burn the midnight oil
Originally referring to the act of burning oil in lamps for light and safety, the noun midnight oil is attested from mid 17th c.
burn the midnight oil (third-person singular simple present burns the midnight oil, present participle burning the midnight oil, simple past and past participle burned the midnight oil or burnt the midnight oil)
- (idiomatic) To work studiously, especially late into the night.
2013, Phil Cousineau, Burning the Midnight Oil: Illuminating Words for the Long Night's Journey Into Day, Simon and Schuster, ISBN 9781936740772:
- You worked at whatever task you were engaged in — churning out steel nuts for the car companies or churning out essays in college — until you were done, even if you had to burn the midnight oil. No excuses, no whining, no cheating.
He was burning the midnight oil to finish his paper.