I've found that I HAVE to stay in shape, sleep well and eat some approximation of healthy to maintain my mental performance for long periods, so many issues that I blamed purely on psychology or "how I was wired" really came down to I wasn't working out at all, was drinking too much on the weekends, and sitting in a cubicle for 8-9 hours a day.

That said, even today I can definitely work myself into a state where my brain just collapses if it attempts to do anything complex, and doing even the simplest chores seems like climbing Mt. Everest.

However I have some morbidly obese co-workers who still manage to keep up with me and stay sharp for longer in some cases. Likewise I've read some blogs about some people who genetically can be at full capacity, or near to it, off of 5 hours of sleep consistently. I'm honestly jealous

I'm sure he has a strong will but I think Carmack is some variety of biologically gifted for sure, lucky bastard.