Exhaust duct... blackflow outside air problem?

Totally new to this, got a 6" duct with duct-fan booster (local grow shop!) 160cfm kicking hot air away. May be overkill, but the box sits at 86-90F all day, and hotter if I close the box. Cant even close the box.... :< SO. Now I've got this great setup to get rid of allll that hot air, I plan to have the duct booster off at night at the same time the lights turn off (12/12 cycle). On the inside of the box, I have a small fan circulating air for her to breath during the night.

So my question. What about the hot backflow air from outside, will it come back in the box during the night? I have the fan set to off during the night and that would allow air to come BACK in from outside? Will this be a problem? Should I leave that particular fan on 24/7? Thanks for the input guys!

My fan is coolest at night with no fans running. The ambient temperature is about 70 degrees F.

Run exhaust 24/7.. plants still need fresh air at night. You can turn off the canopy cooling fans with the lights though. That's what I do.

why dont you have the lights on at night and off during the day. perhaps this might help also. i dont know your situation but just an idea.