Whilst 411A is undoubtedly correct about what some pilots consider to be 'restful activities', the blunt facts are that crew rostering practices are run by a soulless computer which will aim to squeeze every last minute out of them which it legally can.
And yes, the perception of one's performance does indeed go rapidly downhill with the insidious onset of fatigue.
Mind you, 411A, there are also those wet-pants who whine about 'human factors' or 'CRM factors' whenever they're criticised or given direct analysis of their failings - whereas in actual fact they were just cr@p!