what is midnight
For other uses, see Midnight (disambiguation). Midnight is the transition time period from one day to the next: the moment when the date changes. In ancient Roman timekeeping, midnight was halfway between sunset and sunrise (i.e., solar midnight), varying according to the seasons.
burn the midnight oil. Fig. to stay up working, especially studying, late at night. (Alludes to working by the light of an oil lamp late in the night.) I have a big exam tomorrow so I'll be burning the midnight oil tonight. If you burn the midnight oil night after night, you'll probably become ill.