When organizations are setting up their fatigue risk management systems, they will always run into the challenge of assessing fitness to start or continue duty. You will often hear people saying “We cannot simply ask our shift-workers if they are fatigued because people are a poor judge of their own fatigue levels”.
There is some truth to this position for a few reasons. If you ask someone if they are “fit and well rested for duty” they may not answer accurately because they fear negative consequences of reporting for work in an unfit state. Many companies with poor safety cultures send their workers home without pay if they report for duty unfit. Loss of income is a big incentive to work even if you are unfit.
Second, if you ask someone “Are you fatigued?”, and they are not experiencing symptoms of extreme fatigue like head nods, slow eye blinking or micro-sleeps, then they will probably say they are not fatigued because there are no overt cues to tell them they are fatigued.
Third, fatigue is not a binary state. It doesn’t get turned on and off like the flick of a light switch. Fatigue levels range across a spectrum from wide awake to fully asleep. This means that asking people to answer yes or no to “Are you fatigued?” is forcing them to choose between wide awake and almost asleep. Again, if there are no overt cues, then the logical answer will be “No, I am not fatigued”.
What we need to do is rephrase the question and provide the person with the ability to assess their level of fatigue on a spectrum and instead ask “How fatigued are you?”. Studies have shown that questionnaires that ask people to rate their level of fatigue can be accurate and correlate well with objective measures of sleepiness like changes in electroencephalograph patterns (brainwaves). But even then, accuracy of the subjective questionnaires isn’t consistent across fatigue levels. It is more accurate when people are experiencing higher fatigue levels likely because the fatigue symptoms are more recognizable.
Take Home Point
The take home point here is that you can’t simply ask people if they are fatigued. If you are going to use subjective estimates of fatigue in your fitness for duty assessment, you need to use a questionnaire that gets the person to rate their levels of fatigue, and preferably a standardized and well-researched questionnaire. But keep in mind that the subjective fatigue assessment is going to be more accurate when people are experiencing higher levels of fatigue.
 See for examples:
(A) Kaida, K., Takahashi, M., Åkerstedt, T., Nakata, A., Otsuka, Y., Haratani, T., & Fukasawa, K. (2006). Validation of the Karolinska sleepiness scale against performance and EEG variables. Clinical neurophysiology, 117(7), 1574-1581.
(B) Tremaine, R., Dorrian, J., Lack, L., Lovato, N., Ferguson, S., Zhou, X., & Roach, G. (2010). The relationship between subjective and objective sleepiness and performance during a simulated night-shift with a nap countermeasure. Applied Ergonomics, 42, 52-61.
 Tremaine, R., Dorrian, J., Lack, L., Lovato, N., Ferguson, S., Zhou, X., & Roach, G. (2010). The relationship between subjective and objective sleepiness and performance during a simulated night-shift with a nap countermeasure. Applied Ergonomics, 42, 52-61.