Flight: Some get tired just thinking about exercise, or moving (I do). That's the brain protecting the body from harm. No need to say that strengthens fatigue... and pain and stiffness;..
Freeze: You feel powerless against fatigue, you see it as a enemy that can't be beaten, and wait for better days to come when the enemy leaves. Nope, that's also passive coping with AS.
All three are survival instincts, not being lazy, being cowardly, it's NORMAL.
Can fatigue be measured or seen? Yesss. A Canadian study published feb 2014 showed NMR scans proving that AS patients had significant differences in the ratio white/ grey areas compared to healthy test persons. So, indeed it's between our ears, but not in the way most say! Though it could be proven, they still don't know why. BTW, a test with pain gave similar results.
Well meant advise, but wrong advise...
Just try to sleep better. 35 to 90% of AS patients have problems sleeping. Not deep enough (ie no recuperation), problems falling asleep, restless leg syndrome (double as much as in normal people) Anti-TNF normalizes sleep, and improves the quality, but fatigue remains.
Take better anti-inflammatory meds. No, They think the ratio differences in the brain are caused by inflammation inside the braincells. The Anti-TNF molecules can't get passed the blood/ brain wall. NSAID's can get passed that barrier (are smaller), but they bind with proteins before the barrier, so are blocked too. Paradox: these meds work very good against inflammation and pain, but have no effect what so ever against fatigue.
Get better pain-treatment. Ok having pain is tiresome, and when you're tired you can tolerate pain less. So: take away the pain and you'll feel fitter? No... it's a paradox, but the fatigue remains present even when pain-free.
But also read the side effects of most medications.... On top: fatigue, depression, sleeplessness...
So, hopefully this made any sense, and was not too boring.