Adjust the day/night switch-over sensitivity? If the firmware supports it, you can also sometimes schedule the switch-over based on what time it is.
what cameras and dvr do you have? but presuming your right and it gives you no ability to control it.. then it sounds like your SOL.
this one is tricky, even time schedules do not work that well since the sunrise and sunset changes every single day.. I had to figure out API calls and write my own program to do it.