I wonder if there is a correlation of body temperature or parts of a human body and fatigue? If so then a sensitive thermal camera could be a potential measuring device! I can imagine that one could detect the frequency of exhalation with a thermal camera

If you are interested in self reporting  a survey tracking symptoms a participant experiences their daily life with rating scales might be helpful.

For example: At times today I felt like I could not keep my eyes open.

Strongly agree --> strongly disagree. 

With geriatric patients a measure that is used for physical as well as mental issues and could be adapted to other individuals is a measurement of gait speed and number of steps.  You could unobtrusively watch people as they walk over a floor (or outside) area that you know the distance of, and you would collect the time taken and the number of steps.  Physical parameters, including fatigue, should be measurable by a slowing down and increased number of smaller steps.  You can check the geriatric literature as they have a good number of studies on this.

Dear all, thank you all for your responses so far. Really helpful!

@Shahid, I like your idea, but I think that a l ot of factors can influence the body temperature. This concerns both core and surface temperature of the body. However, a device measuring respiratory frequencies might be useful to measure energetic load for example. I will look into that.

@Chelsea, Good suggestion. I was planning to do that as a secondary measure. Main focus however remains on the objective measurement.

@Robert, I will look into that, sounds promising! I have been thinking about using activity trackers as well, but am not completely up to date with literature to determine whether step size or speed is a proxy of physical fatigue.

I should be able to find you better articles about using accelerometer's for tracking activity, but the link below will give you some info.  The gait speed and gait length could be measured unobtrusively, or even filmed for later analysis (just make sure there are clear marking lines on the floor for analysis of distance).

Poster Increasing Early Mobility on a ACE Unit: Tracking the Number...

fatigue is a subjective description, related not only to physical but also to mental status so i dont think any objective indicators could or should attempt to describe it. 

however a response lag to a standard non-threatening stimulus would be a relatively good measure if at all. like responding to a door bell / phone ring / sound of something dropping on the floor from behind. you would have to compare the individual to his / herself though, and that may not be possible unobtrusively. 

While much of fatigue is based upon subjective description, just like pain, it is a subjective symptom that can and should be studied and clearly described.  If we had a rating scale for fatigue, like we do for pain, it would give clinicians additional data on which to base diagnoses, treatments, and/or prognoses.  In the potential study you describe, this would be great, because an unobtrusive observation would be more likely to give valid and comparable data to the clinician. 

To help clarify, I used Taber's Cyclopedic Medical Dictionary and their #1  definition for fatigue is "An overwhelming sustained feeling of exhaustion and diminished capacity for physical and mental work."  While the 'feeling' part is subjective, the 'diminished capacity for physical and mental work' is something that could be studied and needs to have measures developed for proper assessment.  Sander - this give you a lot of good work to do!