middle of the night; dead of night
Refers to the late-night hours when most people are asleep. Can be used as a noun or adverbially.
I woke up in the middle of the night.
夜中まで勉強した。
I studied until late at night.
I was startled when the phone rang in the middle of the night.
I was working until the middle of the night.
I woke up at 3 a.m. in the middle of the night.