I' ve just had a look in the base game file and it is about one third of the way down in my lighting.txt.
projectiveShadowStrength 0.73
# modifiers insideObjects 0 0 0 0
# modifiers insidePortals 0 0 0 0
# modifiers insidePortalSpills 0 0 0 0
# extra ambient for unlit rooms.
unlitRoomColor (.3, .275, .275) -threshold .33
unlitRoomColor (.45, .425, .4) -threshold .4
flatShade (1, .95, .85) .75 -sims 1.7 -objects 1.3 -indoors 1 # for lighting set to low.
environment (1, 1, 1) 1
# max in time (s) max out time (s)
transitionTimes 2 100
end
Then if you scroll down a bit there is another transition Times under night time.
I've just changed both the 2s to 30s and will go and check out what it does.