"Tonight is a work night" is -- most likely -- an allusion to the more common phrase "Tonight is a school night".
"Tonight is a school night" is often used to indicate that one must get to bed early tonight in order to get up early tomorrow for school.
From Cambridge dictionary
noun [ C ]
US /ˈskuːl ˌnaɪt/ UK /ˈskuːl ˌnaɪt/
the evening before a day when children have to go to school. Adults also use this expression informally to describe the evening before a day when they have to get up to go to work. :
I don't usually stay out this late on a school night.