Prescriptive rules dictating hours of service (HOS) limitations often include recommendations to limit the time a person spends on a task. The logic here is that it becomes increasingly difficult to stay focussed on a task the longer you spend on that task. In other words, the longer you spend focussed on one activity, the less likely you are going to be able to keep doing it well. This is usually blamed on fatigue. While this may be a correct attribution, it is being blamed on the wrong type of fatigue.
Focussing on completing one action, is more likely to cause mental fatigue than sleep-related fatigue. For example, if you ask a person how they feel after driving for 6 hours during the middle of the day, they will likely say they are fatigued; or if you ask an air traffic controller (ATC) how they feel after dealing with a very high air traffic volume for 3 or 4 hours, they might say they are fatigued. But if you take the same two people, wire them up with electrodes and sensors to monitor signs of sleep-related fatigue, they won’t show anything different than people who have not been driving or controlling aircraft. The fatigue the driver and ATC are reporting is mental fatigue. This type of fatigue can impair performance, especially when the task being performed is monotonous or intensely demanding. This means that it does have to be managed. One way to manage mental fatigue is to give your brain a break from the mental task. That is, stop doing what you are doing for a while and let your brain rest, and then return to the task.
If however, the driver or ATC is also experiencing sleep-related fatigue due to being awake at night or not sleeping enough, then simply taking a break will not bring the driver’s or ATC’s performance back up to a safe level. For a break to be effective in reducing sleep-related fatigue, it must include sleep or a stimulant like caffeine.
The need for sleep or caffeine during the break becomes even more important when you combine time on the task (i.e,, mental fatigue) with increasing levels of sleep-related fatigue. Studies have shown that even if you perform the same task for the same period of time, your performance gets worse with increasing levels of sleep-related fatigue. In other words, even if you are only driving 6 hours a day or controlling aircraft for 3-4 hours at a time, if you don’t get your full complement of 7-8 hours of sleep every day or if you don’t nap or consume caffeine during your breaks, your risk of a negative outcome like a car crash or loss of separation of aircraft increases as the work week progresses.
While the duration of time we spend on tasks at work might stay the same across a week, without a couple of days off every now and then, most of us build up a sleep debt. The goal of limiting the time on the tasks we perform at work is to reduce how long we are exposed to the negative effects of combining mental fatigue with the inevitable increasing levels of sleep-related fatigue across a work week.
Take Home Points:
- Limiting the time on task, reduces the risk exposure time for workers who are likely already fatigued.
- Maximize the effectiveness of your breaks by taking a nap and consuming caffeine during your break.
 Rest breaks can reduce the likelihood of negative outcomes, see for examples:
 Klaas, E. (1978). Psychological effects of immoral actions: The experimental evidence. Psychological Bulletin, 85, 756-771.
 See for examples:
(A) Philip, P., & Åkerstedt, T. (2006). Transport and industrial safety: How are they affected by sleepiness and sleep restriction? Sleep Medicine Reviews, 10, 347-356.
(B) Sagaspe, P., Taillard, J., Chaumet, G., Moore, N., Bioulac, B., Philip, P. (2007). Aging and nocturnal driving: Better with coffee or a nap? A randomized study. Sleep, 30, 1808-1813.
(C) Schwarz, J., Ingre, M., Fors, C., Anund, A., Kecklund, G., Taillard, J., Philip, P., & Åkerstedt, T. (2012). In‐car countermeasures open window and music revisited on the real road: popular but hardly effective against driver sleepiness. Journal of Sleep Research, 21, 595-599.
 See for example: Haavisto, M., Porkka-Heiskanen, T., Humlin, C., Härmä, M., Mutanen, P., Müller, K., Virkkala, J., Sallinen, M. (2010). Sleep restriction for the duration of a work week impairs multitasking performance. Journal of Sleep Research, 19(3), 444-454.