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Sleep mode refers to a low power mode for electronic devices such as computers, televisions, and remote controlled devices. These modes save significantly on electrical consumption compared to leaving a device fully on and, upon resume, allow the user to avoid having to reissue instructions or to wait for a machine to reboot. Many devices signify this power mode with a pulsed or red coloured LED power light.
In computers, entering a sleep state is roughly equivalent to "pausing" the state of the machine. When restored, the operation continues from the same point, having the same applications and files open.
Sleep mode has gone by various names, including Stand By (for Microsoft Windows 98-Server 2003), Sleep (for Mac OS 8-Mac OS X, Windows Vista, Windows 7, Windows 8, Windows Server 2008), and Suspend (Windows 95, Linux). Machine state is held in RAM and, when placed in sleep mode, the computer cuts power to unneeded subsystems and places the RAM into a minimum power state, just sufficient to retain its data. Because of the large power saving, most laptops automatically enter this mode when the computer is running on batteries and the lid is closed. If undesired, the behavior can be altered in the operating system settings.
A computer must consume some energy while sleeping in order to power the RAM and to be able to respond to a wake-up event. A sleeping PC is a case of a device on standby power, and this is covered by regulations in many countries, limiting such power under the One Watt Initiative, from 2010. In addition to a wake-up press of the power button, PCs can also respond to other wake cues, such as from keyboard and mouse.
Hibernation is functionality allowing a computer to be turned off completely while maintaining its state. On switching back on the state is restored to the way it was, with files open, unsaved data intact, etc. Whereas a computer in standby mode has the computer's state saved in RAM, a system in hibernation has the state of the computer saved on the hard disk (swap partition).
Hybrid sleep 
Sleep mode and hibernation can be combined: the contents of RAM are first copied to non-volatile storage like for regular hibernation, but then, instead of powering down, the computer enters sleep mode. This approach combines the benefits of sleep mode and hibernation: The machine can resume instantaneously, but it can also be powered down completely (e.g. due to loss of power) without loss of data, because it is already effectively in a state of hibernation. This mode is called "hybrid sleep" in Microsoft Windows--except, of course, XP.
ACPI is the current standard for power management, superseding APM and providing the backbone for sleep and hibernation on modern computers. Sleep mode corresponds to ACPI mode S3. When a non-ACPI device is plugged in, Windows will sometimes disable stand-by functionality for the whole operating system. Without ACPI functionality, as seen on older hardware, sleep mode is usually restricted to turning off the monitor and spinning down the hard drive.
When sleep mode was first introduced it was usually not tested as thoroughly as the normal mode, causing various problems especially with peripherals such as computer mice. Partly for these reasons, in a working environment sleep mode was generally avoided because the costs associated with wasted time and materials far outweighed the savings in energy from sleep mode.
Microsoft Windows 
Microsoft Windows 2000 and later support sleep at the operating system level (OS-controlled ACPI S4 sleep state) without special drivers from the hardware manufacturer. Windows Vista's Fast Sleep and Resume feature saves the contents of volatile memory to hard disk before entering sleep mode. If power to memory is lost, it will use the hard disk to wake up. The user has the option of hibernating directly if they wish.
Sleep mode is often under-used in business environments as it is difficult to enable post-deployment without resorting to third-party PC power management software This omission by Microsoft has been criticized as having led to a huge waste in energy.
There is a significant market in third-party PC power management software offering features beyond those present in the Windows operating system. Most products offer Active Directory integration and per-user/per-machine settings with the more advanced offering multiple power plans, scheduled power plans, anti-insomnia features and enterprise power usage reporting. Notable vendors include 1E NightWatchman., Data Synergy PowerMAN (Software) and Verdiem SURVEYOR
See also 
- Advanced Configuration and Power Interface
- Green computing
- Hibernation (computing)
- One Watt Initiative
- PC power management
- Standby power
- Shutdown (computing)
- "About safe sleep". Apple Inc.
- Enable hibernate using group policy
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