You Splitter has made a very valid point, in that crews often don't help themselves, but I will of course acknowledge that rosters have a major impact upon their lives.
The term fatique should not be confused with 'tiredness'. Bader described the difference between the two as being, 'if tired you can be arosed to full awareness and competence', whereas in 'fatique your performance is almost certainly going to be below normal response level'. You shouldn't be flying if you believe yourself to be fatiqued.
Pilots have been flying with CAP371 based FTL schemes for over 30 years now, I believe the profession would have killed it off a long time ago if it did not offer protection from 'Fatique', but readily acknowledging the NOTAOCH on pushing it too the limits of the rules.