Computer whirring after being silent

So when I go to bed at night, I usually leave the laptop online. My room is deathly silent for a while (aside from the very quiet laptop fans), but then suddenly – after about 20 minutes since closing the laptop – the hard disk starts working frantically, as if it’s processing something huge. It does this for about 5 minutes, then sporadically for the next 5 minutes or so after that.

I only notice it up until that point; I usually fall asleep by then. But what could it be doing??? I’ve already run the latest version of AdAware and Norton on it; it’s clean of adware and virii. It IS connected to the cable modem overnight, but the activity lights don’t flash when it’s doing its thing, and the overnight activity doesn’t look high enough to be normal idling.

What could my computer possibly do to itself after 20 minutes of idling?

Dumping the cache(s), unloading programs from memory, virus scans, large (3rd party) screensavers, trojans, failing power management components – any of these can cause that.

Hmm… sounds reasonable. But why would they all wait that long before doing something (aside from the screen saver or scheduled virus scan-- neither of which I have).

Windows operating systems like to “optimize” their swap file if you leave them alone for long periods of time. It’s trying to make your system more efficient when you finally get around to needing the disk again.

A cache contains the most recently / heavily accessed files. After a period of time, they would expire and be purged.

Depending on OS, the paging file and / or swap file will also unload on timed intervals, removing unused data.

Most modern OSes are quite capable of monitoring idle time, and initiating housekeeping tasks if it reaches a threshold value.

If you have the Task Scheduler enabled, it could be defragging your hard drive. Does it always do this at a particular time of night or after you’ve left it idle for a certain length of time?