this needs mentioning again but a common cause of cronic fatigue is an undiagnosed sleep disorder such as sleep apnea
sadly lots of docs dismiss us as just depressed and/or neurotic and dont really try to help
checking a persons blood oxygen level shortly after waking can help although a few sleep disorder sufferers have oxygen levels that dont go down during sleep
a very common sign of a sleep disorder is heavy and loud snoring, (loud and legendary) 
docs should always ask fatigued patients whether they snore, if living alone, tape record yourself when asleep to find out
sometimes there will be pauses as the person struggles for breath but not always as the apnea of blockage can happen on almost every breath, this is very hard on the heart and usually shortens the undiagnosed sufferers life
a big help now is video cameras that work in low light, set one up to tape yourself when asleep or ask someone to tape you when your snoring is severe, show to doc or a nurse to convince them you really do have a problem
sadly most docs learn of new diagnoses from drug company sales reps when the co starts to sell a patented drug for the disorder,with a multi million sales budget. there are no patented drugs for sleep apnea, just CPAP machines, so few docs know about, or consider a sleep disorder when a patient complains of fatigue