Have you ever thought about the different meanings of the word “fatigue”? Here are 7 uses of the term:
- Fatigue = the weakening of metal due to repeated use or bending; the word is often used this way in the physical engineering sciences; I call this “metal fatigue”, don’t confuse this with “mental fatigue”
- Fatigue = a human psychological state resulting from spending extended or intense periods of time on a task like studying for an exam; I call this “mental fatigue”
- Fatigue = a human physical state resulting from body movements like exercise; I call this “physical fatigue”
- Fatigue = a human psychological and physiological condition of lethargy that can result from a number of illnesses such as depression or the common cold; I call this “lethargic fatigue”
- Fatigue = a biological response to over stimulation of one of the senses; for example, the olfactory receptors will fatigue after prolonged exposure to a smell, this results in a temporary inability to distinguish that specific smell; I call this “receptor fatigue”
- Fatigue = a point of diminishing returns, where the effort required to stay on task is greater than the effort required to move forward on the task; I call this “task fatigue”
Finally, the fatigue I am most interested in, I call “sleep related fatigue”:
- Fatigue = a human biological state of sleepiness
This type of fatigue has different levels, ranging between being asleep and being fully awake. At the extreme fatigue end, you would have difficulty maintaining wakefulness and, without someone forcing you to stay active and aroused, you would fall asleep within five minutes. At the other end of the range, you would be wide awake and you would find it hard to fall asleep within ten minutes.
These are just a few types of fatigue. I am sure that with a bit of creative thought you could think of more. All of these types of fatigue, with the exception of “metal fatigue”, can impair human performance. When you are looking for the cause of impaired performance, make sure you know which fatigue you are talking about. You wouldn’t want to go looking into a person’s sleep (sleep related fatigue), for reasons why they could not smell a fuel leak (olfactory receptor fatigue).
 This level of fatigue is referred to as pathological sleepiness in the clinical sleep medicine literature. See for example: Carskadon, M., Dement, W., Mitler, M., Roth, T., Westbrook, P., & Keenan, S. (1986). Guidelines for the multiple sleep latency test (MSLT): A standard measure of sleepiness. Sleep, 9(4), 519-524.
 See for example: Carskadon, M., Dement, W., Mitler, M., Roth, T., Westbrook, P., & Keenan, S. (1986). Guidelines for the multiple sleep latency test (MSLT): A standard measure of sleepiness. Sleep, 9(4), 519-524.