No need to repeat that 'fatigue' can be hard to describe and even harder to demonstrate objectively. A new blog on health rising suggests that deterioration in rapid eye movements (aka saccades where both eyes move rapidly to scan or fixate on a target) may objectively demonstrate increasing fatigue in healthy controls and a range of conditions (including breast cancer survivors) : https://www.healthrising.org/blog/2018/02/20/objective-fatigue-measure-chronic-fatigue-fibromyalgia/ Interestingly a ME/CFS specific study found some impairment in saccades but marked impairment in tracking a smoothly moving target (smooth pursuit) : http://irep.ntu.ac.uk/id/eprint/25859/1/221920_3205preprint.pdf It would be good if we could piggyback on fatigue research in other areas.
This is one of the papers linked in Cort Johnson's article. https://www.ncbi.nlm.nih.gov/pubmed/28619425
And here's the one @Marco linked on ME/CFS patients: http://irep.ntu.ac.uk/id/eprint/25859/1/221920_3205preprint.pdf