Dean Karnazes: the man who can run for ever

http://www.theguardian.com/lifeandstyle/the-running-blog/2013/aug/30/dean-karnazes-man-run-forever

When running, you break down glucose for energy, producing lactate as a byproduct and an additional source of fuel that can also be converted back into energy. However, when you exceed your lactate threshold, your body is no longer able to convert the lactate as rapidly as it is being produced, leading to a buildup of acidity in the muscles. It is your body's way of telling you when to stop – but Karnazes never receives such signals.

"To be honest, what eventually happens is that I get sleepy. I've run through three nights without sleep and the third night of sleepless running was a bit psychotic. I actually experienced bouts of 'sleep running', where I was falling asleep while in motion, and I just willed myself to keep going."