burn the midnight oil

/bˈɜːn ðə mˈɪdnaɪt ˈɔɪl/

to burn the midnight oil

to do something until the very late hours of the night

Examples

1. Lawmakers were burning the midnight oil last night, as the debate continued.

2. Then in radiant secrecy Louisa burned the midnight oil and prepared the little book for the press.

3. This guy's getting that school work done, burning the midnight oil.

4. And we're burning the midnight oil at our small town casino tonight. -