This is the 311th most frequent Russian word.
ночью
At night.
Here, 'ночью' is used as an adverb meaning 'at night', specifying the time of the activity.
Я люблю гулять ночью.
I love walking at night.
'Ночью' is used adverbially to describe the period of the day when the scene takes place.
Ночью в городе очень тихо.
At night, the city is very quiet.
In this sentence, 'ночью' functions as an adverb describing when the action of working occurs.