My Mac is waking up from sleep without reason.

Question

So this is what happens:
Sometimes I send my iMac to sleep and then I go away, but after I return, I've noticed the hard drive is working as if the computer were awake, not sleeping. For some reason it is waking up.
Sometimes I'm sitting at my office, and then I just hear the hard drive started buzzing, just like it does when it comes brack from sleep mode.
Nevertheless the screen remains off. But if I click on the trackpad, or press any key, it turns on and shows that the computer was already on (it's different when it's returning from sleep, it takes a little longer and you can hear the click of the hard drive being activated).
I don't know what is causing it to wake up!
Does anybody know it?

Answer 1

First: reset the > System Management Controller (SMC)
1. Shut down the computer.
2. Unplug the computer's power cord.
3. Wait fifteen seconds.
4. Attach the computer's power cord.
5. Wait five seconds, then press the power button to turn on the computer.
Second: check for a loose power plug and/or have an electrician check your homes power circuits, because it sounds as if your iMac is loosing power and then restarting when the power is restored.
Lastly: to prevent the iMac from restarting when there is power problem, go to: Apple / System Preferences / Energy Saver and uncheck the box for "Startup automatically after a power failure" that way the iMac will remain off when there is a power problem....