Linking fatigue to the amount of time we put into work is very difficult. This is because work hours are almost independent of sleep. Sure, the more we work, the less time we have for sleep. But people who work very little can still be fatigued and people who work a lot can still be fine. For example, I could be a part time worker, putting in as little as three or four hours per day. This leaves me with 20 or 21 hours available for sleep. This does not guarantee that I will get my six to nine hours of sleep per 24 hours of life. What if I am a part time worker because my passion is volunteer work and I spend eight hours at it after my four hour duty day, plus I have a family and household to run? The 20 hours dwindles down significantly and leaves very little time for sleep. The reverse is true as well. I could work seven hours a day, every day for months at a time and still be fine if that is all I do. No volunteer work, no family or household to manage, all I do is work, relax and sleep. I would be fine.
Rather than try to link fatigue to weekly and daily hours of work, I look at areas where the link is a lot tighter. Research on accidents, injuries and occupational illnesses shows that the more we work, the higher the risk of these negative outcomes. Some of this could be due to sleep-related fatigue and some could be due to other types of fatigue like mental and physical fatigue.
We can use this research to set up processes within a Fatigue Risk Management System (FRMS) to mitigate negative outcomes at daily and weekly duty times and to create hard-stop time limits for prescriptive hours of service rules.
Here is what the research shows:
- Negative outcomes become apparent at just over eight hours on duty, reliably evident at 10 hours and increase dramatically to unacceptable levels as daily work hours surpass 12 hours[1].
- Negative outcomes become apparent at about 50 hours and reliably evident as the work week surpasses 60 hours[2].
References
[1] See for examples:
(A) Dembe, A., Erickson, R., Delbos, R., & Banks, S (2005). The impact of overtime and long work hours on occupational injuries and illness: New evidence from the United States. Occupational and Environmental Medicine, 62, 588-57.
(B) 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.
(C) Rosa, R., & Bonnet, M. (2007). Performance and alertness on 8 h and 12 h rotating shifts at a natural gas utility. Ergonomics, 36(10), 117-1193.
(D) Sallinen, M., & Hublin, C. (2015). Fatigue-inducing factors in transportation Operators. In S. Popkin (Ed.) Reviews of Human Factors and Ergonomics: Worker Fatigue and Transportation Safety, 10, 138-173.
(E) Wagstaff, A., & Sigstad Lie, J. (2011). Shift and night work and long working hours – A systematic review of safety implications. Scandinavian Journal of Work, Environment & Health, 37(3). 173-185.
(F) Wong, I., Davies-Schrils, K., Popkin, S., Folkard, S. (2017). Safety interventions to mitigate occupational sleep-related fatigue. Working Time Consensus Paper, retrieved from: http://www.workingtime.org/page-1842455.
[2] See for examples:
(A) Dembe, A., Erickson, R., Delbos, R., & Banks, S (2005). The impact of overtime and long work hours on occupational injuries and illness: New evidence from the United States. Occupational and Environmental Medicine, 62, 588-57.
(B) Sallinen, M., & Hublin, C. (2015). Fatigue-inducing factors in transportation Operators. In S. Popkin (Ed.) Reviews of Human Factors and Ergonomics: Worker Fatigue and Transportation Safety, 10, 138-173.