Student or Learner
He works at night.
Can we say in the night?
What's the difference?
In the morning
In the afternoon
In the evening
However, to some extent it can be explained as follows:
Day and Night refer to a specific time period where as morning or afternoon refers to specific time period of the day. So a definite article ‘the’ is used which preceds the preposition ‘In’. So you can use ‘In’ if you use 'the' to express a specific event that occurs during the time of night, as:
'I heard a frightening sound in the night before going to bed.
Whereas, we usually use "at night" to express an action that occurs at a more or less precise point in that time period.
I go to bed at night.