iBook won't stay asleep anymore

iBook periodically awakens during sleep and sometimes opens the CD tray.
hoarse with no name wrote on :

A few days ago I ran out of disk space on my 2001 iBook and had to restart without the running software being able to save any preferences. This isn't the first time this has happened to me and I knew from experience that my iBook would behave differently afterward and that it would take a while to get everything back the way I like it. This time something new is happening, the iBook periodically awakens during sleep and sometimes opens the CD tray. It will come to life 2-3 times in a row and then go back to sleep for about an hour. What is causing this?

TIA