I wasn't sure if I'd bother responding, but I think it's important because who knows who may be reading this.
The human body needs sleep. People who go without REM sleep lose cognitive ability in a measurable way. That is, they forget stuff, say "huh" a lot, lose focus ("you listening to me?"), and make dumb decisions.
People who go without sleep for long periods of time (many days) start having psychotic episodes. This means they start hearing voices, seeing things, and making bizarre comments, observations, and decisions. This is the basis of real-live "brainwashing" techniques. You want to crack someone? Just muck with their sleeps schedules and be patient.
How much sleep do you need? Like many people here have said, it varies by person (and probably is both nature & nurture). If you look for averages, you'll typically see 7-8 hours. In fact, in a big study in the 60s(?) that tracked mortality rates among people, and associated these with smoking (primary target of the study), they noted that people who slept 7-8 hours had a mortality rate of 1, and that each hour band in *either direction* raised the mortality rate.
Does that mean sleep longer/shorter amounts causes death? No. It's a correlation, not causation. If you're wondering about this, consider: is it possible that people who are very unhealthy (meaning diseased, not simply weak) also happen to sleep more than the average person? Is the mortality rate higher in that group?
Bottom line is what other people here have said: get sleep, find out what times work best for you, and if you're looking for a number, 7-8 seems to be an average out there.
-Jeff
