at night 表示的是在夜晚的某个时间，不是整个夜晚，所以不用in night

at night ：在晚间;在夜里;晚间;晚上

相关介绍

例句：

It was late at night when we finally arrived.

我们最后到达时已是深夜。

I started to feel afraid of going out alone at night.

我开始害怕夜间单独外出了。

A red sky at night often indicates fine weather the next day.

夜空呈红色往往预兆第二天天气晴朗。