It's not really a 'condition' - rather, it's a series of symptoms they just lump together and give a name.
They have no idea what it is, what causes it.
It's very difficult to diagnose, other than patients are always 'tired'. And there are many different kinds of 'tired'.
I thought I had it, but I wasn't sleeping enough and drinking too much red bull, not getting enough water, probably too much stress.
Doctors had no clue. All I did was sleep regular hours, stopped drinking energy drinks, drank water - and over a few months I 'got better'. But it was a really weird experience.
Some people get 'it' bad.
My bet is that there are probably a number of reasons that people get it - some of it is psychosomatic - and that it might have something to do with a messed up endocrine system - i.e. the system that regulates melatonin, insulin etc..