I've noticed that whenever I install updates via Software Update, it will say optimizing hard drive. Is this something like defrag on Windows? Should I manually do it every once in a while, or just let it go whenever I update some piece of system software?

Also, I was checking my free disk space, and noticed a little thing that said "Content Index" on the hard drive. It looked like it hadn't been indexed before. Is this something that is supposed to run at night? Should I leave my computer on at night? Oh yeah, this is an ibook. I usually put it to sleep when I'm not using it.