By default, Windows puts your computer to sleep when you press the power button on your computer. This applies both to the physical power button and to its software counterpart, which displays when you click Start.
You can also click Start and the arrow button for a list of additional options. Sleep automatically preserves your open documents and programs in memory and shuts down all nonessential functions.
Figure : Software power button
The main advantage of sleep is that it takes only seconds to restore your computer to where you left off—and uses only about as much electricity as a night light.
The disadvantage is that sleep mode retains the power to your memory (RAM). Thus, there is no saving of the memory contents to a file and which means no reloading of memory which makes it faster. However, if there is a power interruption for some reason while in this state, all your unsaved memory content is lost.
Hibernation is a deeper “off state” than sleep, and thus offers greater power savings at the cost of a slightly longer reboot time. It is also safer, because, unlike sleep, hibernation not only shuts down the power to peripherals (monitor, etc.) and hard drives, but also turns off the power to the RAM memory chips.
Hibernation saves (writes) all data in memory to a reference file on your hard drive before removing power. It then returns (reads) this file and reloads the data back into memory when you come out of hibernation. Thus restoring you to where you were before. The saving of memory data is why it takes a little longer to return (wake) from hibernation than coming out of sleep.
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However, if you want to power down your computer completely click the arrow next to the Lock button and then click Shut Down. This option also closes any open programs (you'll have the opportunity to save your work), shuts down Windows, and powers down your computer.