yeah, I think it should be rulesetted at the very least. maybe as a "low-light vision reduction modifier"?
something like:
lowLightVisionReduction: vision range in low light conditions is multiplied by this factor. defaults to "1" for vanilla aliens and 0.45 for vanilla humans.
unfortunately, reality is a little bit more complicated...
https://www.ufopaedia.org/index.php?title=Night_Missionsapparently, vision range is always 20 (for every unit, no matter if alien or human). The difference is that aliens can see units "in darkness", but humans can only see units "in the light". and while the graphics make it appear to be gradual, night missions (including twilight missions) operate under a binary system: light sources illuminate everything within 9 squares, everything else is "in darkness".
I think this calls for an optional gradual system... but implementing it may be difficult, because the vanilla system is so different from any "realistic" system