So, I don't usually put general how-tos up here at Shirt Pocket Watch, but since I found this more than a bit frustrating and hard to diagnose, I thought someone else out there would benefit from what I found.

For years my Macs haven't gone to sleep on their own. I honestly don't know why, but there must be some application that's active enough to stop the sleep timer and prevent energy saver from doing its thing.

Annoying, but I'm pretty used to using Cmd+Opt+Eject to put the Mac to sleep. But recently, my Mac recently started waking unexpectedly—basically, I'd put it to sleep and whenever I'd go back (after an hour or so), it'd be awake, screen off.

I hate wasting power like that (not to mention the "fake" presence in AIM or whatever), so I spent some time diagnosing what was wrong.

It seems that the new Bonjour "Proxy" support implemented in Airport Extreme devices requires the proxy to be 'refreshed' on occasion by the proxied Mac. So, every hour, your Mac wakes itself without turning on the screen, updates the proxy, and then goes back to sleep.

Mine hadn't been doing that, but I recently reset my Energy Saver preferences to their defaults, and that turned on "Wake for network access". When that setting is on, this behavior takes hold. And if you Mac doesn't go to sleep on its own (for whatever reason), it stays on after the first wake.

So, in summary, it's a barely Documented Feature with surprising and unexpected behavior. Turning Wake for network access off in the Energy Saver preference pane resolves the issue.