In a nutshell night, caves and closed/underground areas should be really dark in fonline. The darkness would hide many things like players and their names etc and also would drastically reduce the chance to hit with guns, nades, throwable weapons.
- Night person perk would give you something like night vision or just better sight in the darkness
- You'd be able to make flashlights using some metal parts, electrical parts etc if you have enough science, they would use some e-weapon batteries
- You'd be able to make flares with your science skill and some parts
- You'd be able to make torches with wood and any kind of alcohol with enough outdoorsman
- You'd be able to make campfires on the desert to rest or cook some food etc with outdoorsman, wood, alcohol also
- There would be some light sources in cities eg burning barrels, lamps, other stuff, guards would sometimes have flashlights
- Sneaking at night would be extremely effective
- You'd see/hear things better with higher PE
- Lines of sight for characters with no light source would be very short, they also wouldn't see anything behind them.
- Using a lightsource would make you visible - easy target for other players
Wasteland is a dangerous place, especially at night! be sure you have some light source with you.
I know this would change many things, but think about it for a second. It would be very climatic, RP positive, challenging. Traveling in groups during the night, something attacks you suddenly but you can't see it because of the darkness, suddenly A DEATHCLAW! OR EVEN A RAT!!! God, that would be scary and fun! If you don't like the idea you're probably power gamer type of player who wants to one hit KO all living things with 95% hit chance all the time. This modification would force using tactics, hiding etc.