Kelly Hopper, you need to allow the Mac to do some routine cleanup operations every now and then. For some reason these "CRON" jobs are scheduled to run by default at between 3am and 5am. Some every day, some once and week and others once a month. You can download free utilities to schedule these jobs to run manually when you want to, just in case the jobs haven't had a chance to run for a while. I use MacJanitor.
Versions10.6 and later this does not apply. Tasks run automatically when waking from sleep mode. I believe there isa Dashboard widget that keeps track of them if you need confirmation that they have run.