If the desktop machine does not have an “ACPI Wake Alarm” device (or if it is disabled in the BIOS), Windows 8 still uses the Real Time Clock (RTC) to program wake events, assuming the power policy / Advanced settings / Sleep / Allow wake timers is Enabled for AC power.
Windows 8 automatically configures a "Regular Maintenance" event in TaskScheduler to run at 3:00 AM every day. After initial installation of Window 8, Windows Update is preconfigured to initiate the regular maintenance task and wake event to ensure that it is run.
NOTE: On mobile computers, if there is no “ACPI Wake Alarm” device detected, Windows 8 disables “allow wake timers” on all in-box power plans, and “Allow scheduled maintenance to wake up my computer at the scheduled time” is not enabled.
1. From an Administrator command prompt, “Powercfg /waketimers” will indicate that the Regular Maintenance task is scheduled to run at 3:00 AM. For example:
C:\> powercfg /waketimers
Timer set by [PROCESS] \Device\HarddiskVolume1\Windows\System32\services.exe expires at 2:59:29 AM on 12/4/2012
Reason: Windows will execute 'NT TASK\Microsoft\Windows\TaskScheduler\Regular Maintenance' scheduled task that requested waking the computer.
2. Event ID: 808 will be logged in the Application and Services Logs/Microsoft/Windows/TaskScheduler/Maintenance log, to indicate the application which caused the regular maintenance event to be scheduled. For example:
Event ID: 808
Maintenance Task "\Microsoft\Windows\WindowsUpdate\AUScheduledInstall" requests computer wakeup during next regular maintenance run.
Any application can scheduled a regular maintenance task to run automatically, as described by the following MSDN links.
The first application to make use of the Regular Maintenance task scheduling feature is Windows Update. The .NET Framework NGEN v4.0 utility has also been observed to cause the regular maintenance event to be scheduled.