I had long wondered about the Vista power save schemes, and about sleep state — particularly that it did not exist; even when you tried to force the computer into sleep or hibernate, it would just wake up immediately, after a couple of seconds of flicker in screen. This weekend I finally got a half an hour and looked into this, and the problem was in BIOS settings. The default BIOS power save setting in most motherboards/BIOSes is S1 (POS, Power-On-Suspend), and that does not work with Vista. Vista expects the setting S3, which you need to set by pressing Del or F8 key during the power-up sequence, then use keyboard-only to navigate to the “Power” BIOS settings, and change S1 into S3; then press F10 or similar to save settings and exit/reboot. This was the way for me at least to get this damned Vista machine finally to get some sleep. For more, see e.g. these pages:
- http://www.mydigitallife.info/2007/10/30/missing-or-not-found-allow-hybrid-sleep-in-vista-power-options-advanced-settings/
- “Set up S3 sleep state in Vista for significant power savings” (http://articles.techrepublic.com.com/5100-10877_11-6185330.html)
frans goes blog 
great tip-
My vista machine keeps pulling all nighters and I have lost some sleep running through all the vista settings.
We are both sleeping easy.