Your questions: "when should I use Sleep and Hibernate modes on Windows?"
I get the question a lot on sleep vs. hibernate (and as noted in a recent post), and it came up today in a discussion.
"I'm confused by the different selections I have in the Start menu... when should I use Sleep and Hibernate to turn off my computer?"
In Sleep (or standby), you'll see that the computer is generally off, with a trickle of power keeping the computer's memory powered. When you use your PC throughout the day, but leave for extended periods (lunch, meetings, even overnight), Sleep is a good choice. And (depending on your model) all it takes is a mouse movement or keyboard tap to wake the computer up.
One of the reasons I also have a UPS on my main desktop at home is that I use Sleep most often, and if the computer loses power, the work you may have open but not yet saved will be lost. And on our notebooks at home (and at the office) we use Sleep given that the computer's battery acts like a mini UPS and generally has enough power to keep the memory alive. (Note that we generally keep the notebooks connected to a power supply as well.)
When I leave for the weekend, I often use Hibernate on my PCs at the office and at home. In this state, the PC's complete memory state (the contents of RAM) is saved by writing to the hard disc and the computer powers down. When you turn the computer back on (recovering from hibernation) via the hibernation file, you start up where you left off.
When a PC starts up, Windows Vista performs many processing tasks in the background, returning control to the user much sooner than previous versions of Windows. Even this short wait happens less frequently than before, because instead of shutting down the computer to save power, users can use the new Sleep state, the default state for turning off computers running Windows Vista. Sleep combines the resume speed of Standby mode with the data protection and low power-consumption characteristics of Hibernate. When entering the Sleep state, Windows Vista records the contents of memory to the hard disk, just as it would with Hibernate. However, it also maintains the memory for a period of time, just as Windows XP maintains the memory in Standby mode. Windows Vista enters and recovers from Sleep state in seconds, and while the system sleeps, power consumption is extremely low.
The most significant benefit of Sleep is simplicity; users don't have to choose between using Standby or Hibernate because Sleep offers the best of both.
As a follow up, here's a little more information on Sleep and Hibernate modes in the OS, as the Productivity Portfolio weblog has a good post that describes the Windows XP power schemes.
Click here to learn how to use your computer efficiently and to save energy by activating Sleep mode for your monitor.