The horrible thing about chronic fatigue, is that you don't realise how chronically fatigued you are. Bad judgement is often the result of fatigue. Bad judgement like "I feel good enough to fly". Acute fatigue is easy to self assess, I am sure most experienced pilots at some time or another have felt a line has been crossed and called in unfit to fly.
Are we talking about long term chronic fatigue or acute short term went to bed too late had a bad night fatigue?