Whats the deal with sleep?

At the moment I am so insanely tired, I can barely function. It's hard to think, it's hard to talk, it's hard even to move. All because I haven't gotten my sleep.

Why is it that without sleep our bodies will just shut down? Oh you can prolong it for a short period of time. Possibly up to a week, but eventually your body will take over and you will be out. Then a couple hours later, magically you're back to tip top shape. You don't even really have to make up all that lost sleep, just the regular 8 to 10 hours will do! Why can't I just sit here and not do anything to strenuous and get the same benefits? What is it exactly about sleeping that makes you function that much better?

I think it's kinda like a reboot for your mind and body. Like you got to many things going so you need to turn off for awhile and restart all your processes. It's a little weird to think about but it kinda fits. The only part that doesn't, is that you really aren't turned off. The mind is still going. It doesn't turn off. You dream and your subconscious takes over and does it's thing. Which then goes back to the question if you aren't really off, then why can't you sit and not do to much and get the same benefit?

I just can't figure it out. Although that might just be because I am so tired and I can't think so clearly.