… What is fatigue? It's a sensation of sleepiness, muscle weakness, or a feeling that you don't have the energy to do something - either physical or mental. It's the brain that experiences fatigue; that means there are certain chemical changes in the brain that lead to fatigue - even though those chemical changes may be triggered by many different illnesses. …Komaroff, what the heck????
No, no, no. Fatigue is not "sleepiness". Sleepiness is the urge to sleep – and you can be fatigued without being sleepy. Somnolence is "sleepiness". "Tiredeness" is maybe a combination of fatigue and "sleepiness". But one can be very well fatigued without being tired. Being chronicly "sleepy" and being chronicly "fatigued" are two different symptoms! Were you find one, it is not unusual to find the other, but they a separate sensations.
And fatigue is not the feeling of "muscle weakness". Yes, fatigue and the feeling of "muscle weakness" can go together (e.g. after doing way do much physical work) – but one can be very well fatigued without having the feeling of "muscle weakness".
Yes, one could say fatigue is the feeling that one is "lacking energy".
But for crying out loud, what is the problem with saying:
Fatigue is the feeling of EXHAUSTION.
Or you could bloody well say:
Fatigue is having the urge to rest.
And no, no, no. Fatigue (like for example the feeling of pain) is not simply "chemical changes in the brain". Both pain and fatigue are signals of the physical state of your body – if pain is the gauge of the engine temperature, then fatigue is the gauge of the petrol tank. If pain tells you "Mate, if you keep doing that, something is going end up on the fritz pretty soon", then fatigue tells you "Mate, you are running on reserve, take a rest and replenish your energy or you'll end up with power". Pain is (usually) alleviated by stopping the action that causes the pain, and fatigue is (usually) alleviated by resting. The cause is (usually) not in the brain, it is only registered in the brain.
Words fail me how someone like Komaroff, who is so seemingly methodical, can be so confused with elementary definitions of the diseases he deals with. And I find it shocking that he spreads this confusion.
PS: It is however interesting that he mentions both mental and physical fatigue, but alas, he does it not in a way that is helpful.