1、at night 的意思是 during any night，通常表泛指。如：

I often work at night. 我经常在夜间工作。

He's afraid to go out alone at night. 他害怕夜晚独自出去。

It's becoming dangerous to go out alone at night. 夜间独自外出已经越来越危险了。

Please ensure that all the lights are switched off at night. 夜间请务必将所有的灯关掉。

【注意】在非正式文体中，有还可用复数名词不加介词来指反复进行的活动。

Would you rather work nights or days? 你愿意上夜班还是上白班?

2、in the night的意思是 during one particular night，通常表特指。如：

The storm blew over in the night. 风暴在夜里停止了。

I had to get up in the night. 那天夜里我非起来不可。

The temperature fell sharply in the night. 那天夜间温度急剧下降。

I'm exhausted. The baby woke up three times in the night. 我累坏了，孩子夜里醒了三次。

【注意】这一含义有时可用 during the night 来表示。如：

Everything was quiet during the night. 昨夜万籁俱寂