Repeatedly wakes and sleeps

Hey guys,

If I leave my hack in "sleep" overnight, sometimes I wake up to find that it is waking for a few seconds, going back to sleep for a few seconds, then waking etc. The case fans and LEDs come on for a few seconds, then off for a few seconds and this keep repeating until I manually wake the system up. Whats strange however is while this is happening there is no HDMI signal, so the screen remains black throughout. Any ideas?

Check if Wake For Network Access is enabled in Power options in System Preferences and disable Wake On Lan in Bios.