Have you ever been fast asleep only to be woken up by your computer waking itself from sleep? Ever wonder exactly why it woke up? I found a tip on the web recently that gave a great and easy way to try to determine what woke the computer up.
You’ll have to dive into Terminal a bit to get this information, and you may not necessarily get an answer. After your machine wakes up launch Terminal and type the following in:
syslog |grep -i “Wake reason”
You’ll then get a report from your OS that will give a code that tells you why the machine woke up. The article I found give some common error codes:
OHC (Open Host Controller): This likely means an external device like a USB keyboard or mouse woke the machine. It could also be an external FireWire device like a hard drive. To my knowledge, this should also include Thunderbolt devices as well.
EHS (Enhanced Host Controller): Similar to OHC except it’s usually wireless or Bluetooth devices.
USB (Universal Serial Bus): A USB device woke the computer.
LID0: This literally refers to the lid of your laptop. You shouldn’t see this code if you’re on a desktop.
PWRB: This refers to the Power Button of the computer.
RTC (Real Time Clock): This refers to wake-on-demand services like scheduling your computer with sleep and wake times in the Energy Saver settings.
As always, mess with Terminal at your own risk. Before you all start emailing me with other ways to do this, the article does point out that that there are other ways you can monitor this in the Console application, but this way you don’t need to wade through extensive logs filled with gobbledygook.