All the words or phrases I know for people who stay up at night merely describe the habit of not sleeping (e.g.: a night owl or insomniac)
I am looking for a word or phrase with an emphasis on working hard at night. These people work are different from insomniacs because they have no problem sleeping but they refuse sleep to work. They are different from night owl because they wake up early in the morning.
To give more context, suppose a nurse with kids who has a normal day life like most people and takes care of the kids but also works most nights in a hospital.