Weird behaviour, During the night, Mac wakes every hour on it's own and does this - log attached, any idea why?

I think this may be your Bonjour Sleep Proxy waking up your Mac to see if it's alive. Take two of these and call me in the morning:

http://support.apple.com/en-ca/HT201960

And also check this:

http://ispire.me/fix-yosemite-rtc-alarm-wakeup-issue/

Normally a Mac will wake to do some housekeeping, including checking email. However, its not a 'full wake' in that the screen does not come on, external drives don't mount etc.

My iMac does this every hour, but the screen does not come on.

Maury is probably correct that BSP is waking your Mac while it is sleeping...

This log line indicates that you do appear to be connected to an Apple TimeCapsule Airport wifi network:

12/12/2014 04:20:59.662 airportd[84]: _doAutoJoin: Already associated to “Airport - TC1TB (5 GHz)”. Bailing on auto-join.

This line suggests that your Mac may be waking due to a Bonjour/MDNS over the network:

12/12/2014 04:21:46.000 kernel[0]: ARPT: 91381.078889: wl0: powerChange: *** BONJOUR/MDNS OFFLOADS ARE NOT RUNNING.

And this line suggests that your Mac has Wake for Wi-Fi network access enabled in the Energy Saver Panel of System Preferences:

 12/12/2014 04:21:46.000 kernel[0]: ARPT: 91381.566541: wl0: leaveModulePoweredForOffloads: Wi-Fi will stay on.

The line above indicates that it is leaving Wi-Fi enabled while your Mac is sleeping.

The two links Maury provided have information and solutions that work, but they require some interpretation:

• the Apple Support article explains Wake on Demand and Bonjour Sleep Proxy but doesn't explicitly explain how to disable these features:

  • these should be disabled by unchecking the Wake for Wi-Fi network access and Enable Power Nap while plugged into a power adapter in the Energy Saver System Preferences panel
  • these features can be disabled provided you do not have your Mac providing these services while sleeping:
    • iTunes and iPhoto Sharing, Printer Sharing, Back to My Mac, Local File Sharing, Screen Sharing, other sharing services
  • if you are using these features some can be done with your TimeCapsule instead

• the 2nd link provides instructions to deactivate these features by disabling multicast for the discoveryd service daemon, which seems like a poorer solution than turning off these settings in the Energy Saver panel