- The origin of "at night" to indicate a point of time and the usage of prepositions "in" and"at" In olden times, when the time expression "at night" was originated, night might have been thought as a point of time in the day because there wasn't any activity going on and people were sleeping that time unlike daytime.
At Night or In the Night? - English Language & Usage Stack Exchange
The spoken use of "night" as an informal, familiar version of "good night" (wishing one a restful sleep) is common, but I'm not sure what the proper written equivalent is - if there is one.
Adding Time brings the two into a comparison of time periods. Saying just Day and Night compares the non-time/instantaneous status; light and dark, up and down, inside and outside. This is the purpose of the usage, not a time period.
Can we use "day and night time" instead of "day and night"?
Do you think '2 o'clock in the morning' might somehow actually mean '2 o'clock in the afternoon', as that's the only alternative? I suppose I can see your point if someone says '11 o'clock at night' for 11pm, but again, unless you're above the arctic circle, the distinction with '11 o'clock in the morning', or any normal representation of 11am, is surely clear.
word usage - 1 o'clock in the morning OR 1 o'clock at night? - English ...
If it's 7:30pm, which of these phrases is correct, Good night or Good evening?