AT vs IT

[NavyGuero83]

I was writing and I wrote "at night" in a sentance and I realized that I wanted this to be done in the afternoon. So I had to change night to "afternoon". Then I looked at it and realized I had to change "at" to "in the" also.
I was wundering why we say "AT" before night and "in the" befoer morning, day, and afternoon. It seems that by the rules of AT and IN we "should" be saying IN before night. hummm??? Im just curious so if any one could give me a logical answer I would be happy. Thank you.

[5jj]

In the morning/afternoon/evening; at night.That's just the way it is.

We can also say during/in the night.

Prepositions are a law unto themselves, and not just in English.