The Origin of Tiresomeness (the Mental Kind)
Perhaps it is not the specific attributes of certain individuals to feel tired out of the blue, for no reason in particular. There was no exercise of any sort done recently, nor has there been any particular mental task that required so much thinking that it strains the body for just supplying oxygen to the brain. A person can just sit there, surf the Internet, be completely passive for a couple of hours, and still manage to feel absolutely tired out in a short period of time. It really makes the person think about exactly what is causing the energy drain.