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I have my iMac set to set the display to sleep after 15 minutes. I notice at night that, even when no one is touching the computer, the display will come on for a while and then to go back to sleep after it's usually set time (10 minutes).
Is there any way to find out what's waking it up? Or even better, to force it to keep the display asleep?

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Try this in your Terminal:

syslog |grep -i "Wake reason"

Then you'll get something like:

Sat Sep 08 09:28:12 MacBookPro kernel[0] : Wake reason = OHC1
Mon Sep 10 21:14:54 MacBookPro kernel[0] : Wake reason = PWRB
...

Depending on the text after the =-sign, you have the reason why your Mac went to sleep. And it can have multiple causes:

  • OHC: some USB device. If you see OHC1 or OHC2, it's very likely a USB keyboard or mouse has woken up your iMac.
  • EHC: USB interface too, but if you see this, it will likely be a wireless device.
  • USB: another USB device (drive, printer,...)
  • PWRB: If your powerbutton gets pressed (very unlikely)
  • RTC: a scheduled wake-up (energy saver, backups, iCal events,...)

I hope your item is in this list :)

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Applications folder -> Utilities folder -> "Console" application will show you logs.

Maybe you see something like USB wake up

Do you have some bluetooth HID like keyboard mouse ?

found at http://forums.macrumors.com/archive/index.php/t-428248.html

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