|MadSci Network: Neuroscience|
Pretty much the same thing it does while you're awake. It sends signals down your spinal cord to control your heartbeat, your breathing, all the important stuff that keeps you alive and functioning properly. Sleep is composed of 2 easily distinguished stages or phases. One is characterized by slow EEG activity (a measure of brain activity), general muscle relaxation but with sporadic movements of the body, and deep breathing, and is usually referred to as slow wave sleep; while the other stage shows very fast EEG activity, similar to how our brain patterns look when we're awake, and skeletal muscle paralysis--those are the ones in your arms and legs which you can consciously control (but not your heart, and not the smooth muscles in your stomach or intestines, and also not the eyes, which move around rapidly--none of those are paralyzed). It does this so you won't move around and hurt yourself in your sleep by falling out of the bed or something. Sometimes this paralysis fails in people who sleepwalk (noone is exactly sure how or why). This stage is known as rapid eye movement sleep REM sleep) or as you may have heard, the main type of sleep where you do the most vivid dreaming. Let's talk more about the changes in brainwaves, the patterns of electricity that carries the signals between different parts of your brain. While you're awake, if you recorded your brainwaves and played them back through your stereo, they would sound like static. (If you listened to only one brain cell at a time, the signals might make sense, but that's another story.) The same thing would happen if you tried to listen to all the traffic on the Internet at once. All of those different signals mixed together is called desynchronized activity. While you're in slow wave sleep, all of that furious activity collapses down to three or four powerful rhythms. It becomes synchronized, meaning most of the cells start to fire electrical signals together--on, off, on, off, on, off, on, off... instead of off, off, on, on, on, on, off, off... However when you enter REM sleep your brain waves start to look like you're awake again! Dreaming may be a way of making your memories permanent, like writing them from computer memory onto the hard drive, but nobody knows for sure. Try running a search on Google. Here's a couple of sites I found there: A brief Introduction to Sleep
If you look at their graph of sleep, you can see REM sleep represented by the blue bars, and slow wave sleep by the red lines. As you will see, there are different levels within slow wave sleep, 4 being the deepest sleep and 1 being the lightest sleep (at 0 you are awake). http://www.stanford.edu/~dement/ http://www.sleepnet.com/ (This one looks especially good, because you can ask them questions, too)
Try the links in the MadSci Library for more information on Neuroscience.