The phrase 'burning the midnight oil' implies working overtime and working over normal standards.
Example of use: “The assignment was very difficult, so I was forced to burn the midnight oil to get it done on time"
Though this is a very old phrase, since oil lamps have become almost redundant today, it is still commonly used. It simply means working hard doing something late at night. However, nowadays, it also implies sleep deprivation, which is considered very unhealthy.