Sleep, the Final Frontier

As you get older, you seem to be become more aware of the things that science can't explain rather than those it can. Two of these I've always been fascinated by - Sleep and dreaming.
Specifically, the fact that nobody knows exactly what sleep is for, how it works and why you must have it.

This is an interesting article on the danger of not sleeping for a prolonged time -

http://www.slate.com/id/2218092/

It references these experiments too -

http://www.nytimes.com/1997/01/05/magazine/awakening-to-sleep.html?sec=health&pagewanted=5