I have also a homebuilt system and it never goes to sleep. My Synology Disk Station DS213j goes to sleep. If I start a PC it wakes up and I guess that somehow it monitors LAN traffic, any LAN traffic. The only thing I can think of is that the NIC is set to detect traffic and wakes the disk station.
Does anybody know how it works? Is there a way to find out what NIC is used in the disk station?