by night

by night

During the night. Often used to refer to one's typical nighttime activities, in contrast with what they are apt to do "by day." By day, he was an average office clerk, but by night he was a crime-fighting vigilante. She's a teacher by day and an actress at the local theater by night. We'll need to travel by night so that we remain undetected.
See also: by, night
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
See also:
References in classic literature ?
The luxurious bed, the well-ventilated room, the delicious tranquillity of Venice by night, all were in favour of his sleeping well.
With that refuge as a base, I could face this strange world with some of that confidence I had lost in realizing to what creatures night by night I lay exposed.
And yet, though herds of whales were seen by night, not one whaleman in a hundred would venture a lowering for them.
There was no reason, however, why I should refrain from seeing the person who had inconsiderately sent her to so great a distance by night and alone, and as it was not improbable that if she found herself near home she might take farewell of me and deprive me of the opportunity, I avoided the most frequented ways and took the most intricate, and thus it was not until we arrived in the street itself that she knew where we were.
But waking or sleeping, by night or day, in sickness or health, she is the one object of my care, and if you knew of how much care, you would look on me with different eyes, you would indeed.
'Stay here of course,' the child had said in answer to my question, 'I always do!' What could take him from home by night, and every night!