Is it difficult to ask a science-based question without an answer? E.g. Should the relevant literature be known in detail to ask such a question?

It can be psychological, emotional, physical, intellectual, for instance!

I am no expert in this field but have heard about "fatigue syndrome", the underlying (despite ) mechanisms mostly are unclear / not elucidated yet. At least when it comes to (in hum. diagnostics, e.g. muscle biopsies) with a clinical diagnosis of "fatigue".

Usually this syndrome is said to include a variety of symptoms as a consequence of diverse chronic diseases (e. g. cancer, and others) perhaps can be seen as one kind of independent .

@ Marcel: Sorry for not being exhaustive...one for sure should "dig" into literature references and reports available. Regards, Wolfgang

fatigue is often linked to changes in body measurements, but it does not explain the exact proximate cause why people feel fatigue. It must something have to do with the nervous system, or not? For instance is the feeling of fatigue reflected in the activation of a specific part in the brain?

Dear Marcel thank you for sharing this question with me.  According to http://en.wikipedia.org/wiki/Central_nervous_system_fatigue  “Controlling central nervous system fatigue can help scientists develop a deeper understanding of fatigue as a whole”. When I am really tired, I drink coffee and caffeine allows me to hide my fatigue and to reach a performance on work that would be difficult to reach without this stimulant. But naturally the best is to have enough rest between our journeys of work or enjoy holidays. I agree with the hypotheses enumerated by Marwan and Wolfgang.

http://en.wikipedia.org/wiki/Central_nervous_system_fatigue

People with super-sensory abilities (medium) provide alternative explanations.

Perhaps stimulation of well-defined brain parts might induce the impression of fatigue in the absence of fatigue due to physical activity?

Do people with so-called burn-out are victims of mentally-based fatigue, e.g. excessive stimulation of well-defined brain parts independent from physically-based  fatigue, e.g. due to excessive physical activity?

See more on BBC World News: Why do we feel tired? (6 December 2014)

The feeling of fatigue of body X is a signal only addressed to the owner of body X, ignored by others in a communication Framework? How can feelings of fatigue be synchronized across individuals in a cooperation Framework?

Communication networks: different scales of analysis involved?

1) A neuroendocrine communication network based on exchange of signals via neurons/chemicals within the individual body X, e.g. brain versus muscle facing different levels of 'fatigue' but brain and muscles somehow have to 'collaborate' to ensure longterm body performance versus

2) A communication network based on signalling at the between-individual level where different individuals belonging to the same collaboration network might face different levels of 'fatigue' because of X reasons but somehow have to 'collaborate' to ensure longterm group performance?

The fallacy of measuring a feeling

http://estebankolsky.com/2009/05/the-fallacy-of-measuring-feelings-or-how-to-capture-lighting-in-a-bottle/