djstirling: That's a very good and somewhat complex question. Exhaust fans have multiple functionality. They help cool by removing hot air, exchange stale air, and funnel out odors.
For temperature considerations, if it's too warm then the fans should stay on. That was easy enough...
During the dark time your plants will still need the air to be exchanged so that oxygen/water can still be used efficiently for respiration, which releases energy from sugar and makes the plant grow. This always happens, in light and dark times. I'm sorry Slipon, as much as I respect your opinion across the many topics you address, I'm afraid I must disagree slightly with your latest response. In my opinion there should always be enough movement of air across the entirety of the plant so that the appropriate amount of oxygen and CO2 can be utilized. So I guess I'm saying there is a need for some windy weather at night...
When it comes to odors, it seems to me that "negative pressure" is pretty important here. Negative pressure guarantees that all odors are routed through the exhaust filters. Therefore, running the exhaust fans 24/7 would be the right thing to do IF you have negative pressure in your grow space and use exhaust filters, because odors still occur during the dark period.
Stealth is also a prime ingredient to a successful run. If fan noise is high then you may not have a choice but to cut the fans when the local area gets quiet at night.
Hope this is helpful and I don't get blasted too badly for my hasty response...