iMac won't stay asleep

I have a mid-2011 27" iMac with a SSD and hard drive. Whenever I put my iMac to sleep the screen turns off after a few seconds but it takes around 15 seconds before I hear the fans and hard drive die down - is that normal? Then, after maybe 5-10 minutes, I hear the fans spin back up together with the hard drive although the screen stays off; the fans/HD go back to sleep after a little while but then come back, over and over.

I have an external hard drive that I thought might be the culprit but this behaviour happens even with that drive disconnected. My iMac's hard drive is my main drive at the moment (waiting for Yosemite before using my newly installed SSD), but I have installed Mavericks on my SSD to test it out and the behaviour occurs then as well. And this happened before I had my SSD installed too, so I don't think it's part of the problem.

What am I missing here?

Saileshbreaker said:

You probably have "Power nap" enabled?

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Power Nap is only supported on Late-2012 and later iMac models, so mine does not have it.

DanielCoffey said:

Is Wake on LAN active in the power settings? It is only really needed for the Bonjour service.

Here is a link to find the Reason for Wake codes...

http://osxdaily.com/2010/07/17/why-mac-wakes-from-sleep/

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Could Wake on LAN be interferring although I have my iMac on wifi (nothing connected to the ethernet port)? And thanks for the wake codes link - I'll check those when I get home tonight.