Burn the Midnight Oil

A literal description of working by lamplight before electric lighting existed. The specific phrase "midnight oil" was coined by the poet Francis Quarles in his 1635 collection Emblems, describing nocturnal labour, and the image of literally "burning" it, rather than just using or consuming it, took hold by the 19th century. The picture is simple and accurate: oil lamps needed refilling and tending the later into the night you worked, so working late genuinely meant burning through more oil.