“In a primary shift schedule, employees work a fixed shift and occasionally work the opposite shift. This type of shift is most susceptible to workplace accidents and efficiency problems (Owen, 1976)” (Oexman, Knotts, & Koch, 2002, p. 153).

Oexman, R.D., Knotts, T.L., & Koch, J. (2002, December). Working while the world sleeps: A consideration of sleep and shift work design. Employee Responsibilities and Rights Journal, 14(4), 145 - 157.