Duration of Irregular Night Shifts – Limits to Consider

Shorter shifts reduce the risk of a fatigue-related incident according to the recent consensus statement from the Working Time Society (WTS)[1].  The WTS consensus is that longer shifts, night shifts, successive shifts and sequences of shift structure increase the risk of a fatigue-related incident.  This means that an 8 hour night shift is safer than a 12 hour night shift.  It also means that day shifts are safer than night shifts and that less shifts in a row is safer than more shifts in a row and that the type of shift rotation can also play a role in safety.

It seems pretty clear then that we should be shortening the duration of night shifts as much as possible and keeping the number of night shifts in a row to as low as possible.  Unfortunately, it is not that simple.  There are unintended consequences, advantages and disadvantages of this approach that must be considered.  For example, one unintended consequence of shortening the night shift to 8 hours is that if the shift ends at say 0400, workers may be driving home at the worst time…during their circadian trough, a period of time when fatigue is at its highest and performance is at its lowest.  While the shift worker may be safer at work, this approach puts the worker at higher risk of a motor vehicle accident on the drive home.

night driving
An advantage of a shorter night shift is that it is easier to stay focussed on a task if you have to do so for only 8 hours versus 12 hours.  This can be particularly important in safety critical tasks where constant monitoring is required like pipeline control room operations, long surgical procedures, operating a train, or flying a plane.

A disadvantage of a shorter night shift is that you have to work more night shifts to make the same amount of money and reach your weekly hourly limit.  In other words, longer shifts (days or nights) are more efficient from an earnings perspective and provide more time off in between consecutive duties and this is usually preferred by the shift worker. 

If you carefully consider all the advantages and disadvantages[2] of the 12 hour duties versus 8 hour duties, engage the workforce in the process of deciding on shift duration[3], and you expertly manage risks through safety management systems or fatigue risk management systems, you can probably make either duration of night duty work.  For example, you may still be able to end a night shift at 04:00 if you assess worker fatigue levels at the end of duty and provide adequate rest facilities if workers are too fatigued to drive home. However, this all flies out the window when night shifts are irregular.

The problem with irregular night shifts is that there is no well-established scientific findings on them to guide us.  That is, all the good research on night shifts is on fixed schedules where the workers had advanced notice of the days they would work and the shifts started and finished at consistent times.  This means we really don’t know what risks we are dealing with for irregular night shifts.  Risks could be extreme or risks could be lower than expected.

So now what do we do for industries like healthcare, rail and aviation where irregular night shifts are required?  We prioritize safety over everything else and exercise caution by limiting night shift duration and consecutiveness.  Transport Canada has taken this cautious approach for aviation by reducing night duties for pilots to as little as 9 hours and 3 consecutive night shifts.  Flight schedules are known well in advance and these limitations might be sufficient in similar industries with greater degrees of predictability to night shifts. However, when night shifts are irregular and there is limited predictability to work days and inconsistent shift start and finish times, more cautious limitations are needed. To prioritize safety, a cautious approach would be to reduce night duties to the standard work day of 8 hours and to limit consecutiveness to 1 or 2 nights until we have a well-established body of scientific findings to guide us.


[1] 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.

[2] 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.htmlWhen 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[1] which states that longer duties increase the risk of a fatigue-related incident.

[3] 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.