Why would you want to do that? Linux is not like Windows where it needs a fresh boot to clear out the crap. Your NAS is designed to be run continuously, if you so desire. Let it do that.
There is no provision for a re-boot, just a power off/on. You could create your own process to do it, but there is no guarantee that it won't interrupt an important task, making things worse instead of better. The auto power-off does have provisions to insure it does not occur during processes over which it has control. For example, it will wait till a backup job initiated by the NAS completes. But if you have a backup job initiated by a PC, the NAS will not know to wait for its completion.
But to answer your question: you can't do exactly what you want from the NAS admin UI. But you can create a power schedule, where you turn off the NAS for an hour and then restart it.
