Morning, Afternoon, Evening, & Night
Rule 5.2: Use the definite article with in the morning, in the afternoon, and in the evening.
I go to school in the morning.
My grandmother usually takes a nap in the afternoon.
A lot of people surf the Internet in the evening.
Rule 5.3: Do not use the definite article with at night.
Correct: Astronomers have to work at night.
Incorrect: Astronomers have to work at the night.