As practical examples, by the way... There's a Theravadin monk named Dhammanando who used to post at our sister-site, Dhammawheel. He was extraordinary -- he only slept a few hours a night, then it was just constant work. He got malaria several times. I worry that one day a great man like him will die early of a heart attack or stroke because he pushes himself too hard.
Because I can give you another example... I had a co-worker at Wal-Mart who also only slept a few hours. Instead of being a monk, this guy went to parties. He didn't use drugs or alcohol, though -- just talked and had sex. Well, I was amazed at how he was able to stand up all the time, looking as if he was about to pass out but he was fine. After working with him for a while, though, he eventually started falling asleep in the break room. It was only a few times, so he didn't get in too much trouble, but it did happen and in the long-term, it would definitely cause him trouble.
Another example... Another woman I knew at Wal-mart was the same way. Except instead of teaching dhamma or partying, her burden was even greater... She had kids and had to support herself with two jobs. On certain nights of the week, she got no sleep at all -- every week. That day of the week, she simply didn't sleep. On other nights, it was just a few hours here and there. I never saw her slip up, but I wonder if she managed to get through it.