Chronic fatigue gets used a lot as a catch-all diagnosis. My dad got "diagnosed" with it when he started feeling tired all the time and stopped having the energy to run. His normal weekly mileage went from 15-20 down to half that or less, very suddenly. Turns out he had injured his knee and developed a rare auto-immune disorder. Doctors have tried to pin chronic fatigue on my mom too. Nope. Habitual insomnia due to sleep apnea is the major cause, helped along by her tendency towards major depression and (at the time) her need for a hip replacement. Mom basically ignored her doc, since there are plenty of good explanations for why she's tired. Dad listened for a while, but kept harping on the symptoms that didn't fit. In both cases the doctor had decided they were middle aged semi-hypochondriacs and was trying to get rid of them with an easy diagnosis.