“The day is divided into three working periods – morning, afternoon and night. This kind of shiftwork usually, but not always, involves one or more weeks of mornings, followed by one or more weeks of afternoons, followed by one or more weeks of nights” (Hurrell, 2003, no page).
Hurrell, K. (2003, March). Statistics on shiftwork: Survey questions, definitions and classifications. Retrieved August 3, 2005, from http://www.iwh.on.ca/media/images/Shiftwork.pdf