What do you mean by "sleep mode"?
Sleep Mode allows the local repeater receiver to be effectively disabled after a set period of inactivity (keeping the associated transmitter quiet). The repeater receiver will be re-enabled if any of the following events occur:
- An "awaken from sleep" (
cop,53) command is decoded on any command source- There is traffic coming in from a remote node
- There is telemetry coming in from a remote node
Huh... Now that you point it out I remember that but I completely forgot that existed. 
I've never used it.
Sleep mode will not engage until everything in the cue is played.
However, if you are playing any messages on schedule, it will cause it to awaken.
I think you can monitor the status though ‘stats’ on the given node.