To “burn the midnight oil” means to work late into the night. Originally this was by the light of an oil lamp or candle. Now that electricity has been invented, the phrase is simply figurative.
The first instance of “the midnight oil” in print appears to be from the English author Francis Quarles. in Emblemes, 1635:
“Wee spend our mid-day sweat, or mid-night oyle;
Wee tyre the night in thought; the day in toyle.”
The phrase is connected to another old word, “elucubrate,” which means to work late by candlelight. Henry Cockeram defined that in his The English dictionarie, or an interpreter of hard English words, 1623: “Elucubrate, to doe a thing by candlelight.” The word is in my Random House Unabridged Dictionary (2003), and the prime meaning now is, “to produce, (especially literary work) by long and intensive effort.”
A word of caution is in order. Burning the midnight oil, causing sleep deprivation, may result in insomnia or sleeplessness over time, as the body becomes confused. However, I’m going to ignore that. For one thing, I’ve always been confused about this and that. For another, I like working late at night or early in the morning when it’s quiet and the rest of the world is asleep.
The drawback to that is having to go to bed very, very early, so I can wake up at midnight. Still, we all have to make sacrifices of one kind or another. Drinking coffee or an energy drink with enormous amounts of caffeine helps ease the pain.
The phrase is still in everyday use. Porter Wagoner and Dolly Parton released an album called The Right Combination/Burning the Midnight Oil, in 1972. And, Midnight Oil is the name of an Australian rock band.
I suppose the Australian rock band is playing while I’m working. Or are they sleeping? Is it tomorrow or yesterday there? What the time may be in other parts of the world is one of the things that I always find confusing.