You are "burning the midnight oil" when you are working hard late into the night. Example: "I could see the light in your window last night. It looks like you have been burning the midnight oil." Before they had electric lamps, people used oil-burning lamps to read or work in the darkness at night. When you are working very hard until late at night you are "burning the midnight oil." "Has your son been working hard to prepare for his final exams?" Reply: "He sure has. He's been up each night burning the midnight oil".

Burning the midnight oil 的意思就是挑灯夜读,也就是我们平常所说的熬夜,开夜车。在英文中,这个用法也由来已久了,可以追溯到人们最初开始使用油灯的时候。如果在午夜时分燃烧灯油,那也就意味着熬夜苦读、看书了。例如:I have a big test tomorrow morning, so I plan to burn the midnight oil tonight.(因为明天我有一个重要的考试,所以我计划挑灯夜读了。)