For Windows 8.1 desktops or All-in-one computers, under Action Center / Automatic Maintenance, the checkbox is automatically enabled for “Allow scheduled maintenance to wake up my computer at the scheduled time”. Also, the power policy / Advanced settings / Sleep / Allow wake timers will default to Enabled for AC power.
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.
Resolution:
To prevent the Regular Maintenance task from waking the machine at 3:00am, go to Action Center / Automatic Maintenance and disable the checkbox for “Allow scheduled maintenance to wake up my computer at the scheduled time”.