A lot of very good points in this thread for sure.I have always found that the majority of my fatigue is more mental than physical especially if I am instructing on these days and I do not claim to be in the best physical shape at 58 years of age.If we run 40 minute sessions I generally stop about halfway through if I do not have people to "play" with or to chase down during the session.I guess I get bored with just lapping after I get the track figured out as well as I am going to.
When I was racing it was not unusual to do a 3 hour shift in the endurance series we raced in and I never had a great problem with this from a physical point of view but mentally I would be bagged when I got out of the car especially at night.Often my best lap times where at 3 in the morning late in my shift in the dense cool air.But then again that was 20 years ago and I am not sure if I could still do that today for that long a time period
It was a lot easier to maintain a pace with constant communication from our pit and to help with keeping you mentally alert.
Knowing your limits of physical & mental endurance is very important at trackdays for experienced lappers & students alike.I really notice the degradation with 1st time students during the last sessions on their 1st day which of course is being mentally fatigued.