Shift work is defined as schedules outside the typical 9 a.m. to 5 p.m. day. Roughly 15 percent of full-time U.S. employees work on shifts outside this traditional schedule. For many, shift work is part of the job as service occupations like healthcare professionals and protective services  are needed 365 days a year 24 hours a day.

The Sleep Health Institute defines two types of shift work schedules: