Wiktionary
burn the midnight oil
vb. (context idiomatic English) To work studiously, especially late into the night.
Usage examples of "burn the midnight oil".
So off she fluttered, blowing me a kiss, and I snarled away to the Horse Guards, where I had to burn the midnight oil over sapper transports.
The two brothers had evidently come to the lab directly from a session of evening flyfishing, and had felt impelled to burn the midnight oil.
The authorities have provided me with a certain amount of work in my own field, but it seems to be as much an occupational therapy as anything else, and I'm never forced to burn the midnight oil.