The expression by night is typically used to contrast someone's nighttime activities to their daytime activities, especially when the nighttime activities are unusual or unexpected. You will see it very frequently in the construction X by day, Y by night. Some examples: ...treated it like a greasy spoon by day and a hotel by night I mean, if it's -- if it's somebody that is -- is living as a ...
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.
What can I say about a thing happened at night? Someone stole my phone at night. OR Someone stole my phone in the night. Which one is right to say?
Mediterranean? Lots of Mediterraneans have an afternoon siesta, so they could indulge themselves in such a preference. But Anglophones by and large don't go in for siestas, so there's probably not much call for a single term covering both "morning person, lark, early bird" and "night owl".
What is a word for someone who is both an early bird and a night owl ...
- 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