You take a deep breath, rub your tired eyes and prepare to push away from your personal computer after a lengthy instant message exchange, video viewing or analysis of your monthly budget—maybe all three. But before you exit cyberspace, a decision must be made: Should you shut the machine down, place it into "sleep" mode or do nothing at all?

How you end a computer session depends on how often you use the computer, your views on energy conservation (the amount of juice it uses while sitting idle), and what you have been told about how your decision will affect your investment's longevity: Will frequent starting and stopping cause its circuits to burn out sooner?

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.

When a computer goes into sleep mode, it shuts down everything but its random access memory (RAM), a group of memory cells (which represent bits of data) that retains short-term data for easy access, thus preserving the computer's last active state—the running software, used log-ons and other settings—so that the user does not have to reboot when active use resumes. Sleep mode has gotten a bad rap in the past because, "with Windows operating systems prior to Vista [which debuted in late January 2007] the 'resume from sleep' mode has not been that reliable," says Ken Bosley, Hewlett-Packard brand manager for consumer desktop PCs. "Sometimes the resume fails and you have to reboot anyway."

"The usability benefits of leaving a desktop on appear to be growing as indicated in our focus groups," says Glenn Jystad, senior manager for desktop products at PC-maker Gateway, Inc., which in October 2007 was bought by Taiwan-based computer company Acer, Inc. "Nevertheless, it is prudent for home owners to manage their PCs power settings so as to not unnecessarily draw too much power and grow their electric bills." One trick Microsoft Windows Vista users can try is to select the "balanced" power setting, which causes the PC to go into sleep mode with one hour of nonuse, he adds.

The U.S. Environmental Protection Agency (EPA) confirms that putting desktop PCs in a low-power sleep mode after a period of inactivity can lead to annual energy savings of $15 to $45 per computer. The EPA breaks sleep mode down into two categories: "system standby" and "hibernate." System standby wakes up faster than hibernate (five to 10 seconds compared with 20 or more seconds) but does not save work in the event power is interrupted or lost. This is because in system standby the PC saves work to RAM, whereas in hibernate it saves to the hard disk, which records data magnetically, thereby retaining it even when the power is cut.

If the EPA's projected cost savings fail to dazzle, there are other incentives for using computers more efficiently. The EPA says that PCs adhering to its latest Energy Star specifications are expected to save consumers and businesses more than $1.8 billion in energy costs over the next five years and prevent greenhouse gas emissions equal to the annual output of 2.7 million vehicles. Desktop PCs meet Energy Star qualifications if they use two watts or less of electricity in system standby mode, five watts or less in sleep mode and no more than 60 watts in active mode. Laptops qualify under Energy Star if they use 0.5 watt or less in standby, five watts or less in sleep and 15 watts or less in active mode.

When it comes to judging whether sleep or standby causes more wear and tear on your computer, pick your poison. Whereas disk hard drives are most likely to crash during the process of turning off the computer, leaving the PC on causes the microprocessor to generate heat—more heat than if the system is shut down—that will wear down the electronics over time. "Some components will last longer if you shut down your computer, others won't," Bosley says.

As a general rule of thumb, he says, most electronics have some failure rate linked to the amount of hours they're in use. A few hundred dollars will buy you either a blazing-fast new microprocessor or a spacious, terabyte-size hard drive, so the replacement cost differences are negligible.

Of course, the importance of whether to power down or put your computer to sleep depends a lot on how much you use it. If you spend 20 minutes each night reviewing your e-mail, it's a waste of energy to leave your PC on all day. If you're continually on your computer, or go back several times a day, it's best to leave it in sleep mode between sessions. Another variable is the efficiency of the PC's processor. Slower ones use less energy but have to work longer and harder than more powerful CPUs. "It's like a car," Bosley says, "you can't talk about gas mileage without talking about the vehicle's performance."

Ultimately, if you want to leave your PC on most of the time, your best move is to buy one that meets the EPA's Energy Star efficiency standards—Energy Star–approved PCs consume less than half the amount of energy as products without this designation—and also to make sure your computer defaults into sleep mode if it is inactive for any length of time. That sort of compromise will make sure your computer is ready for action on a moment's notice without padding your utility bills.