"Night" is special when it comes to prepositions. "In the night" suggests you are or should be sleeping. "At night" suggests you are or normally would be awake. Otherwise, we usually use "in the" before times of the day: In the morning / afternoon / evening.

Both can be used. Depending on the context. The sky is dark in the night time. It was late at night Early in the morning. We ate dinner late at night. I was driving my car late in the night/ late at night - can also be used.

Phil is right that "night" is treated differently from other extended time periods (such as "morning", "day", "afternoon"). If I take these two sentences: "He did his work at night." "He did his work in the night." It's a very subtle difference in tone. The "at night" one to me seems to be simply stating the fact. The "in the night" one seems to be implying that other people might not be aware because they were sleeping. Or another way of looking at it is that "at night" makes night seem like a moment in time (same as "at sunrise" or "at sunset") rather than as an extended period. But "in the night" makes night feel an extended period of time that you can operate within (same as "day" or "morning). In other words, using "in the night" makes it sound like some action is occurring during part of the night.