Just call it "evening" if it's before midnight. That way you get four nicely subdivided periods.
00-06 - night
06-12 - morning
12-18 - afternoon
18-24 - evening
Would need to refactor the name of "midnight" tho to minimize confusion. But if we use it as a starting point to count hours in a day from, it doesn't make sense to simultaneously designate it as a middle of anything.
(BTW, such subdivision is actually common in many places of the world.)
I feel we've just rediscovered why appeals to grammar[0] don't actually solve much of anything. (Not saying you're appealing to grammar, just that grammar is generally not that useful for discerning... anything really.)
Anyway, to be a bit more substantive: What really baked my noodle when I was younger is the fact that the seasons and the night/day cycle are much more disconnected than it appears when you live on Earth. Of course, it makes sense when you understand the tilt/rotation thing, but still... it really weirds me out sometimes.
[0] English in this case, but any language, really.
