Most Hours of Service (HOS) rules designed with fatigue science in mind, state that there must be a limit to the number of consecutive night shifts. This is because there is a tight relationship between the risk of fatigue-related incidents and the number of night shifts someone works in a row.
Fatigue increases across successive night shifts for a number of reasons. But the two most likely contributors are shorter daytime sleep and circadian disruption. Many shift-workers find it difficult to sleep more than 6 hours during the day. Sleeping 6 hours per day for a week results in a sleep debt of 21 hours. A sleep debt this big could make you as unsafe as someone with a blood alcohol level of 0.095% or higher. Circadian disruption has been linked to fatigue as well as decreased well-being and performance, and increased risk of insomnia, and gastrointestinal disorders.
Good HOS rules also put limits on the duration of night shifts. This is because fatigue-related incident risk increases exponentially as shift duration exceeds 8 hours. For example, a 10 hour shift increases the likelihood of a fatigue-related incident by 13% and a 12 hour shift increases the risk by 27%.
Where you place your limits for duration and consecutive night shifts ultimately depends on your level of risk tolerance. Not just your tolerance for safety risk, but your tolerance for risks to the health and productivity of your workforce. If you absolutely have to include a night shift in your rotation, two principles, based on the science, to keep in mind are:
1-Shorter shifts are always better, even for day shifts.
2-Less consecutive shifts are always better, even for day shifts.
With a safety conscious approach, and a well-developed fatigue risk management system (FRMS) that includes training on fatigue prevention strategies and countermeasures, one or two 8 hour night shifts followed by at least two nights of recovery sleep is a good limit to consider.
At the other end of risk tolerance, you may be thinking that permanent nights would solve the problem because people would adapt to the night work. Don’t do it. Even permanent night shift workers are at risk, with 42% of them experiencing excessive sleepiness at work and 15% having sleep attacks (sudden episodes of falling asleep).
The upper limit for the duration and number of consecutive night shifts seems to be four 12 hour shifts, if you are comfortable with an increase in risk. Going beyond this and pushing it to six night shifts in a row can make it dangerous for the shift worker to drive home after work
 See for example: Wong, I., Popkin, S., & Folkard, S. (2019). Working Time Society consensus statements: A multi-level approach to managing occupational sleep-related fatigue. Industrial Health, 57, 228-244.
 See for example: Ohayon, M., Smolensky, M. & Roth, T. (2010). Consequences of shiftworking on sleep duration, and sleep attacks. Chronobiology International, 27, 575-589.
 See for example: Roehrs,T., Burduvali, E., Bonahoom, A., Drake, C., & Roth, T. (2003) Ethanol and sleep loss: A “dose” comparison of impairing effects. Sleep, 26(8), 981-985.
 Anch, A., Browman, C., Mitler, M., & Walsh, J. (1988). Sleep: A scientific perspective. New Jersey: Prentice-Hall.
 Most studies show that limiting shifts to 5 consecutive traditional diurnal shifts and 2 consecutive night shifts is an effective fatigue risk management. See for examples:
(A) Wong, I., Popkin, S., & Folkard, S. (2019). Working Time Society consensus statements: A multi-level approach to managing occupational sleep-related fatigue. Industrial Health, 57, 228-244.
(B) For a discussion of the advantages and disadvantages of 12 hour shifts, see this white paper: https://www.circadian.com/blog/item/15-8-major-disadvantages-of-12-hour-shifts-a-manager-s-perspective.html. When reading this paper, beware that it suggests there is no difference in fatigue levels between 8 and 12 hour duties and that this is in stark contrast to the WTS consensus statement which states that longer duties increase the risk of a fatigue-related incident.
(C) For an example of the negative effects of instituting a shift system that was not desired by the workforce see: Frick, B., Simmons, R., & Stein, F. (2018). The cost of shift work: Absenteeism in a large German automobile plant. German Journal of Human Resource Management, 32(3-4), 236-256.
 See for example: Tepas D. I., & Monk, T. H. (1987). Work schedules. In G. Salvendy (Ed.), Handbook of human factors (pp. 819-843). New York: John Wiley & Sons.
 If you extend the duration of the shift from 8 to 12 hours and decrease the consecutive shifts from 5 or 6 to only 4, the risk of a work place incidents increases by 25% and greater for night shifts, see Folkard, S., & Lombardi, D. (2006). Modeling the impact of the components of long work hours on injuries and “accidents”. American Journal of Industrial Medicine, 49, 953-963.
 See for example: Huffmyer, J., Moncrief, M., Tashjian, J., Kleiman, A., Scalzo, D., Cox, D., Nemergut, E. (2016). Driving performance of residents after six consecutive overnight work shifts. Anesthesiology, 124(6), 1396-1403.