Margaret Thatcher was famous for needing only four hours of sleep. That's the popular wisdom, at least; her critics might argue that chronic sleep deprivation explains some of the crazier policies of her latter year.

What researchers now appreciate about sleep is that, as your brain tires, it sucks energy (in the form of glucose) away from the areas needed from critical thinking, in favor of those areas that keep you alert. So you can turn up--you just can't think very well.

What is striking about leaders, however, is that even those who do get a decent eight hours a night are mostly early risers. Helena Morrissey, CEO of Newton Investment, gets up at 5 AM. Vittorio Colao, CEO of Vodafone gets up at 6. Tim Armstrong, CEO of AOL, gets up at 5 because, he says: "Life is too exciting to sleep."

I suspect there's a virtuous circle here: Great jobs make you want to get up early--and the better the job you do, the more exciting getting up early becomes. Waking late could be a sign not that you're indolent but that work is, after all, not so thrilling.

What intrigues me about leaders' daily schedules is what they do with those early hours. The very disciplined have learned not to start shooting out emails at 5 AM. At the very least, they park them in the inbox until 8. Some go to the gym, others use the time to think and collect their thoughts before the day begins. For myself, I like to wake up early and lie in bed thinking through the work that lies ahead of me. It means that, when I do get up, I feel mentally prepared. I love working in the house while everyone else dozes. But I also recognize that I'm lucky to do work I love.

If you're sleeping late; what does that mean? Are you exhausted--or just disengaged?