To "burn the midnight oil" means to stay up very late working, studying, or applying yourself diligently to a task. It implies exhausting effort and sacrifice of sleep. Students burn the midnight oil before exams; writers burn the midnight oil to finish manuscripts; entrepreneurs burn the midnight oil building their businesses.
Before electric lighting — which only became widely available in the late 19th and early 20th centuries — artificial light came from candles and oil lamps. Both had a cost. Using lamp oil or candles after dark was a measurable expense. Working late literally meant spending money on light.
The phrase captured this reality perfectly: to burn the midnight oil was to consume a resource, to pay a price for your dedication. Late-night effort had a visible, tangible cost.
The earliest known use comes from the English poet Francis Quarles in his 1635 emblem book Emblems, Divine and Moral: "We spend our midday sweat, our midnight oil." The context is one of human effort and labour — burning oil at midnight as a symbol of hard work and sacrifice.
The phrase continued to appear through the 17th, 18th, and 19th centuries, always carrying the same sense of diligent, late-night effort. When electric light made oil lamps obsolete, the phrase survived — its meaning was too vivid and useful to abandon.
This is a common pattern in language: a phrase outlives the technology or practice that created it. Nobody cranks a car to start it anymore, but "cranking it up" still means starting with energy. Nobody uses a literal deadline — a line beyond which prisoners would be shot in Civil War camps — but we still have deadlines. The midnight oil lamp is gone; the phrase remains.
The earliest known use is by the English poet Francis Quarles in his 1635 work Emblems, Divine and Moral: 'We spend our midday sweat, our midnight oil.'
Primarily yes — it refers to intense, late-night effort on any demanding task: studying, writing, business, creative work. The implication is always that significant effort is being applied at great personal cost.