Rest easy, your computer is more likely to be damaged by a virus picked up from the Internet than by being turned off and on too much. They are also energy efficient: Such efficiency has reached the point where most PCs place themselves in sleep mode if they remain idle for a certain period of time. So your PC will likely slip into sleep mode anyway, even if you leave it on overnight.
Sleep mode itself, once a pretty unreliable option—you never knew if you would be able to wake your PC without having to reboot it—has been vastly improved with newer operating systems. If you want your PC to consume as little energy as possible when not in use, shut it down. If you want it to consume zero energy, you're going to have to unplug it.
Your PC can be in only three states: on, sleep or off (also called standby)—each of which draws some level of electric current. A PC that is "on" will either be actively processing information or sitting idle, depending on whether the user is typing a document, reading e-mail or has stepped away briefly. The amount of wattage drawn when the computer is on varies greatly depending on whether it is a laptop or a desktop PC. (The latter uses more energy because desktop power supplies are less efficient and require a separate and often larger, power-hungry monitor.) It also varies based on the type of work being done: Complex calculations requiring intensive processing are more power hungry, whereas writing or Web browsing consume far less electricity.
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