I have seen some studies about this, and what Sraaz said is mostly correct. You body itself does not need that much rest in order to maintain functions, it is your brain that really needs the sleep. Throughout your day your subconscious gets full of stuff, and when you sleep your brain vents those by the way of dreams. Son in actuality, it is not sleep, but rather the REM cycle that your brain needs.
The study that I read entailed having two groups of 10. They each slept for 8 hours a night, but one group was woken up just as soon as their REM cycle started, and the other group was woken up just after REM ended. The group that was not allowed to have REM sleep started experience hallucinations and other psychological problems after 5 or 6 days.
So, your body can do it (it only really needs about 30-60 minutes of downtime a day apparently) but your brain won't be able to handle it. Especially if you spend that time studying, or whatever... things that really work your brain will shorten that amount of time that you will have before your brain goes on strike.