I think the distinction needs to be made between short term tiredness and long term fatigue.
If you are doing a night duty (or not acclimatised to local time) you might expect to be "tired" at certain times.
Longer term fatigue though is more "battle damage" brought on by successive changes from day to night flights coupled with long duty days.
There have been occasions over the last 40 years of my flying where I have been unable to get any meaningful sleep prior to (for example) an early start. I remember one when I said to the First Officer (as we checked in for a 11 hour duty 4 sector early at 0500) "I hardly got any sleep last night" to which he replied "I've been called from standby and only got 2 hours sleep myself!" So what to do? We completed the duty but I would not like to profess that we were in the best state to do so! Legally we should have called in not fit to fly but this seems to be unofficially frowned upon under the veneer of "we care about safety".
Also I would like to comment on another aspect. Will the new pilots who are entering the profession now be "burned out" by the time they get into their thirties? This constant abuse of the body clock and long duty cycles over many years must eventually exact a toll.