Automatic shutdown hindered by sleep mode

I have an iMac (late 2009 model) running Yosemite. Recently, I have set up automatic shutdown in the Energy Saver settings under System Preferences. However, if my computer goes into sleep mode before the automatic shutdown - which I have scheduled to happen after twenty minutes of inactivity - my computer will not shutdown automatically at the scheduled time.

The question: is there a way to make my iMac shutdown at the scheduled time even when it is in sleep mode, or is there no way to circumvent this and I'm better off with manual shutdowns or disabling sleep mode?

You can use a third party tool like Power Manager to automate reliable shut downs.

By default, events will wake a sleeping Mac before being performed. Typically your Mac will be woken 15 – 30 seconds before the scheduled shut down begins. Ultimately, the wake up is performed by OS X's IOPMQueue which you can see and edit through the pmset tool.

I am the engineer behind Power Manager, so do ask if you need more details.

You could set a start up / wake right before the shutdown time. There should be enough time before sleep kicks back in to have the normal shutdown be processed.

I'd say 5 minutes of gap on a 15 minute sleep timer would be ideal.