O que significa a expressão "burning the midnight oil"?

Today we're going to talk about a very common practice among hard working students: burning the midnight oil. Do you know what that means? Better yet, are you one of those students that are always burning the midnight oil? If you have an upcoming test and you've been working very long hours, it's possible you also study late into the night. When something like that happens, you say you're burning the midnight oil. Let's think of an example to make things easier: your friend calls you up on the phone to invite you to a party. You would really love to go, but unfortunately you have to stay at home working long hours on a project from school. So you say: I'm sorry, but today I'm burning the midnight oil. Maybe next time! If the expression still sounds odd to you and you're curious to know about its etymology, just know that it comes from the use of oil to provide light before the invention of electricity. Up to this day, people still say they're burning the midnight oil to refer to any kind of work they're doing late into the night. That's it for today guys. See you!