I find that my machine (incidentally, a 1974 G5 on OS 10.5.8, son to be replaced by the new iMac) goes slightly crazy now and then if it has been running (including sleeping) for about a 1 1/2 or 2 days. Eg., the optical disc drive re-closes immediately, applications respond in an odd way, a file does not appear on the Desktop despite being saved there, the spinning beachball goes on for ever.) I restart and all is well. This has been a feature of my Mac experience since 1995 with several macs. With the Classic OS (predecessor of OS X) an American Apple tech guru at a Mac World conference advised restarting a Mac every 5 or so hours, but Classic was a notoriously unreliable OS. I suspect that (even OS X) collects gunge after a day or so which needs clearing out. That may be one of the reasons that servers (always on) need a server version of the OS.