Mac mini won't wake from sleep. A SOLUTION.

I've been having the same problem everyone else has:

Mac mini won't come out of sleep, requiring a shutdown and reboot.

I found the cause of my problem. This solution has been tested.

Your system may differ from mine, but my solution may give you ideas to try.

My system: Mac mini (mid 2011), running Lion.

On March 19, the day before the trouble started, I sent a print job to my printer.

The next morning (March 20) the trouble began, and happened again March 21.

Exact Symptoms Seen: Display goes into a power save mode; however the computer never enters sleep (front panel LED remains on, all night long, not doing its usual slow-breathing sleep pattern).

Notably, the file "system.log" does not show the usual "maintenance alarm" wakeup every 2 hours. The logs just go quiet.

Printing triggered the problem:

My printer is a late-model Brother laser printer.

Beginning on March 19, I had been using Command-P to print documents;

however, at no time did I turn the printer on; my intention was to let my print jobs accumulate for several days,

after which I planned to turn the printer on and do my printing and mailing all at once, for efficiency.

This helped me: This solved my problem:

Late on March 21, I wondered if something in the print queue was preventing sleep.

Experimentally, I opened Brother’s print queue app and deleted the print job in the queue,

and BINGO, I instantly regained the ability to sleep and wake my Mac mini.

Further testing showed that the problem is repeatable at will. :^)

On March 22 I tested the mini to make sure it sleeps and wakes.

Then I sent a print job to the queue, leaving the printer off, and tried to sleep the computer.

The problem returned immediately: The display went to sleep but the computer did not.

Next, I logged in via ssh over the LAN and issued:

% lpq

to get the job number of the print job, and then

% lprm [jobnumber]

to clear the printer queue.

This deleted the print job, and the computer went to sleep immediately.

Moral: They say peripherals can prevent sleep.

Well, this shows that a peripheral can prevent sleep without even being turned on!

Tip: If you *must* shut your mini down, first try logging in over the LAN via ssh and issue

% sudo shutdown -h now

for a more orderly shutdown with less risk of data loss.

PS. Usually I can solve any sleep problem if I ssh in from another computer and issue

% killall ScreenSaverEngine

but that didn’t work in this case.