Support was committed to Mir on Friday for handling Display Power Management Signaling (DPMS) so that monitors will be able to enter down their power-down/sleep state.
While the Ubuntu 13.10 release is only one month away, a Mir Display Server feature not arriving until yesterday was support for DPMS. Display Power Management Signaling is the VESA standard for GPUs and monitors to communicate about display power savings, like putting the monitor to sleep after it's been idling for a certain period of time.
When using XMir with the unity-system-compositor on Ubuntu 13.10, the display would never be put to sleep due to DPMS not being implemented, per this bug report
. Fortunately, that's resolved with Mir Revision 1054
. Adding a DPMS configuration API to Mir also bumped the client library version to libmirclient3.