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."