Hibernate is a feature of many computer operating systems where the contents of RAM are written to non-volatile storage before powering off the computer. When the computer is restarted it reloads the content of memory and is restored to the state it was in when hibernation was invoked. Hibernating and later restarting is faster than booting up every time. Most importantly it saves power and hence lessens your carbon footprint.
Follow these steps :
1. Open Power Options in Control Panel >>Performance and Maintenance
2. Click the Hibernate tab, select the Enable hibernate support check box, and then click Apply.
3. Click OK to close the Power Options dialog box.
Now next time you shutdown your PC ,it will be in hibernate mode
If you are using Windows XP , the Shut Down menu will present the options to Stand By, Turn Off, or Restart your computer. Hold down the Shift key, and the Stand By button will change to Hibernate.
To Automatically shutdown your PC in Hibernate mode, go to "Power Schemes" Tab in power options window and then set the appropriate time in which you want your PC to move in to Hibernate mode.