Power Saving States

Windows Vista and Windows 7 implement three different power saving states designed to extend the life of a user’s battery or reduce power consumption: Sleep, Hibernate, and Hybrid sleep.

What is Sleep

Sleep is a power-saving state that allows a computer to quickly resume full-power operation (typically within several seconds) when you want to start working again. Putting your computer into the sleep state is like pausing a DVD player—the computer immediately stops what it’s doing and is ready to start again when you want to resume working.

Windows includes a default setting to put the PC into a sleep state after a period of inactivity. This is done to save power, but without risking your open programs or unsaved data. When you return to the PC and press the power button, you should find that it returns to your Window session in the state that you left it.

You can choose to put the PC to sleep manually as well, using the shut down control in the Start Menu

The disadvantage of Sleep is that power is still being used by the PC. Power must continually be supplied to things like the CPU, RAM, and any lights indicating the sleep state. A laptop on battery power that is left in Sleep will eventually run out of power and lose the data it is keeping in memory.

You can be in sleep indefinitely on a desktop PC or a mobile PC that is plugged in.

What is Hibernate

Another option in the menu above is Hibernate. Hibernation is a power-saving state designed primarily for laptops. While sleep puts your work and settings in memory and draws a small amount of power, hibernation puts your open documents and programs on your hard disk, and then turns off your computer.

Of all the power-saving states in Windows, hibernation uses the least amount of power. On a laptop, use hibernation when you know that you won’t use your laptop for an extended period and won’t have an opportunity to charge the battery during that time.

Hibernation saves the Windows state and the contents of memory to a special file on the hard disk. This file – C:\hiberfil.sys – is then loaded during startup to restore your Windows state where you left it.

Because this process of going into hibernate involves writing a large amount of data to the disk, it will take longer to go to hibernate than it will to go into sleep mode. This can be anywhere from 30 seconds to 5 minutes depending on the speed of the PC, and mainly the speed of the hard disk

The disadvantage to Hibernate mode is that it takes longer to both go into and come out of. Each is a disk intensive process and will take longer than their Sleep counterparts. Despite this disadvantage, Hibernate can be preferable to Sleep, because a PC can stay in hibernate mode much longer than it could in sleep since the battery is not actually being used.

Note: A mobile PC that is either switched off or in Hibernate mode will still see that the amount of stored power in the battery decrease if left for a period of weeks or months. This is expected, and is the nature of current battery technology.

What is Hybrid Sleep

Hybrid sleep is designed primarily for desktop computers. Hybrid sleep is a combination of sleep and hibernate—it puts any open documents and programs in memory and on your hard disk, and then puts your computer into a low-power state so that you can quickly resume your work. That way, if a power failure occurs, Windows can restore your work from your hard disk.

When hybrid sleep is turned on, putting your computer into sleep automatically puts your computer into hybrid sleep. Hybrid can be thought of as a combination of Sleep and Hibernate. When a user puts a PC with Hybrid Sleep enabled into Sleep mode, it will immediately save the current state both to the hiberfil.sys on disk and to memory, at which point the PC will go into normal sleep. If the user comes back to the PC a few hours later, resuming from sleep will be just as quick as the normal Sleep state. However, in the event of a power failure, because the state of the PC is also saved to the hard disk, Windows will be able to resume from disk and get back to the same state.

Hybrid sleep is enabled by default on desktop PCs only, as that is the configuration where a power interruption during sleep is more likely, and where the extra disk access to the hibernation file should not cause a problem.