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I have an iMac which I sometimes use as an external display for my Macbook Air. Normally, I want the iMac to go to sleep after about 30mins except when I have the Air attached.

Is there any app (or Applescript?) out there which will change/disable the sleep time when it's being used as a display, but still let the iMac sleep when the Air is not connected?

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Wait, if you're using your iMac as an external display, won't it go to sleep only if your Air goes to sleep? – theAmateurProgrammer Sep 7 '12 at 9:47
That's what I was hoping but it didn't seem to be the case yesterday when I was using it. – ideasasylum Sep 7 '12 at 13:08

Theres a very popular app called caffeine that sits an icon in your menu bar, and when you click it the app prevents your computer from sleeping either indefinitely or for a predetermined period of time.

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Thanks. I already use Caffeine and it's a reasonable solution but I was hoping there would be something more automated – ideasasylum Sep 7 '12 at 13:09

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