"Ми спимо вночі."
Переклад:We sleep at night.
In the night is not commonly used. It sounds poetic or literary. "The wolves howled in the night."
At night is more common. "Our neighbor's dog always wakes me up at night with its barking." "He works at night so he sleeps in the daytime."
According to OXFORD guide to English grammar:
//At night= when it is night, "The windows are shut at night." //IN THE NIGHT = in the middle of the night, "I heard a noise in the night."
I hope it will help.