone from 84 f to 59 f. Should i try and put my inline on a timer so its only bringing cold air when the lights are on? Should I get a heater? Its 34 outside right now and its only going to get colder from here on out

Im 6 weeks in and my temps are just like yours and my chernobyl is purple as hell.My outside plants made it in 28 degree temps I know they can stand 58.If it gets below 50 Ill throw a small heater in there

Ive started to cut my fresh air intake off after 12 at night due to extreme cold air

If i was you, i would put your inline fan on the same timer as your lighting. Also i would add an oscillating fan inside your grow room just to help with air flow inside the room without cooling. good luck.