Change runlevel instead of restart?

Lets say for arguement's sake that I have a bet going with a friend that I can have longer uptime than they can on their Window XP SP2 box (we know how this is gonna turn out but anyway...)

Assuming that I want to still use my machine and keep up with all updates, is there any simple way that I can turn off the Aqua interface and drop down to a lower runlevel, instead of doing a "reset" on the system and then bring it back up from there, just as you can in *nix with X11? This I assume would keep my uptime clock unchanged and unaltered since I have still been "up"

Is there any downside to doing this (assuming you can, which it seems on a BSD based system you should be able to do easily)? How do you quit out of Aqua (or whatever the technical name for the graphical interface is anway) to begin with? How would I start it back up? Pretty simple task in *nix that doesn't seem to have a clear option in OS X.