‘night’ and ‘at night’
Night is the period during each twenty-four hours when it is dark. If something happens regularly during this period, you say that it happens at night.
The doors were kept closed at night.
I used to lie awake at night, listening to the rain.
A night is one of these periods of darkness. You usually refer to a particular period as the night.
He went to a hotel and spent the night there.
I got a phone call in the middle of the night.
the previous night
If something happened during the night before the present day, you say that it happened in the night, during the night, or last night.
I didn't hear Sheila in the night.
I had the strangest dream last night.
You can also say that a situation existed last night.
I didn't manage to sleep much last night.
Last night is also used for saying that something happened during the previous evening.
I met your husband last night.
If you are talking about a day in the past and you want to say that something happened the night before that day, you say that it happened in the night, during the night, or the previous night.
His father had died in the night.
This was the hotel where they had stayed the previous night.
If you want to make it clear that you are talking about a particular time in the early part of the night rather than the morning, you add at night.
This took place at eleven o'clock at night on our second day.
However, if you are talking about a time after midnight and you want to make it clear that you are talking about the night and not the afternoon, you say in the morning.
It was five o'clock in the morning.