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Turns out that OS X Yosemite on Mac Mini does not support standard Wake-On-Lan (“WOL”), at least not when using the built-in Ethernet port.
Which is strange – but when trying to make the media server attached to the household’s TV a bit more energy efficient, I just couldn’t get that Mac Mini to come out of sleep using a WOL magic packet. Others out there report the same thing, so it’s not an isolated issue for me. Just to rule out issues with the WOL client used, I tried waking Windows and Linux machines from sleep – worked flawlessly. Weird.
There was however an easy workaround:
Connect the Mac Mini to the LAN via a cheap, ubiquitously available USB Ethernet dongle, instead of the built-in Ethernet adapter. True, you might only get 100 MBit instead of 1 GBit, but that was ok in this case. I tried using both an Apple branded Ethernet dongle and a generic branded one, both worked well.
With that insight it was then just a matter of setting the system settings on the Mac Mini to enter sleep mode after desired amount of time, as well as enable WOL to take the computer out of sleep.
To send the actual WOL packet, you can use any of various Windows, OSX or Linux tools written for this purpose. Many home/SOHO routers can also do this for devices on the network controlled by the router.