There seems to be some confusion about Sleep. It's a new feature introduced
in Vista and combines the functions of Standby and Hibernate. It saves an
image of RAM to the hard drive (traditional hibernate function) AND keeps
RAM refreshed while powering down most PC functionality (standby). This
allows the very fast wake up time of standby while protecting against data
loss in the event of a power outage.
That being said, personally I like to turn the computer off overnight, but
use sleep if it will be inactive for some hours during the day but used
again later. This purely a matter of personal preference, but for users who
use Sleep all the time I recommend an occasional reboot. Vista seems quite
stable, but all versions of Windows have tended to develop some system
corruption over time if run for extended periods.
http://www.microsoft.com/windows/products/windowsvista/features/details/fastsleepandresume.mspx