Prepositions in Expressions of Time

Time phrases are words that show the verb tense we need to use

Note: We say at night when we are talking about all of the night:
Example: He sleeps during the day and works at night.

But we say in the night when we are talking about a short time during the night:
Example: He woke up twice in the night.

taken from:

http://languagelearningbase.com/88672/prepositions-in-expression-of-time-in-at-on-ago