Currently, Windows 8 and Windows Server 2012 RTM computers check for updates from Windows Update or Windows Software Update Services (WSUS) daily at a default time of 3:00 AM (configurable) as part of an automatic maintenance task which runs every day.
If any of the updates applied required a reboot, clients and servers may reboot during production hours instead of during maintenance windows which can be defined in the following group policy:
Computer Configuration\Administrative Templates\Windows Components\Windows Update\Configure Automatic Updates
We have released an update KB2885694 which ships as part of cumulative rollup KB2883201. This update will change the behavior so computers honor fix installation times defined in the policy instead of the automatic maintenance task.
For a complete technical explanation, please visit our WSUS blog below:
Enabling a more predictable Windows Update experience for Windows 8 and Windows Server 2012 (KB 2885694)
Senior Support Escalation Engineer
Microsoft Commercial Technical Support