Wat exactly is sleep? What's makes us fall asleep? What makes us need sleep?

Our body is telling us that it needs to recooperate from the day, and rest up for the next. There's a lot of things that have yet to be determined about sleep. All I know is that its something we all do, and its something that we need to feel refreshed for the next day. So enjoy it!

Yes but what is going on in your head that puts you to "sleep"

I found the information on the following site to be particularly helpful concerning the mechanics of sleep. I would have copied and pasted some it here and also given credit to the author, but I think that a full reading of what is on that site would be best.

http://library.thinkquest.org/11189/nfmechanics.htm