If you leave your PS3 on all the time, or if it's running at 4 am you may see the HD light come one and if you look at whats running you see processes running.
Linux machines often do system maintenance at 4 am 0400 hours. It knows to do so because of cron, a daemon that runs all the time and performs jobs at specific times. You can use cron yourself to schedule jobs. (Though I've never done it.) For example, if an open source project released nightly tarballs at say 3 in the morning. Suppose you wanted the newest tarball downloaded, built and installed, every day. You could wait till you wake up and do it manually, or you could use cron to download the tarball, untar it, and compile and install it while you were sleeping so it would be already ready when you wake up. To learn how to do it:
man cron (tells you about the daemon)
man crontab (manual about the program that sets up the crontab file)
man 5 crontab (manual about the crontab file itself.
You may be wondering what happens if your system is not on at 4 am. In that case, a program called anacron will run jobs that were supposed to run but couldn't because your system is off, when it is back on again.
Ron Rogers Jr. (CronoCloud)