burn the midnight oil To study or work late into the night; to lucubrate. In the days before electricity students and scholars who wished to read or study at night used oil lamps for light. The term midnight oil for late-night study was in use as early as 1635; the entire phrase appeared somewhat later.

keep one’s nose to the grindstone See PERSEVERANCE

a lot on one’s plate British slang for a lot to do, much to think or worry about.

moonlighting Working a job at night to supplement one’s daytime income. Although it was formerly used in Ireland and other countries to describe nighttime excursions of violence, the expression’s current figurative sense is of American origin. The term is now used frequently in the United States and Great Britain, always in reference to a second job.

salt mines One’s place of employment; any unnamed place, real or imaginary, that represents habitual punishment, confinement, isolation, or drudgery. This expression alludes to the salt mines of Siberia (U.S.S.R.) where political and other prisoners were sent to serve sentences at hard labor. Salt mines often appears in back to the salt mines, a jocular and somewhat derogatory reference to returning to work.

snowed under Overwhelmed; inundated, buried, or overburdened by work or other responsibilities. This expression alludes to the fact that while a single snowflake seems completely innocuous, a large amount of snow can be totally overpowering.