Vivien Bonvin is correct in that you can safely ignore the night budget.
I tested this out by setting one of my daytime service budgets to a different value than the night so I could record when the game considered changing from day to night. After several cycles I disabled the day-night cycle option and ran for another few months, long enough for the game to have normally cycled through the entire night phase. Only the daytime values were in effect.
Ordinarily, the game cycles through 10 weeks of daytime followed by six weeks of nighttime, with the last week of each phase serving as the visual transition from one mode to the other (where the streetlights turn on and off). You can track where you are in the cycle by looking at the day/night graphic around the Areas button, where you'd normally buy additional plots of land:
If you disable the day/night cycle, it immediately reverts to the middle of the day phase and services will go to their daytime budget. Enabling day-night mode again will not continue where the cycle left off, but rather continue from the midday setting you were in with the permanent day mode.
Even though the graphic in permanent daytime will look like the sun is at 11 AM, it is actually midday. You'll enter night mode five weeks after re-enabling day-night mode again, which places the time halfway through the ten-week day cycle. (As a bonus tidbit, weeks for the day/night cycle appear to start on a Tuesday. Days also last 10s of real-world time on the slowest speed setting.)
This testing was done with only the Parklife DLC installed, so I don't know if After Dark modifies this, but I would speculate not.