Why Does My iMac Wake Itself Up?

I have an early 2009 24" iMac. Lately, when in sleep mode it will occasionally wake up for around ten seconds with the screen off, then go back to sleep. Wake on LAN is turned off so I can't figure out why this is happening!

Solution 1:

There are a number of possible reasons, and the system actually logs the cause of each 'wake from sleep'.

Open a terminal and enter:

syslog | grep -i "wake reason"

(or, on Sierra or newer log show | grep -i "wake reason")

This may take some time to return a result, but when it does, you should get a series of lines that look something like the following:

Jan 24 08:52:19 hostname kernel[0] : Wake reason: OHC1

Here are the possible 'Wake Reason' codes (lifted from OSXDaily)

• OHC: stands for Open Host Controller, is usually USB or Firewire. If you see OHC1 or OHC2 it is almost certainly an external USB keyboard or mouse that has woken up the machine.
• EHC: standing for Enhanced Host Controller, is another USB interface, but can also be wireless devices and bluetooth since they are also on the USB bus of a Mac.
• USB: a USB device woke the machine up
• LID0: this is literally the lid of your MacBook or MacBook Pro, when you open the lid the machine wakes up from sleep.
• PWRB: PWRB stands for Power Button, which is the physical power button on your Mac
• RTC: Real Time Clock Alarm, is generally from wake-on-demand services like when you schedule sleep and wake on a Mac via the Energy Saver control panel. It can also be from launchd setting, user applications, backups, and other scheduled events.

Happy hunting!

Solution 2:

A common reason for random wake-ups is network accesses.

If you go to System Preferences>Energy Saver, there you will see an option saying "Wake for network access". If this option is enabled, then your sleeping Mac will be able to wake-up for network accesses. For example, when you initiate a screen sharing from another Mac on your local network.

Anyway, this is all okay BUT when this option is enabled it's also documented on Apple's site that it will cause random wake-ups by just running the hard-drives but not starting up the screen; and it will go immediately back to sleep.

Also @macaco have putten it beautifully that you'll probably get a code for this reason.