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Sometimes, I leave my MacBook Air on for thirty minutes. When I return, and move my mouse around, I see the cursor on the screen moving around, but the screen is extremely dim. In fact, I could barely recognize that the computer was on. I could still click on icons and interact with windows though.

Why did that happen?

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It's likely in your screen saver settings. –  afragen Apr 10 '12 at 2:04
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Check System Preferernces > Displays > Automatically adjust brightness. –  timothymh Apr 10 '12 at 11:44
    
Can you provide additional details of your setup? Was your MacBook Air connected to an AC adaptor? Does the MacBook Air connect to a thunderbolt display or other external display? When returning to your MacBook Air, does the screen light up again once you interact with the trackpad? –  MrDaniel Apr 10 '12 at 21:11
    
Thanks. Yes, it was connected to an AC adapter. I am using no external monitor, and the screen does not light up as I move my mouse. However, if I press the power button for a while, the monitor lights up again. –  David Faux Apr 11 '12 at 0:06
    
I have had the same experience ad Dave. I looked that the answer above and took a look at my energy saver settings. The screen sleep and computer sleep were set to the same amount of time. - Could this be a glitch? –  Nohl Apr 11 '12 at 1:39

1 Answer 1

With the information provided my Initial impression points to your "Energy Saver" preferences where the "Automatically reduce brightness before going to sleep" option is enabled. The dimming not occurring before 30 mins is likely due to the "Computer Sleep" settings your MacBook Air is set to.

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Upvoted - this worked for me on Lion 10.7.4, and is still working after some months. Make sure to change the prefs for both Powered and Battery. –  RichVel Nov 2 '12 at 9:35

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