I once worked with a guy who only ever slept about 4 hours a night for years. He would then run a full marathon into work every morning. I can tell you right now, his body was there but his ability to do anything other than make small tactical decisions was destroyed. Having a strategic meeting about the direction of the company was pointless, as you could nearly see has brain unable to compute and think at that level. The end of the story was that he died on Mount Everest and the company has basically collapsed because of his poor management and some other financial irregularities.