Set Computers to Sleep Automatically
If you have a PC
Windows Vista: Control Panel → Power Options → Choose any preferred plan → Click Change Preferred Settings → Choose Turn off the Display after 10 minutes and Put the Computer to Sleep after 15 minutes. To save slightly more energy, enable Hibernation (click Change Advanced Power Settings, click the + next to Sleep and the + next to Allow Hybrid Sleep and choose On) and turn your monitor off when you leave for the day.
Windows XP: Control Panel → Power Options → Choose Energy Star power scheme. To save slightly more energy, enable Hibernation (check the box on the Hibernation tab under Power Options) and turn your monitor off when you leave for the day.
To Hibernate or Not to Hibernate: In addition to the energy savings, the advantage of Hibernation is that it protects any unsaved files in case of a power failure; the disadvantage is that it takes a little longer to start back up. If you both enable hibernation and turn your monitor off when you leave, you will be saving as much energy as if you turned your computer off. But if for some reason Hibernation interferes with your work or computer function, don’t feel too bad – your PC’s sleep mode saves almost as much energy.
If you have a Mac
Apple → System Preferences → Energy Saver. Choose to put the display to sleep after 10 minutes and put the computer to sleep after 15 minutes. Check “Put the hard disk(s) to sleep when possible.”