Probably the best point in time tool to assess sleep-related fatigue is the Karolinska Sleepiness Scale (KSS). It is well researched, easy to use in practical settings, and has good face validity, which means it appears to be measuring what it is supposed to measure. Research has shown that fatigue scores from the KSS are correlated with brain (EEG) and behavioural (Psychomotor Vigilance Task/PVT) indicators of sleepiness/sleep-related fatigue. This means the KSS is a valid measurement of sleepiness[1].
Karolinska Sleepiness Scale
There are a few different versions of the KSS being used in practical settings. The original scale is a nine point scale with sleepiness levels ranging from “Extremely alert” to “Very sleepy, great effort to keep awake, fighting sleep”. One modified version includes a 10th level of sleepiness using the label “Extremely sleepy, can’t keep awake”[2]. Another modified version of the KSS has labels on every second step rather than on every step like the original nine point scale[3].
Being a point in time assessment tool, it can be used in fitness for duty assessments. In fact, some regulators, like Transport Canada, require its use for assessing fatigue before starting work. If you are searching for a conclusive answer to “Where should we set the acceptable limit to fatigue for starting a shift?”, you won’t find it. The reason you will not find a limit to an acceptable fatigue level is because the level is dependent on the tasks being performed and the level of desired safety or level of risk you are willing to accept. Plus, there could be a liability issue if an organization sets an acceptable level and then people following that rule experience a negative fatigue-related outcome…they could potentially blame the organization that set the acceptable level.
The best guidance in selecting an acceptable fatigue level for fitness for duty assessments is to use 7 as a beginning point only. At a KSS score of 7, a person will describe their state of fatigue as “Sleepy, but no difficulty staying awake”. At a KSS score of 8, they will describe their state of fatigue as “Sleepy, some effort to stay awake”. This means that, as KSS scores surpassed 7, there is increasing difficulty in maintaining a state of wakefulness. Performing safety critical activities while experiencing this difficulty is undesirable. Setting an acceptable maximum level of 7 as a fitness for duty cut off, is therefore, a good place to start. But your continuous improvement process should aim to reduce sleep-related fatigue and drive the KSS lower before all shifts. This really is just a beginning point for your fitness for duty assessment development.