Having done way too much of this type of flying hauling both pax and freight, i know that if I had flown the schedule that they (especially the captain) had flown, that my cognitive capabilities would be significatly impaired compared to my well rested sefl.
Here is what the FAA said about it, in FAA Circular 117-3, which "was developed to demonstrate acceptable methods of compliance with Title 14 Code of Federal Regulations (14 CFR) part 117, § 117.5, Fitness for Duty".
"Fatigue is characterized by a general lack of alertness and degradation in mental and physical performance. Fatigue manifests in the aviation context not only when pilots fall asleep in the cockpit during flight, but perhaps more importantly, during the task-critical takeoff and landing phases of flight. Reported fatigue-related events have included procedural errors, unstable approaches, lining up with the wrong runway, landing without clearances, and poor decisionmaking."
Stress has the same affect, but adding fatigue to the equation will magnify any stress related congnitive impaiment. I can only speak for myself, but I know that I am much more at the "top of my game" if I am well rested.