burn the midnight oil

To burn the midnight oil means to stay up late working, such as at work or studying for school. It is usually said pertaining to school work, such as in college when you stay up very late working on homework or school projects. The phrase dates back to when people would stay up light at night using an oil lamp for lighting in their room. The phrase is still used today when describing how they stayed up late working or studying, even though using an oil lamp is not what is assumed.