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First off, let's begin with some research that I did before asking this question.

Now, in my situation, I was using my Macbook Pro (Mountain Lion) with an Apple USB keyboard connected to the computer. Also, a USB mouse (Logitech) was connected to the computer via a USB port on the keyboard. The computer was plugged into the adapter. I had to step away from my computer while using it (I left the lid open while I was away). Whatever I had to do actually took me away from my computer longer than I thought; I took a three hour nap. When I returned to my computer....it was neither in screen saver nor in display, much to my dismay because I had set the previously brightness to high. I certainly do not want to damage my screen by leaving it on for long periods of time and there could've been an extra risk from not going to sleep had the computer not been plugged into the adapter.

Hence, I would like to know what prevented my computer from going to sleep. Even if it went into screen saver or at least dimmed the brightness during those three hours, I would've been happy. My first diagnosis of the problem when I got back to the computer was to check my Power Settings:

Power settings when plugged into adapter

Power settings when using the battery

Hence, my power settings would've already allowed my computer to sleep after > 15 minutes. This led me to believe that perhaps me having my keyboard and/or mouse connected to the computer prevented it from sleeping, but I found that to be far-fetched because I'm sure many people (especially in the workplace) leave their laptop, connected to a keyboard and/or mouse, for long periods of time. Certainly during those time the computer will be able to sleep displays.

The only other reason why my computer may have not slept was because of an open application running a process that disabled the computer from sleeping. These were the only applications that were open:

  • Chrome (there was a blog post draft open in one of my tabs)
  • Messages
  • Dash
  • Inkscape
  • XQuartz (because Inkscape is an X11 program)
  • Terminal (but no shells were active)
  • Google Drive

There were no file transfers in-progress, no movies or music being played. I am not sure how any of these applications could've prevented my computer from sleeping.

The final factor that I determine could have maybe (but very unlikely) played a factor was the way I positioned my laptop. IN order to provide myself with a more ergonomic workspace, I place my laptop on top of a shoebox on my desk in order to keep the screen near parallel to my straight eye sight.

Okay, this is all the diagnostic information that I can think of in order to help other out with debugging this issue. Thank you for all responses!

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Use the command line tool pmset to show your power management log: "pmset -g log". This will show assertions blocking sleep and possible errors: developer.apple.com/library/mac/#documentation/Darwin/Reference/… –  Graham Miln Oct 30 '12 at 6:56
    
This is the only line I see between me stepping away from my computer and returning to it 3 hours later: 2012-10-29 10:38:02 PM E Assertions PID 240(coreaudiod) Released NoIdleSleepAssertion "com.apple.audio.'AppleHDAEngineOutput:1B,0,1,1:0'.noidlesleep" 00:00:15 id:0x100000386 Aggregate:0x1040 –  ecbrodie Oct 30 '12 at 12:17
1  
The assertion being released at 10:38 told OS X not to idle sleep. Look back earlier in the log for when the assertion – and maybe others – were created. Was a process actively telling your Mac not to sleep? –  Graham Miln Oct 31 '12 at 8:18
    
Thank for that. I'll try digging into the logs later on. I hope that it is not too late to look at the logs from a couple days ago. –  ecbrodie Oct 31 '12 at 13:03
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