He is a night owl.
他是个夜猫子。

Scene
BILL: I saw Rik sleep in class several times. Does he often stay up late ?
Jany: Well, he has always been a night owl.

Notes
night owl 通常用来形容经常熬夜或通宵不睡的人. 也有night-bird的说法. Frankly speaking, I am a night owl too! and what about you ? ^_^

Exercises
下面我们来做看两个跟night有关的用法,你知道它们的意思吗?

1. He walked into the room like a thief in the night,
2. The meetings were held in secret at the dead of night.