Day time tuning is probably going to get you closer but once you get it dialed in it really shouldnt have much of any variations. I do a lot of late night tuning and dont have any trouble but sometimes for WOT fuel and timing adjustments you have to stay a bit extra conservative as the air temps have dropped off quite a bit compared to the extra heat during the day. One thing that will also help you out and can drive you nutz.... Unplug the evap solenoid connector under the hood. It will throw a code but its not a big deal and you can temporarily turn the code off while tuning. What happens when its warm out and you do not unplug the evap solenoid is that you THINK you finally have your trims where you want them and then pull up to a red light and then it hits, BOOM -15 fuel trims!! You get agrevated and it doesnt make sense how a good tune all of the sudden shows really screw fuel trims. When it is warmer outside, especially hot days sitting in traffic, the vapor levels in the tank build up much more so than when it is cooler outside and the pcm monitors this gas tank pressure/vacuum with a sensor and uses this data to cycle the evap purge solenoid. This is one reason sometimes you may notice in traffic the car wants to hiccup a bit coming to a stop or starts to idle slightly rough out of the blue. The fuel trims will go negative because once the evap solenoid opens up there is extra fuel entering the system and making the car run too rich, so the fuel trims drop to temporarily compensate until the solenoid is off again. I hope this helps you out, good luck with the tune!