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I've recently upgraded my MacBook Air to Mountain Lion. Now whenever the machine sleeps for a long time (more than two hours or so) while disconnected from the mains, it won't wake up. I had to hold the power button to reboot the machine.

Is there a solution to this problem?

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This is a known issue that hasn't been fixed as of 10.8.1. When Mountain Lion hibernates (either directly or as part of standby mode), it couldn't wake up and thus need to be restarted.

The current workarounds are:

  • Disable hibernate mode.
  • Disable standby mode.

I've written an article how to get around this issue by preventing OS X from hibernating.

For Apple folks: please see rdar://1212084 - Thanks.

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    Apple still hasn't fix this in 10.8.2. – adib Sep 20 '12 at 12:45
  • or 10.8.4, as far as I can see. – tekknolagi Aug 30 '13 at 14:24
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I encountered the same problem, maybe you can try the following ways to wake up your Mac.

Hold the power button for two seconds, then release the button, not until reboot. If this way does not work, try this:
Adjust the light button F2, maybe just the screen does not light.

Good luck!

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