"I was always tired. From the moment I would wake up in the morning until the time I went to bed I would have a lack of energy. It did not make sense for me to feel always tired yet I did. I have not ever led an extravagant lifestyle, I do not make it a habit of being on the run all the time, and I certainly do not have the stress level of other folks the way a single parent does."
That quote was from a patient suffering from Chronic Fatigue Syndrome, but needless to say there are many causes of illness which have similar effects. Sadly doctors frequently have difficulty differentiating the ultimate causes and aren't always able to provide relief from fatigue.