Sleep hours
Let me preface what I am about to say that I am a board certified neurologist and Assistant Professor of Neurology at a prestigious American medical school. I see patients for sleep problems at least once per week.
Most of what has been said here is true. Each person needs the amount of sleep that he needs. Most people's need requirement in the normal distribution of 7-9 hours. The amount of sleep that people need as they age decreases, and elderly people have a harder time getting more sleep.
There are a few people who run on a higher amount of energy than the rest of us, they need less sleep, they perform well in lots of circumstances when the rest of us are tired, etc. These people are labeled "hypomanic". We had a joke in medical school about what to call these people, because many of them excel in academic medicine where intensity and time shortages rule. We called them "damn lucky."
It wouldn't surprise me that the person who wrote that article about getting only 4 hours of sleep really only needs 4 hours a night. A faculty member I work with in my department is always on the go, has tons of NIH grants and recruits lots of private money, has very many people working for her, and travels nationally an average of once every two weeks. She needs 3-4 hours of sleep a night and can function well on that. She is hypomanic. Most of us are not. I am not. That puts us at a comparative disadvantage in our achievement addicted society. Oh, well, I guess I am a mere mortal.
The post immediately prior to this provided a good example that there are serious neurological and psychiatric effects to not getting enough sleep. The reactions of hearing voices and seeing things that aren't there are called psychotic reactions, and having them is psychosis. This is very serious and can get you hospitalized on a psychiatric ward. People in our society are chronically sleep deprived, and driving when drowsy is more dangerous than driving after drinking. More dangerous. That is NOT an invitation to drink and drive, just a point to convince you that it is dangerous.
In my opinion, the purpose of GTD is not to make you a billionaire. The purpose is that that you do not go insane. Keeping track of agreements, projects, tasks, having them all in your sight so that you can make informed decisions and stay relaxed with your many commitments to yourself and others is the purpose. Use GTD in such a way that you can get the amount of sleep that you need.