Length of sessions help or hurt psychology?

I just finished an overnighter at Fallsview Casino in Niagara Falls. 4 friends got a few hotel rooms. The long and the short of it is this. I played two sessions, both about 6-8 hours, and got some sleep overnight. A few friends played through the night, around 20 hours straight. My question is what is everyones optimal point of effectiveness? Between what hours are you maximizing your poker playing abilities at? When is your body at its maximum effectivemness to pay attention/make correct reads/etc.

First, it takes me personally about 45 mins to get comfortable, and reads on players. I can play fairly well for upwards of 8 hours, but then start losing focus. My mental game slips, and I become pretty robotic, playing cards only.

My friend told me he has no problem lasting 20+ hours at one session, but I find this hard to beleive. His bloodshot eyes tell me something different.

Howie