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Does anybody know or have a link to documentation on how the Power Manager of Windows, determines to go to the ACPI power states S1-S4? Are there any settings that can be configured? I know that by default states S1-S3 are disabled in Vista and later (they can be enabled by a group policy). Can Windows cycle through these states while for instance an application is actively running?

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System Power States

To the user, the system appears to be either on or off. There are no other detectable states. However, the system supports multiple power states that correspond to the power states defined in the Advanced Configuration and Power Interface (ACPI) specification. The following table lists the power states from highest to lowest power consumption.

Working

S0

The system is fully usable. Devices that are not in use can save power by entering a lower power state.

Sleep

S1

S2

S3

The system appears to be off. Power consumption is reduced to one of several levels, depending on how the system is to be used. The lower the level of power consumption, the more time it takes the system to return to the working state.

Hibernation

S4

The system appears to be off. Power consumption is reduced to the lowest level. The system saves the contents of memory to a hibernation file, preserving the state of the operating system, applications, and open documents.

Soft Off

S5

The system appears to be off. Some components remain powered so the computer can wake from input from the keyboard, LAN, or a USB device. The working context can be restored if it is stored on nonvolatile media.

Mechanical Off

G3

The system is completely off and consumes no power. The system returns to the working state only after a full reboot.

More here:

ACPI / Power Management - Architecture and Driver Support

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Okay, but how does the Power Manager decide to cycle through those states? Are there internal timers, does it look for certain activity (like network activity, mouse activity, CPU load, ...)? –  Kurt Pattyn Dec 10 '09 at 21:50
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