Windows won't wake up from Sleep

[Karaim]

I have Vista 64. Vista is programmed to put the PC to sleep after 10 minutes. After about 30 minutes or so of being in sleep mode, I can't wake the PC up from sleep mode.

Does anyone have an idea of what this can be?

I am thinking it might that that when my harddrive turns off (after 20 minutes), that that's what's causing the problem.

 

[GameMaster]

Here, read some of the articles below, those may help you.
http://www.vistax64.com/vista-performance-maintenance/41838-cant-wake-sleep-hib.html
http://www.eggheadcafe.com/software/aspnet/30988006/sleep-and-wont-wake-up.aspx
http://www.tomshardware.co.uk/forum/247388-13-computer-wake-sleep

There are many ways to fix it, you choose the one you feel is the best for you.
If you ask me, I'd suggest you to disable Hibernating on your Vista OS.