Burn the midnight oil
Definition 1 of 1
Pro Tip 1/3Metaphorical Use
This phrase uses 'burning oil' metaphorically to mean working hard late at night. 
During the project, we had to burn the midnight oil to meet the deadline.
Pro Tip 2/3Figurative Expression
It's a figurative way to say someone is staying up late, not literally burning oil. 
She burned the midnight oil to finish her thesis.
Pro Tip 3/3Intensity Implied
Implies intense focus or effort, not just being awake at night. 
He's burning the midnight oil every night to become a top athlete.